Rocinela sp.

The bonefish is a popular recreational species for catch-and-release fishing. It is targeted by anglers using fly rods or light tackle, and requires some skills to do so as they're easily startled, and once hooked can put up quite a struggle. But if you are wading on a beach while fly fishing for bonefish, you might in turn become the target, because one of the bonefish's parasites may have its eyes on you too.

Left: Rocinela isopod feeding on a bonefish just above its right eye, Centre: Rocinela isopods on bonefish at the base of the dorsal fin and left flank, Right: Rocinela isopod dorsal view.
Photos from Figure 2 of the paper.

This blog has previously featured Cymothoidae isopods, which tend to be somewhat picky about what types of fish they parasitised But the isopod being featured in today's post isn't picky at all, in fact,  when it comes to its next meal, and it doesn't always have to be a fish. Rocinela is a genus of isopods that belongs to the Aegidae family, and unlike the cymothoids which tend to stay on their hosts for extended periods of time, these isopods are temporary blood feeders, rather like land-dwelling leeches or bed bugs. On rare occasions, they can even feed on human blood. But adopting this kind of free-wheeling blood-sucking can open yourself up to becoming an unwitting carrier of many microscopic passengers.

The study we're looking at in this post investigated the health and microbes of bonefish at Belize. The scientists in this study captured bonefish around Ambergris Caye, and examined each fish for scars and ectoparasites (such as Rocinela), then collected some blood samples for genetic analyses. The scientists also analysed the blood present in the gut of the isopods they collected, to identify what kind of fish they had been feeding on. Genetic analyses of blood-suckers' meals have previously provided valuable insights into the hosts of ectoparasites.

Two of the three sites the scientists sampled from were frequented by Rocinela, and about 70 percent of the isopods they found on the bonefish had plump bellies that were full of blood. As expected, most of the isopods were filled with bonefish blood, but one of the Rocinela also had blood from a type of small killifish called the mangrove rivulus, and somewhat alarmingly, there was an isopod in the sample which had fed on human blood at some point.

What's even more interesting were the plethora of virus sequences that were found. Possibly because of its indiscriminate feeding habits, Rocinela has inadvertently picked up about 11 different types of viruses. Most of those were viruses that usually infect arthropods. One of them, XKRV-2, is related to a group of viruses which have been previously reported from a range of crustaceans, including parasitic isopods, so its presence was to be expected. 

But one of the Rocinela also carried a less expected virus called XKRV-1, which is more related to a common genus of fish virus called Aquareovirus. None of the bonefish sampled had XKRV-1 in their blood, which means Rocinela has picked up the virus from one of other fish species that it had fed on. And rather than just being a transient, XKRV-1 has been persisting in the isopod's belly for a while - which is a common adaptation for vector-borne viruses such as those found in ticks and mosquitoes.

Given Rocinela can feed from a variety of fish, its payload of viruses may disembark into one of its hosts during feeding, so it could be transferring viruses between different species at sea. While Rocinela is also known to feed on humans, the likelihood of those fish viruses jumping into us is comparatively low - viruses that jump into humans tend to come from mammals and other warm-blooded animals, especially those that are evolutionarily closer to us, such as non-human primates. But a much bigger concern is that since Rocinela harbours so many different viruses and it is so indiscriminate about the type of hosts that it feeds on, it might end up acting like a transmission hub for viruses to jump from wild fish into aquaculture species.

Most studies looking at vector-transmitted diseases focus on land-dwelling arthropods such as ticks, fleas, and mosquitoes, but crustaceans like Rocinela and other parasitic isopods might be overlooked vectors that are providing a taxi service for pathogens under the waves.

Reference:

Goldberg, T. L., Perez, A. U., & Campbell, L. J. (2024). Isopods infesting Atlantic bonefish (Albula vulpes) host novel viruses, including reoviruses related to global pathogens, and opportunistically feed on humans. Parasitology 151:1386–1396.

Cymothoa indica (et al.)

Tongue-biters are among the most (in)famous parasites found in fish, but they aren't the only type of isopods that parasitise fish, nor is the mouth the only spot ripe for parasitism - there are many other parts of a fish's body where an isopod can make itself at home. Why, right behind the fish's mouth are its gills, and this cosy, well-aerated and blood-rich location is where some isopods reside. There are also others that cling to the fish's skin where they gnaw and suck on host tissue, and even some that just burrow into the fish's body cavity for extra coziness.

Photo collage showing a range of cymothoid isopods on various fishes: (a) Cymothoa indica male (smaller one in the photo) and female attaching to the buccal chamber of Datnoides polota; (b) Cymothoa indica attaching to the mouth of Jonhius sp.; (c) Nerocila loveni attaching to the skin in the ventrolateral region of Deveximentum Interruptum; (d) Nerocila orbignyi attaching to the tail skin of Mugil cephalus; (e) Agarna malayi attaching to the gill cavity of Nematolosus nasus; (f) Joryma sawayah male (smaller one in the photo) and female attaching to the gill cavity of Nematolosus nasus.

From Figure 1 of the paper

So there are many different ways to parasitise a fish and cymothoid isopods are particularly adept at doing so. But some isopods are pickier than others when it comes to which fish they parasitise, and it seems to have something to do with where they live on a fish. The study featured in this post looked at factors that may have driven the preference of these parasites. To do this, the researchers studied fish collected from commercial trawlers at harbours and fish landing centres along the north-eastern coast of India, from Petuaghat down to Gopalpur.

The researchers examined a total of 5798 fish, of which 923 (from 59 fish species) were parasitised by 21 different species of cymothoid isopods. With this massive dataset, they were able to compare the host preference of tongue-biters, gill-biters, and the skin-biters, noting how many different species of fish each of them parasitise, and the characteristics of the fish they infect. From their analyses, it seems that generally speaking gill-biters tend to be most specific - they stick to a single fish species and are mostly found in pelagic schooling fish. In contrast, tongue-biters tend to infect fish that hang out near the seafloor, and are less selective about their host species. And the skin-biters are happy to just go after whatever fish they come across.

This trend might have something to do with the life histories of those isopods. The gill-biters have free-swimming larvae that reach their host by getting sucked into the respiratory current of fish swimming through the water column, and if those fish are in a school, there would be plenty of hosts available nearby for the next generation of gill-biters. On another hand, tongue biters have larvae that hang out on the seafloor, waiting to ambush any foraging fish that come near, so they are more likely to encounter a wider range of fish. But even though tongue-biters can infect more fish species than the gill-biters, each species of tongue-biter definitely has a "type".

For example, take Cymothoa indica, a tongue-biter which is found in a wide range of fish species across seven different families - while that seems like it has pretty broad taste, its hosts all tend to be shallow water fish that live and feed near the seafloor. Similarly, another tongue-biter - Catoessa boscii - infect seven different fish species, but all those fishes are similarly shaped, as they are mostly deep-bodied fishes such as jacks and scads. Meanwhile, the skin-biters have larvae that roam freely around the water, and can launch its attack from the seafloor or while rapidly looping in the water column. Essentially if it runs into a fish, it just latches on and starts gnawing.

Parasitic isopods are found in/on fish all over the world, and they have significant impact on fisheries and aquaculture. But despite their ubiquity, they are relatively under-studied, with most of the published research on their taxonomy, biogeography and patterns of host associations coming from only a handful of specialist researchers across the globe. Studies like the one featured in this post can provide us with some much needed insight into the secret lives of these widely found parasites.

Reference:

Mohapatra, S. K., Swain, A., Ray, D., Behera, R. K., Tripathy, B., Seth, J. K., & Mohapatra, A. (2024). Niche partitioning and host specialisation in fish‐parasitising isopods: Trait‐dependent patterns from three ecosystems on the east coast of India. Ecology and Evolution 14: e70298.

Diexanthema hakuhomaruae

The study in this post takes us to one of the darkest corners of the deep sea, over 7000 m below sea level in the Kuril-Kamchatka Trench, located in the northwestern Pacific. Living in this dark and oppressive environment are isopods called Eugerdella kurabyssalis. And despite the crushing pressure, these crustaceans like it just fine, in fact they are the most abundant isopod down in those depths. But such success and abundance can also attract the attention of parasites, and this post is about a newly described parasitic copepod called Diexanthema hakuhomaruae.

Left: Diexanthema hakuhomaruae (indicated by white arrow) attached to the leg of its Eugerdella kurabyssalis isopod host. Right: Close-up of D. hakuhomaruae, the arrow indicating the copepod's ovaries. Photos from Figure 1 of the paper

Those who are familiar with this blog would know that parasitic copepods come in all kinds of shapes  that would defy most people's idea of what a crustacean is "supposed" to look like. And D. hakuhomaruae is no different - its tiny body is ROUND and if anything, it looks almost like a legless tick. And much like a tick, D. hakuhomaruae attaches itself stubbornly to the leg of its host.

Diexanthema hakuhomaruae belongs to the Nicothoidae family, a group of parasitic copepods that contains about 140 known species. They live on a variety of crustacean hosts, including tanaidaceans, ostracods, amphipods, cumaceans, mysid shrimps, and lobsters. Most of them have a rotund, almost spherical body, greatly reduced or no legs at all, and a specialised mouthpart that ends in a sucker with syringe-like mandibles. And much like the ticks that they resemble, these copepods feed by stabbing their mouth syringe into their host's body and sucking up that crustacean blood (hemolymph) on tap. Some species such as Choniomyzon infaltus are specialised egg parasites - their balloon-shaped bodies allow them to hide amidst broods of their hosts and feed on their eggs without being discovered.

There are currently six other known species of Diexanthema, all of them are parasites of deep sea isopods. And Diexanthema is not alone in its preference - there are other nicothoid copepods that have also been found parasitising deep sea isopods. What makes D. hakuhomaruae special is that it is the first to be found from the Hadal Zone. All other Diexanthema species have been reported from depths of 1300 to 3500 metres below sea level, but none of them had gone down as deep as D. hakuhomaruae.

It is unknown whether D. hakuhomaruae feeds on the host's fluid or if it is an egg parasite, or how it even completes itself life cycle in the hadal zone - as you can imagine, discovering and describing such parasites in an environment like the deep sea is challenging enough as it is. Studying the life style and ecology of these deep sea parasites with current technology is next to impossible. Even so, this description shows that parasitism is indeed ubiquitous on this planet, and wherever you find life, you can be sure that some of them will be parasites

Reference:

Kakui, K., Fukuchi, J., & Ohta, M. (2023). Diexanthema hakuhomaruae sp. nov.(Copepoda: Siphonostomatoida: Nicothoidae) from the Hadal Zone in the Northwestern Pacific, with an 18S Molecular Phylogeny. Acta Parasitologica 68: 413-419.

Cyclocotyla bellones

At the top of this blog, there is a quote by Jonathan Swift about how fleas have smaller fleas that bite them. Indeed, parasites becoming host to other types of parasites is actually a rather common phenomenon in the natural world. Those who would parasitise the parasites are called "hyperparasites".

Left: Cyclocotyla bellones on the back of a Ceratothoa isopod, Right: C. bellones coloured red with Carmine staining.
Photos from Figure 1 and 5 of the paper.

The parasite featured in this post was once suspected of being a hyperparasite. Cyclocotyla bellones is a species of monogenean - it belongs to a diverse group of parasitic flatworms that mostly live on the body of fish, parasitising the fins, skins, and gills of their hosts. But unlike other monogeneans, C. bellones does not attach itself to any part of a fish's body, instead it prefers to stick its suckers onto the carapace of parasitic isopods, such Ceratothoa - the infamous tongue biter. Since Ceratothoa is itself a fish parasite, and C. bellones is routinely found attached to those tongue-biters, this has led some to think that it might be a hyperparasite of those parasitic crustaceans.

But it takes more than simply sticking yourself onto another organism to be considered as a parasite of it. After all, there are algae that grow on the body of various aquatic creatures, or barnacles that are found on the backs of large marine animals like whales and turtles. But those are not considered as parasites as they don't treat their host as a food source, merely as a sturdy surface they can cling to - they're known as epibionts.

So strictly speaking, for Cyclocotyla to be a parasite of the isopod, it needs to be feeding on or obtaining its nutrient directly from its isopod mount. When scientists examine the bodies of the tongue-biters with C. bellones on them, they seem to be pretty unscathed. There aren't any scratches or holes on the isopod's body which you'd expect if C. bellones had been feeding on it. Indeed, the monogenean's mouthpart seems ill-suited for scraping through the isopod's carapace.

Additionally, C. bellones' gut is filled with some kind of dark substance similar to those found in other, related monogenean species. This is most likely digested blood from the fish, which the monogenean has either sucked directly from the fish's gills, or indirectly via the feeding action of its isopod mount. Let's not forget that the isopod itself is a fish parasite that feeds on its host's blood, so if it gets a bit messy during mealtime, perhaps Cyclocotyla is there to suck up any spilled blood. Or it might be doing a bit of both.

The researcher noted that Cyclocotyla is not alone in its habit of riding isopods. Other monogeneans in its family (Diclidophoridae) have also been recorded as attaching to parasitic isopods of fish. And aside from riding isopods, they all share one thing in common - a long, stretchy forebody, looking somewhat like the neck of sauropod dinosaurs. Much like how the neck of those dinosaurs allowed them to browse vegetation from a wide area, the long forebody of Cyclocotyla allows it to graze on the fish's gills while sitting high on the back of an isopod. So fish blood is what C. bellone is really after - the isopod is merely a convenient platform for it to sit on.

But why should these monogeneans even ride on an isopod in the first place? Cyclocotyla and others like it have perfectly good sets of suckers for clinging to a fish's gills. Indeed, there are other similarly-equipped monogeneans that live just fine as fish ectoparasites without doing so from the back of an isopod. Well, that's because the fish themselves don't take too kindly to the monogeneans' presence. These flatworms are constantly under attack from the fish's immune system, which bombards them with all kinds of enzymes, antibodies, and immune cells. By avoiding direct contact with the fish's tissue, Cyclocotyla and other isopod-riders can avoid being ravaged by the host's immune system - which is something that other monogeneans have to deal with on a constant basis.

So it seems that Cyclocotyla and other isopod-riding monogeneans are no hyperparasites - they're all just regular fish parasites that happen to prefer doing so while sitting on the backs of isopods. Cyclocotyla bellones prefers to share in the feast of fish blood with its isopod mount, while sitting high above the wrath of the host's immune response.

Reference:

Bouguerche, C., Tazerouti, F., & Justine, J. L. (2022). Truly a hyperparasite, or simply an epibiont on a parasite? The case of Cyclocotyla bellones (Monogenea, Diclidophoridae). Parasite 29: 28.

Thaumastognathia bicorniger

Gnathiidae is a family of parasitic isopods that can be considered as ticks of the sea. I make that comparison not only because gnathiids are blood-feeding arthropods, but like ticks, their life cycle involves going through a series of feeding and non-feeding stages. The blood-hungry fish-seeking stage is called a zuphea that, much like how a tick would on land, attaches itself onto passing fish and starts feeding to its heart's content. Once it is fully engorged with a belly full of blood, it becomes what's called a praniza, which drops off the fish to grow and moult into its next stage. Gnathiid isopods need to go through alternating between the zuphea and the praniza stage at least three consecutive times before they can reach full maturity.

Thaumastognathia bicorniger stripe (left) and spots (centre) pigemented third stage praniza, and adult male (right)
From Fig. 2. of the paper

The paper featured today is about Thaumastognathia bicorniger, a gnathiid isopod which has recently been described from the waters of Japan. The researchers who described this species found the isopod on various chimaera and sharks that were caught by fishing vessels operating in the waters of Suruga Bay and around Kumejima Island. Additionally, they were also able to obtain previously collected specimens of this isopod that had been stored at the laboratory of fish pathology at Nihon University. Those specimens were originally collected from various different cartilaginous fishes that were caught by fishing vessels off the southern coast of central Japan.

Based on their samples, this isopod has been recorded to feast on the blood of at least ten different species of cartilaginous fishes including nine species of sharks from six different families, along with one species of chimaera (also known as ratfish, in this case the Silver Chimaera). Thaumastognathia bicorniger larvae were always found in the gill chamber of their hosts, where they attached themselves to the blood-rich gill filaments. These isopods are tiny, with the third stage praniza larva measuring about 3.7-4.8 mm long, so having one or two of them would merely pose a minor inconvenience to the host. 

However, some sharks were found to be infected with dozens or even hundreds of those tiny blood-suckers. Of those, the Blotchy Swellshark (Cephaloscyllium umbratile), the Shortspine Spurdog (Squalus mitsukurii), and the Starspotted smooth-hound (Mustelus manazo) appeared to be among this gnathiid's favourite hosts, as they were commonly found to be infected with at least 50 T. bicorniger larvae and some even harboured hundreds of those blood-sucking isopods in their gill chambers. Additionally, much like how ticks are known to carry various pathogens, gnathiid isopods have also been implicated in the transmission of blood-borne parasites in coral reef fishes.

The juvenile stages of T. bicorniger seem to come in two different colour patterns - spotty and stripey. This was only visible in the live or freshly caught specimens as the colour faded rapidly when they are preserved in ethanol. Genetic analysis revealed that despite their superficial differences, those two colour morphs belong to the same species, and it is unclear whether the different colour patterns signify anything, as they're not associated with a particular haplotype, sex, nor host species.

The researchers kept some of the gnathiid larvae alive in captivity to see if any of them would metamorphose into an adult stage - but only one successfully moulted into an adult male. Among gnathiid isopods, there is a high degree of sexual dimorphism - the male gnathiids have squat body with big mandibles, while in contrast, female gnathiid have a larger rotund body for brooding eggs into larvae. Neither of which look anything like a "typical" isopod like a woodlouse or even the infamous tongue-biter parasite and its cymothoid relatives.

For other species of gnathiid isopods, metamorphosing from the third-stage praniza into a mature adult is a relatively brief process. After their last feeding session, some species would take just a week or two to mature into a reproductive adult, while others may take up to two months at most. However, T. bicorniger took a whooping 204 days to moult from a third stage praniza into an adult. So why does T. bicorniger take so long to mature compared with other species of gnathiid isopods?

Gnathiid metabolism and growth is greatly affected by water temperature, and many of the gnathiids that have very short development time are found in warmer, tropical waters. In this study researchers kept their T. bicorniger at 10-20°C in their lab, which is slightly cooler than the water temperature that those other known gnathiids are regularly exposed to. However, there is a species of Antarctic gnathiid - Gnathiia calva - which only took 6 weeks to transform into an adult despite living in waters that were kept at 0 to -1°C.

Alternatively it might have something to do with the fishes that they were feeding on. Many sharks have high levels of urea in their blood, which may make their blood more difficult to digest for any would-be blood-suckers. Lamprey that feed on basking sharks are specially adapted to excrete large volumes of urea which is found in their host's blood. The need to detoxify your food would most likely complicate the digestion process, decrease the blood's nutritional value, which would result in cost to development time. But then again there is another gnathiid species - Gnathia trimaculata - which infects Blacktip reef shark (Carcharinus melanopterus) and it only takes 6 (for males) or 24 days (for female) to moult into an adult.

So for now, the reason(s) why T. bicorniger seems to take such a long time to grow into an adult compared with other species of gnathiid isopods, remains a unsolved mystery.

Reference:

Ota, Y., Kurashima, A., & Horie, T. (2022). First Record of Elasmobranch Hosts for the Gnathiid Isopod Crustacean Thaumastognathia: Description of Thaumastognathia bicorniger sp. nov. Zoological Science, 39: 124-139

Allokepon hendersoni

Crabs have some pretty scary parasites infecting them. They range from worms that use them as vehicles to complete their complex life cycles, to parasitic dinoflagellates that turn their muscles into bitter slurry, and on top of those, there are also other crustaceans that can take over their body, and in some cases, castrate them in the process. These body-snatching crustaceans come in two main types - bopyrids and rhizocephalans. 

Top: Bopyrid isopod and an infected crab, Bottom: Rhizocephalan barnacle with infected crab.
Photos modified from the graphical abstract of the paper

Bopyrids are parasitic isopods in the same suborder as the infamous tongue biter parasite, but instead of going into a fish's mouth, they go inside the body of crabs and make themselves at home, often causing a characteristic bulge on the infected crab's carapace. And then, you have the rhizocephalans, which are freaky barnacles that have a body composed of a network of roots which wrap themselves around the crab's internal organs.

Each of them inflict their own respective flavour of pain on their crab hosts.

This study looked at the effects that these parasitic crustaceans have on the two-spotted swimming crab (Charybdis bimaculata), which is host to both parasitic isopods and barnacles. Here representing the bopyrid isopods, we have a species named Allokepon hendersoni. And fronting for the barnacles, is an as yet undescribed species of rhizocephalan. As hinted at earlier, these two body-snatcher parasites seem to have different effects on the crabs - but what exactly are they?

When scientists compared infected crabs with uninfected crabs, they found the effects to be most pronounced in male crabs, with both species of parasites causing a reduction in weight and claw size of their hosts. This is most likely due to the energetic drain associated with hosting these crustaceans, since they can grow to alarmingly large sizes when compared with their hosts. 

While reducing the claw size may leave the host crab less able to compete with uninfected males, on another hand (or claw as the case may be), it would not be in the interest of the parasite for its host to be getting into too many fights and risk injuries anyway, so it can be a beneficial side-effect from the parasite's perspective

But there were some changes which were more specific to particular parasite species. Male crabs infected with Allokepon had a narrower abdominal flap (the triangular flap on the "belly" of a crab). In male crabs, this flap would usually serve to protect the gonopods - which are specialised appendages that arthropods use in reproduction - but given the crab is already hosting such a demanding resident in its body, it wouldn't be getting up to any of that any time soon.

In contrast, the rhizocephalan barnacle had the opposite effect on the crab and widened that flap - this is part of a whole suite of changes that these parasites induce in male hosts. Male crabs that are infected by rhizocephalans develop characteristics which are associated with female crabs in both appearance and behaviour. In female crabs, the wider abdominal flap serves to cradle and brood the eggs before they hatch. So the barnacle essentially "feminise" the male crabs so that they can become better babysitters for the barnacle's offspring.

Fortunately for this crab population, infection rate was very low. Of the 2601 crabs the scientists examined, only 14 were infected with the isopod, and 21 infected with the rhizocephalan barnacle, though the isopod seems to have a preference for infecting male crabs, whereas the barnacle was less discriminate.

But if you are the unfortunate crab that gets infected, you are in for a bad time either way.

Reference:

Corral, J. M., Henmi, Y., & Itani, G. (2021). Differences in the parasitic effects of a bopyrid isopod and rhizocephalan barnacle on the portunid crab, Charybdis bimaculata. Parasitology International, 81: 102283.

Pseudoacanthocephalus toshimai

Parasites with complex life-cycles often use predator-prey interactions to facilitate their transmission. They have larval stages which infect the body of prey animals, where they wait to be eaten by predators that act as the parasite's final host. But the thing about relying on such interactions to reach their destinations, is that they don't always end up where they are supposed to.

Left: Adult P. toshimai in a fish's gut, Centre: Adult P. toshimai in a frog's gut, Right: Larval P. toshimai from a woodlouse
Photos from the graphical abstract of the paper

Pseudoacanthocephalus toshimai is a thorny-headed worm which is found in Hokkaidō, in the northern part of Japan. The adult stage of this parasitic worm usually infects amphibians such as the Ezo brown frog and the Ezo salamander, while the larval stage parasitises a species of woodlouse called Ligidium japonicum. While it is primarily an amphibian parasite, P. toshimai is sometimes also found in a range of stream fishes. So how does an amphibian parasite end up in the belly of a fish? 

A pair of researchers from Asahikawa Medical University conducted a survey on the prevalence and abundance of P. toshimai at the mountain streams of the Ishikari River around the Kamikawa basin. They caught both fish and amphibians, and examined their guts for the presence of P. toshimai. Of the 174 stream fish that they caught, 56 were infected with P. toshimai, all of them were salmonids and were all from one specific stream. The infected salmonid species included the iwana, Dolly Varden trout, masu salmon, and rainbow trout.

While P. toshimai appears to be fairly common among those salmonids, they were only present in relatively low numbers. On average, each fish was infected with only two or three worms, and none of the female worms carried any eggs. In contrast, the researchers found the parasite to be much more abundant in amphibians. About two-thirds of the salamanders in their sample were infected with P. toshimai, with an average of about four worms per host. Additionally, all the frogs that they examined were infected, with each frog harbouring an average of about five worms. The highest number of worms recorded from a single host was a salamander which had 22 P. toshimai in its gut. Furthermore, all the female worms in those amphibians were brimming with mature eggs, all ready to go.

So while the fish's gut is a hospitable enough environment for the parasite to grow into an adult worm, it is lacking a certain je ne sais quoi that the female worms need to start producing eggs and complete the life-cycle. It is not entirely clear what exactly that might be - it could be that the fish's gut does not produce the right type of nutrients for egg production, or there is simply not enough mating opportunities for the parasite in the gut of a fish - since they are not as commonly nor heavily infected as the amphibians. Either way those salmonids are ultimately dead-end hosts for P. toshimai. So how are the worms ending up in those fish in the first place?

This is where we have to consider the other animal involved in the parasite's life-cycle which is the woodlouse. Woodlice - also known as slaters - are terrestrial crustaceans commonly found under rocks and among leaf litter. As mentioned above, P. toshimai uses a species of woodlouse as intermediate host, where their eggs develop into larval stages known as cystacanths. Since those crustaceans are commonly eaten by frogs and salamanders, they also act as a vehicle to transport the parasite to its final host.

The researchers noticed that P. toshimai is only ever found in fish from one particular stream which is surrounded by bushes. These bushes are habitats for woodlice and amphibians which are the usual hosts for P. toshimai, and provide the necessary conditions for the parasite to complete its life-cycle. But every now and then, instead of getting eaten by a frog or a salamander, an infected woodlouse would fall into the stream, and become a tasty snack for a hungry fish. Indeed, the researchers did find a few woodlice in some of the fishes that they caught. 

This study shows that for parasites with complex life-cycles, things don't always work out the way that they are supposed to. Even when all the necessary condition are present and accounted for, once in a while, your intermediate host might get knocked into a stream, and you end up in the belly of a fish.

Reference:
Nakao, M., & Sasaki, M. (2020). Frequent infections of mountain stream fish with the amphibian acanthocephalan, Pseudoacanthocephalus toshimai (Acanthocephala: Echinorhynchidae). Parasitology International 81: 102262.

Pinnixion sexdecennia

Pea crabs (Pinnotheridae) are tiny crabs that have evolved to live with or within larger aquatic invertebrates. Some species take up residency in the body of various marine animals such as mussels and sea cucumbers. Others (those in the Pinnothereliinae subfamily) merely share the same burrows as their host, living more of a housemate (the scientific term for that is an inquiline) than a bodily symbiont.

Living in the cosy interior of a marine animal (or at least their burrows) where you are sheltered and fed seems like a good life (though it can make finding a mate a bit difficult). But pea crabs are themselves susceptible to a range of their own symbionts and parasites - after all, they're just crabs, and there are plenty of parasites that covet the body of crabs.

Mature female (left) and mature male (right) Pinnixion sexdecennia [photos from Figure 3 of the paper]

The parasite featured in this post is Pinnixion sexdecennia, a parasitic isopod. It belongs in the same group of crustaceans as slaters and the deep sea giant isopod Bathynomus - not that you'd know if you look at the adult stage of P. sexdecennia. The adult female P. sexdecennia looks more like a wrinkly bag than what most people would think a crustacean would look like. The parasite takes up most of the room inside the the crab and is encased in a body bag made out of the host crab's blood cells. As for the males, they are very different to the female -  for one thing, they still look recognisably like an isopod with all the usual segmentations one would expect, and also, they are only half the size of their wrinkly blob-shaped mate.

When the larvae of P. sexdecennia initially enters the crab's body, and metamorphose into a juvenile, it has no determined sex. Instead, the sex that it matures into is determined by the presence of other individuals inside the host. Usually when there are multiple juvenile P. sexdecennia inside the crab, one of them will grow into a female while others develop into male that then attach to her. This kind of environmental sex determination is somewhat comparable to that found in another parasitic isopod - the infamous tongue-biter parasite.

The adult female P. sexdecennia takes up a substantial amount of room inside the crab's body. In fact, most of the internal space in the infected crab's body are taken up by the parasite, which shoves aside most the crab's internal organs. Despite all this, the infected crabs are able to carry on reproducing and moulting as usual and doesn't seem to suffer from hosting the parasitic isopod, though their carapace does end up developing a noticeable bulge. This parasite seems to be fairly common in the pea crab population - on the Florida and North Carolina coast, about one-third to almost half of the crabs that were examined were infected, and in some populations, the isopod seems to be more common in female crabs, though it is not entirely clear why that might be the case.

So what's with this parasite's species name - sexdecennia? Well, the species name translates to "six decades" and that's how long it took to get this species scientifically described. These parasite were originally collected in the 1960s along the coast of New Jersey, North Carolina, and Florida, as a part of a larger study looking at the life history and reproductive habits of the pea crabs themselves. For whatever reason, the result of that study on pea crabs was not published until 2005, and the parasites that were collected during that study got placed into specimen vials, and there they sat until sixty years later when they were finally formally described.

Just how many other tiny invertebrates are currently sitting in vials or slides in laboratories and museums around the world, awaiting scientific description? Unfortunately the scientific community has been suffering from a steady loss of taxonomic expertise over the decades. The number of trained taxonomists have been declining over the decades, due in no small part to a modern academic career structure and incentives, which makes a career pathway in taxonomy more difficult to pursue comparing with one in other life sciences.

And in the age of molecular and genetic technology, even other biologists are disregarding taxonomists and their unique skills, under the misguided notion that taxonomists are rendered obsolete by "DNA barcoding" and automated sequencing. But there is a lot about an organism that one cannot tell simply from its DNA alone, and with at least one million species of plants and animals threatened with extinction, many of which may disappear within the next few decades, we need taxonomists more than ever to document life on earth. With the current state of the planet, the question is - how many species will even get described before they become extinct in the wild?

Reference:
McDermott, J. J., Williams, J. D., & Boyko, C. B. (2020). A new genus and species of parasitic isopod (Bopyroidea: Entoniscidae) infesting pinnotherid crabs (Brachyura: Pinnotheridae) on the Atlantic coast of the USA, with notes on the life cycle of entoniscids. Journal of Crustacean Biology, 40: 97-114.

Riggia puyensis

It is no secret that I am a big fan of parasitic isopods, especially those in the Cymothoidae family - the most well-known of which is the tongue biter parasite, and my love for these adorable crustaceans has even manifest itself in some of my artwork. But while the tongue-biters are no doubt the most (in)famous representatives of that family, to the extent that they even made an appearance on an episode of the Colbert Report, it is their less well-known cousins - the belly-dwellers/burrowers - that turn the horror factor up a notch (or four, or eleven) and as a result, really earned my adoration.

Left: Adult female Riggia puyensis (scale bar = 10 mm), Right: Adult make Riggia puyensis (scale bar = 1 mm)
From Fig. 3 and Fig. 9 of the paper

Imagine if the chest-burster xenomorph from Aliens didn't just explode through your ribcage and leave you for dead - instead, it stays inside your torso for the rest of your life, laying a steady stream of eggs that trickle out through a small(ish) hole in you belly. That's how these belly-dwelling isopod live their lives. So let's kick off the year with a recently described species of these belly-dwellers!

I've previously written a post about a species of belly-dweller call Artysone trysibia which lives in the body cavity of an armoured catfish from the Amazon. This post features Riggia puyensis, which is quite similar to A. trysibia in that it was also found to be parasitising armoured catfish, specifically two species from the Bobonaza River and Puyo River in central Ecuador - Chaetostoma breve and Chaetostoma microps - both of which are better known as suckermouth armoured catfish.

Most of the R. puyensis specimens that the scientists found in this study were females, but the scientists did come across three male specimens which were clinging to the limbs of the female isopods. These male isopods are comparatively tiny reaching only one-tenth the length of the adult female R. puyenesis. The small size and relative rarity of males is par for the course for Riggia. In other studies on this genus of parasite, male isopods are rarely found, if at all. It is possible that this is because the mating strategy of the male isopod is to scoot in, mate with the larger female, then go off and find another infected host.

Riggia puyensis inside its host, from Fig. 2 of the paper

In this study, each infected fish was only parasitised by a single female isopod - which is probably just as well since R. puyensis is quite large in relation to the host. The female R. puyensis reaches over an inch in length and considering one of the host catfish is a species that grows to about four inches long at most, that parasite is a hefty load to be carrying around. It would be like having a corgi living inside you.

So it may seem rather surprising that the survival of these fish does not seem to be compromised by the parasite. In fact, a previous study have shown that the parasite may in fact enhance the infected fish's growth. But this parasite-induced growth spurt comes at a price - after all, there is no free lunch in nature and for the gain in body growth, the parasite incurs a severe penalty on the fish's reproductive functions. A study on bonefish parasitised by Riggia paranensis found that infected fish has reduced level of sex hormones and undeveloped gonads.

So Riggia render its fish host impotent in order to free up more resources for body growth, and a bigger host means more for the parasite to consume. So while a chest-bursting xenomorph invokes a more immediate visceral reaction, the way that R. puyensis and other parasitic castrators modify their hosts' body to fuel their own reproduction presents a more existential form of lingering horror.

Reference:
Haro, C. R., Montes, M. M., Marcotegui, P., & Martorelli, S. R. (2017). Riggia puyensis n. sp.(Isopoda: Cymothoidae) parasitizing Chaetostoma breve and Chaetostoma microps (Siluriformes: Loricariidae) from Ecuador. Acta Tropica 166: 328-335.

Artystone trysibia

The tongue-biter Cymothoa exigua is arguable one of the most (in)famous fish parasite in the world. It was famous enough to get a mention on the Colbert Report, and while the world recoil in collective horror at the sight of a fish which had its tongue replaced by a parasite, among its fellow parasitic crustaceans, tongue-biter's modus operandi is actually rather quaint. It can easily be upstaged by other parasitic isopods in the horror department, and today's post is about one those species.

The parasite we are featuring today is Artystone trysibia - it is in the same taxonomic group as the tongue-biter (Cymothoidae), and it parasitises a number of freshwater fish in the Amazonian basin. But unlike its more famous cousin which is content with merely living in the host's mouth, A. trysibia cranks the nightmare fuel up to eleven and lives inside a fleshy capsule in the host's body cavity.

Photo from Figure 2 of this paper

This parasite and others like it are actually relatively common. This parasite has been documented from a range of freshwater fish from South America, and has also been reported from aquarium ornamental fish. They're so common that A. trysibia has gained a common name - "ghili" - among the Kichwa people.

Female A. trysibia (right) and Female A. trysibia with larvae
Photo from Figure 3 of this paper

This study documented its presence in the Bristlemouth Armoured Catfish (Chaetostoma dermorhynchum). Despite its armouring, this fish has no protection against A. trysibia. Usually, the only sign of the parasite's presence is a small, gaping hole on their belly or their flank. But that hole serves as a window for the parasite within. For this study, these catfish were sampled from three pristine sites at the Tena River in the Amazonian region of Ecuador.

These catfish are fairly small fish, and most of them are about 15 - 20 cm long (6-8 inches), but A. trysibia can grow to 1.5 - 3 cm (0.6-1.2 inches) long and takes up quite a lot of space within the catfish. For comparison, it would be the equivalent of having something the size of a pet rabbit living in your torso. As mentioned above, the only contact the parasite has with the outside world is with a tiny hole through which they breath and release their offspring, and they can reproduce in prodigious numbers - one female isopod was recorded to be carrying 828 larvae. Each catfish was found to (thankfully?) only ever have a single A. trysibia, and it seems that the bigger the host, the bigger the parasite, possibly because a larger host would give the parasite more room to grow.

Artystone trysibia is not alone in its style of parasitism, there are other similar species which are found in both freshwater and marine fish around the world (for example, see here). So the next fish which you come across might not just have a tongue-biter in its mouth, it might also have a ghili in its belly.

Reference:
Junoy, J. (2016). Parasitism of the isopod Artystone trysibia in the fish Chaetostoma dermorhynchum from the Tena River (Amazonian region, Ecuador). Acta Tropica 153, 36-45.

Gnathia maxillaris

Today's blog post features a study in which an infestation at an aquarium allowed a group of scientists to work out the life cycle of a common parasite. Now, we are not talking about your lounge room fish tank, but the biggest exhibition tank at Aquarium of Barcelona. The exhibition aquarium, call Oceanarium, measures 37000 cubic metres and is home to over 3000 fish of 80 different species. But amidst those 80 different species, they have a parasite which has made its way into the mix.

Adult female with larval brood (left) and newly-hatched zuphea (right)
Photos from Fig. 1 of the paper

The parasite in question - Gnathia maxillaris - belongs to a family of little blood-sucking crustaceans call Gnathiidae (we have previously featured gnathiids on this blog here). You can think of them as being like ticks of the sea - not only are they blood suckers, but they also alternates between a blood-feeding and a free-living stage during their development (like a tick). The parasitic stage of a gnathiid is called a Zuphea - it needs to attach and feed on a host for a while before it drops off to moult into its next stage call a Pranzia. The pranzia is free-living stage, but it doesn't stay that way for long, as the next step of its development is to grow into a slightly larger zuphea which jumps right back onboard a fish for a blood meal. A gnathiid needs to go through this parasitic-then-not-parasitic-then-parasitic-again development cycle three consecutive times (each successive stages are called Z1, P1, Z2, P2, Z3, P3) before it can become an adult (and you thought going through puberty was bad!)

There are over 190 known species of gnathiids from all across the world, but the full life-cycle has only been described for four of those species, and now G. maxillaris join that very short list. Even though G. maxillaris is relatively well-studied and fairly widespread across the Atlantic Ocean as well as the Baltic and Mediterranean seas, the complete life-cycle of G. maxillaris was unknown until now because much of this parasite's development takes place out of sight on the open sea.

But the infestation at Aquarium of Barcelona provided scientists with a great opportunity to study this life-cycle. They harvested G. maxillaris larvae by exploiting their natural attraction to light; at night, they turned on a set of light installed at the bottom of the aquarium, then pump the sea water through a fine-meshed plankton net that have also been placed there to trap the parasite larvae.

Clockwise from upper left:
Adult female, adult male, female carrying eggs
From Fig. 2 of the paper

With the harvested parasites, they exposed them to different species of potential fish hosts to observe their behaviour. They noticed that newly-hatched zuphea (Z1) cannot feed on blood because their mouthpart is so small the fish blood cells cannot fit through them. Instead, they feed on lymph and have to subsequently grow into the larger zuphea stages before they can incorporate blood into their diet.

They also discovered that G. maxillaris has different preference for specific parts of the fish's body, and this has consequences for the parasite's growth. While they can attach pretty much anywhere on the fish's body, they have a taste for the base of the fins, near the gill covers, or around the eyes - basically areas of high blood flow and where it would be harder for the fish to rub them off. They also noticed zuphea that attach themselves to the fish's fin feed for longer and takes more time to develop into a pranzia, most likely because there is less blood flow there than other parts of the body, so the parasite needs to stick around for longer to get a full meal.

In all, G. maxillaris' entire life-cycle takes about three months to complete, but that is if the water temperature is at 17.5 °C; if the surround temperature is 20 °C, then the parasite would take only two months to complete this cycle. At higher temperature, the female parasites also grew larger and produced more eggs. This is particularly pertinent to the current situation because one of the (many) consequences of increasing ocean temperature might mean in the future, the seas will be filled with more gnathiids that grow faster than ever before, which is bad news for fish. Not only are they blood-suckers, like ticks on land, gnathiids can also act as vectors for various other parasites.

While an infestation of tiny "ticks of the sea" might not be the best news for a national aquarium, when life hands you an infestation - you might as well do some science with it!

Reference:
Hispano, C., Bulto, P., & Blanch, A. R. (2014). Life cycle of the fish parasite Gnathia maxillaris (Crustacea: Isopoda: Gnathiidae). Folia Parasitologica 61: 277-284.

Anilocra nemipteri

Photo from Figure 1 of the paper

The parasite in the study being featured today makes a living riding around on the top of a fish's head and occasionally gnawing on its face. It is in the same family as the infamous tongue-biter, the Cymothoidae, though technically this one is more of a face-hugger.

Anilocra nemipteri is found on the Great Barrier Reef of Australia and it makes a living by hitching a ride (and feeds) from the bridled monocle bream, Scolopsis bilineata. It is a pretty common parasite - in some areas, up to 30 percent of monocle bream carry one of these crustaceans on their head like a nasty blood-sucking beret that stay attached for years.

As you can see from the photo, A. nemipteri is a fairly big parasites comparing with the size of the fish (in some case they can reach as almost one-third the length of the host fish!), and having a parasite of that size hanging off your face is going to be quite a drag - literally. That is bad news for a little fish like the monocle bream that needs to make a quick getaway from any hungry predators on the reef. So just how much of a drag is A. nemipteri? A related species - Anilocra apogonae - which clings to the cardinal fish (Cheilodipterus quinquelineatus) is known to cause their host to swim slower and have lower endurance. Does the same apply for A. nemipteri and the monocle bream?

To find out, scientists compared how quickly the fish can respond to an attack and their Flight Initiation Distance (FID) in both a laboratory setting and in the field. The FID is the distance from a predator at which an animal decides to flee - risk-takers have a shorter FID. They divided the monocle bream into three different groups: parasite-free fish, fish carrying an A. nemipteri, infected fish which just had their parasite removed.

Photo from Figure 1 of the paper

The research team simulated an attack by a bird (with a weighted PVC pipe) on fish in specially-designed experimental tanks and filmed the response to measure the fish's reaction time to the attack. Even though one would think all that face-gnawing from A. nemipteri would have weakened their host, and not to mention the body of the parasite itself causing significant drag, the escape performance of parasitised fish was not all that different from unparasitised - they reacted and got away from the attack just as quickly as their unburdened buddies. In the field experiment, the scientists donned snorkelling gear and tried to approach any monocle breams they spotted and measured how close they could get to the fish before they fled. There, they found parasitised fish have a slightly shorter FID than parasite-free fish, but not significantly so.

Fish that are infected by A. nemipteri are smaller than uninfected ones, and it just so happen that smaller fish tend to allow predators to get closer to them before fleeing. But whether this is due to the parasite is another matter. Are parasitised fish smaller because their growth have been stunt by A. nemipteri? Or does this face-hugger simply prefer smaller fish because larger and older fish might have built up an immunity to it?

Though it may seem less exciting when we find a parasite doesn't cause much behavioural changes in its host, it is vital to our understanding of host-parasite relationships. Perhaps it means the host is able to compensate for the presence of the parasite. Also it is not clear what the long term cost of having A. nemipteri might be over the life time of the fish. It is also important to treat such a case in its context. Unlike other parasite which have a complex life-cycle and depend upon its host getting eaten by a predator to reach maturity, A. nemipteri is an external parasite that simply sticks to a host and stay for life - if the parasitised fish is eaten by a predator, it'll go down with the host like a bit of garnish and be digested too.

So it is probably just as well that A. nemipteri is not too much of a drag to have around.

Reference:
Binning, S. A., Barnes, J. I., Davies, J. N., Backwell, P. R., Keogh, J. S., & Roche, D. G. (2014). Ectoparasites modify escape behaviour, but not performance, in a coral reef fish. Animal Behaviour 93: 1-7.

Acanthocephalus dirus

The word parasite has a lot of connotations associated with it, and "maternal" is certainly not one of them. To most people, the term "freeloader" comes to mind (hopefully, this blog will show you that parasitism is actually a very challenging way of life). They also have a reputation as being pretty lousy parents. In most textbooks, parasites are usually considered as "r-strategists" - which produce many, many offspring and don't take good care of them (as opposed to a K-strategist which produces fewer offspring, but invest a lot into parental care - like an elephant). But not all parasites are bad parents, and today, I am going to tell you about a study on a maternal parasite which sacrifices everything (literally) for her offspring.

Acanthocephalus dirus has a reproductive strategy that is unusual for its group - the acanthocephalans or the thorny-headed worms (Acantho = "thorns", Cephala = "head"). In fact it is unusual compared to most intestinal parasites. Unlike some tapeworms, which profligately cast off segments (each containing hundreds of eggs) into the wilderness with abandonment, A. dirus has rather different approach. The impetus that spurred on this piece of research were two separate observations: (1) fish that are infected with A. dirus do not have any worm eggs in their feces (unlike most animals infected with intestinal parasites) and (2) perfectly healthy and intact female worms were often expelled from the definitive host. What the researchers found was that instead of simply laying eggs that are expelled from the worm and from the host, a female A. dirus actually retains her eggs until she become completely bloated with them - at which point she exits gracefully from the host fish's digestive tract. Some readers might recall a nematode that has a similar reproductive strategy, and that both lineages have evolved such a reproductive strategy independently. So why has A. dirus evolved such an extreme strategy instead of just laying eggs normally like other thorny-head worms?

One reason could be that A. dirus infects creek chub - which, as its name indicates - lives in flowing creeks. The chub acquire the worm through eating infected isopods in the stream (the picture shows the light-coloured infected isopod on the right, and the darker uninfected individual on the left), which become infected when they ingest worm eggs resting on the creek bed. Acanthocephalan eggs tend to float - so if the eggs are simply expelled into the environment, they would get washed away downstream and deposited where the isopods do not occur. Whereas with A. dirus, the worm's own body can act like a weight belt which would carry the eggs down to the sediment layer, so by the time the worm herself decays, the eggs are already in the sediment where isopods can pick them up.

Furthermore, laboratory tests showed that isopods like to eat egg-filled female worms as much as their usual food - leaf litter - and the worm body itself actually enhances the infection success of the eggs. Researchers found that when exposed to fresh eggs alone, fewer than one in four isopods became infected, whereas when exposed to gravid females, over 80% became infected (natural infection comes somewhere in between those at about 60%). By making the ultimate maternal sacrifice, A. dirus gives her offspring the best possible start in life.

Image from figure in: Seidenberg (1973) Journal of Parasitology 59: 957-962

Reference:

Kopp, D.A., Elke, D.A., Caddigan, S.C., Raj, A., Rodriguez, L., Young, M.L. and Sparkes, T.C. (2011) Dispersal in the acanthocephalan Acanthocephalus dirus. Journal of Parasitology 97: 101-105

Gnathia auresmaculosa

The harmfulness of parasites to their host is not always so straightforward, there are often many factors which contribute to the pathology of an infection. The parasite we are looking at today is Gnathia auresmaculosa - a type of blood-sucking crustacean with an interesting life cycle (which you can read about in this post from last year). These little gnathiids are like ticks of the sea, clinging onto passing fish and gorging themselves on blood before dropping off to continue developing. For adult fish, a few gnathiid here and there is probably not a big deal, but for growing juveniles, that is another matter.

Settlement is a critical transitional stage for coral reef fishes, and that is also when they are most vulnerable to parasites like G. auresmaculosa. A recent study by the lab group of Dr. Alexandra Grutter revealed just how costly these ticks of the sea can be to juvenile fishes. Dr. Grutter and her colleagues found that juvenile damselfish which have been fed on by just one of those little blood-suckers exhibit significantly decreased swimming ability, far higher oxygen consumption rate, and are about half as likely to survive than uninfected fishes.

So if you happen to find yourself on a beautiful tropical reef, take a moment to think about all the little baby fishes which are swimming for their lives through the gauntlet of gnathiids - they never mentioned that in Finding Nemo!

Reference:
Grutter, A.S., Crean, A.J., Curtis, L.M., Kuris, A.M., Warner, R.R. and McCormick, M.I. (2011) Indirect effects of an ectoparasite reduce successful establishment of a damselfish at settlement. Functional Ecology 25: 586-594

October 15 - Nerocila acuminata

Nerocila acuminata is a parasitic isopod related to Cymothoa exigua, the infamous "tongue-replacer". While N. acuminata doesn't have the morbid habit of replacing the tongue of its host with itself, that certainly doesn't make it more endearing. This isopod clings onto the skin of its fish host, feeding on blood and tissue. When it detects a potential host, this parasites becomes a fish-seeking missile - it launches itself at the target fish like a guided torpedo, making precise directional and speed adjustments to ensure it lands on its target with claws outstretched . Upon contact, the isopod starts digging in, causing terrible, terrible damage to the skin of its fish. In addition to damaged tissue and blood loss, such aggravated injuries can often lead to secondary infection by bacterial infection. Compared with the "tongue-biter", this parasite is one nasty customer.


Contributed by Tommy Leung and photo by Peter Bryant.

September 3 - Liriopsis pygmaea

Parasites don't always have things go their own ways. Even in the parasite world, sometimes the hustler gets hustled. There are parasites which specifically infects other parasites, called "hyperparasites" and Liriopsis pygmaea is one such example. The false king crab Paralomis granulosa is host to a rhizocephalan parasite called Briariosaccus callosus which belongs in the same group of parasitic barnacles as Sacculina carcini (which we met back in January 7).

Liriopsis pygmaea attaches itself to the externa of B. callosus and parasitises it (see pale blobs in photo, arrow indicating externa of B. callosus). L. pygmaea belongs to the group of isopods call the cryptoniscid. While most people are familiar with isopods in the form of slaters and pillbugs you see in the garden, adult L. pygmaea bears a closer resemblance to the cherry tomatoes which might be growing in the said garden than their isopod cousins. Just as B. callosus castrate its crab host, L. pygmaea does the same to the rhizocephalan - drawing resources away from the parasitic barnacle and using it for its own reproduction. So in this case, the castrator, becomes the castrated.


The photo and the info for write up came from this paper:

Lovrich, G. A., Roccatagliata, D., Peresan, L. (2004) Hyperparasitism of the cryptoniscid isopod Liriopsis pygmaea on the lithodid Paralomis granulosa from the Beagle Channel, Argentina. Diseases of Aquatic Organisms 58:71-77.

Contributed by Tommy Leung.

August 2 - Gnathia trimaculata

TThe parasite for today is a parasitic isopod belonging to the family Gnathiidae - the larvae of this particular species feed upon the requiem shark (Carcharinus melanopterus). There are many different species of gnathiids parasitising many different species of fish, and they have an interesting life-cycle which involve "protelian parasitism" where only the juvenile stages (called a praniza) are parasitic, while the adult stages are free-living. They go through several stages of development, alternating between feeding and non-feeding developing stages (when they are engorged with blood) before reaching sexual maturity.

They are almost like a functional equivalent of ticks for fishes - they wait in ambush for a passing host, and when one arrives, it climbs onboard, sucks blood for a few days until full, then drops off to develop into the next stage. And like ticks, they can also act as vectors which can transmit blood parasites between the fishes they feed upon.

The photo shows a pair of third-stage pranizae, scale bar is 1 mm and it came from this paper:

Coetzee, M.L., Smit, N.J., Grutter, A.S., Davies, A.J. (2009) Gnathia trimaculata n. sp. (Crustacea: Isopoda: Gnathiidae), an ectoparasite found parasitising requiem sharks from off Lizard Island, Great Barrier Reef, Australia. Systematic Parasitology 79:97-112


Contributed by Tommy Leung.

January 26 - Cymothoa exigua

Cymothoa exigua is a parasitic isopod with a very odd and gruesome life cycle. Juveniles first attach to the gills of a fish and become males. As they mature, they become females, with mating likely occurring on the gills. The female then makes its way to the fish’s mouth where it uses its front claws to attach to the fish’s tongue. It begins to feed on the blood in the tongue until the tongue eventually atrophies completely. The isopod then takes the place of the tongue in the mouth, attaching to the floor of the mouth and the stub of what is left of the tongue with its hind pereopods. There it lives out the rest of its life – even letting the fish use it like its old tongue, holding prey against its teeth. Although these parasites were long thought to be restricted to the Gulf of California and environs, recently one turned up in the mouth of a fish that was caught off the coast of the U.K.