Sacculina pugettiae

Sacculina pilosella is a parasite of spider crabs (Scyra ferox), and while it is technically a barnacle, you're going to have to abandon all your preconceived notion of what a barnacle, or for that matter, an animal looks like in order to understand these parasites. These parasitic barnacles are called rhizocephalans and they are sometimes visible as a blob poking out of a crab's belly. While that is already far from what a conventional barnacle looks like, that's just the parasite's reproductive organ - the rest of its body is composed of an extensive network of roots that spread deep into the crab's body.

Spider crabs infected with Sacculina pugettiae (left) and Parasacculina pilosella (right).
Photos from Figure 2 of the paper

It was previously thought that those spider crabs only have a single species of rhizocephalan barnacle parasitising them, the aforementioned Sacculina pilosella, but DNA analyses of rhizocephalan specimens revealed that the spider crab is actually being tag-teamed by TWO parasitic barnacles hiding in plain sight. Turns out that what scientists have been calling "Sacculina pilosella" is actually two entirely different rhizocephalan species - Sacculina pugettiae and Parasacculina pilosella. It shouldn't be a surprise that their differences have gone unnoticed considering the body of these barnacles is just a blob with a mass of fine roots. Both species share the same breeding season between June to September, during the summer months, and sometimes they even infect the same crab simultaneously.

They do have some minor anatomical differences on the blob-like reproductive organ, but even when compared side-by-side, they can be tricky to tell apart. There is another anatomical feature which might provide a more reliable clue to the parasite's true identity, but that is only visible on a microscopic level. As previously mentioned, the body of a rhizocephalan is a massive network of rootlets, but not all those roots are made the same. Some of the roots, called trophic roots, absorb nutrients and are situated in the host's body cavity.

But the barnacle also grows a different type of roots that invade the host's brain. And it is those brain-invading roots that offer a way of distinguishing those two different species. Sacculina pugettiae has roots that end in microscopic goblets whereas P. pilosella has regularly shaped tapered ends to their brain-invading roots. While the significance of those microscope goblets is not clear, their presence is a reliable way to tell those barnacle species apart.

Often, when two different species of parasites are sharing the same host, this can result in a turf war, especially if they are body-snatchers that take up much of the host's body. Some parasites have even evolved specialised stages to fight off competitors. Since rhizocephalans have extensive roots that proliferate throughout the host's body, you would think that two such parasites living in the same crabs would inevitably end up butting heads (well, roots) with each other. But that's not what the scientists found. Somehow, these barnacles were able to share the same crab without conflict, both being able to successfully grow and reproduce, their rootlets intertwined with each other as they tickled the host's brain stem and absorb the crab's lifeblood in harmony.

Reference:

Lianguzova, A. D., Poliushkevich, L. O., Laskova, E. P., Golubinskaya, D. D., Arbuzova, N. A., Petruniak, A. M., & Miroliubov, A. M. (2025). Two in one: A case study of two rhizocephalan species invading the nervous tissue of one host. Journal of Zoology 325: 185-195.

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.

Biospeedotrema spp. (et al.)

The deep sea is home to unique communities of organisms, and wherever there is life, there are parasites, and deep sea habitats such as hydrothermal vents with their multitude of species can provide the conditions to support parasites with complex, multi-host life cycles. 

But not all such parasites are equal in their requirements, for example digenean flukes can be demanding prima donnas when it comes to the necessary hosts for their complex life cycles.  Not only does the adult fluke needs a vertebrate host to live in, they also have an asexual stage that needs to go into a specific type of invertebrate, usually some kind of snail, and maybe even other small animal to act as go-betweens to carry the larval stage from the snail to the vertebrate host. So while habitats like hydrothermal vents are teeming with life - do they have what it takes to support those fastidious flukes?

Left: Different types and stages of digenean flukes found at hydrothermal vents Including adult and sporocysts of Neolebouria (top left), adult and metacercariae of Biospeedotrema (top right), adult and metacercariae of Caudotestis (bottom), and unknown cercaria (bottom insert) All scale bars: 500 μm. Right: Proposed life cycle of digenean flukes at hydrothermal vents, with the adult living in vent fish, snails as first intermediate hosts, shrimps and other invertebrates as the second intermediate host. Photos of flukes from Fig. 5 of the paper, life cycle diagram from Fig. 1 of the paper. 

To answer that question, scientists from the Woods Hole Oceanographic Institution looked for parasites in samples of organisms collected from hydrothermal vent sites located about 2500 metres below sea level, along the East Pacific Rise. They ended up with a mixed bag of different animals composed of various fish and invertebrates, and when the scientists dissected those deep sea creatures, it turns out many of them were filled with all kinds of flukes at various stages of their life cycles. This range from adult flukes in the guts of vent fishes, to the sausage-like asexual stages in snails, to larval cysts embedded in the bodies of invertebrates, and even the short-lived, free-roaming cercaria stages crawling about in the samples.

Seven different types of flukes were present in the guts and gall bladder of vent fishes such as the pink vent fish (Thermarces cerberus) and viviparous brotulas (Thermichthys hollisi). And by sequencing selective sections of their DNA, the scientists were able to match those adult flukes with corresponding larval stages in a wide range of vent animals including shrimps, crabs, snails, and polychaete worms. For example, the fluke asexual stages found in glass limpets turned out to be a match with the adult flukes found in one of the vent fish.

In total, the researchers were able to identify three distinct genera - Biospeedotrema, Caudotestis, and Neolebouria - but they also found free-living cercaria stages of an unknown fluke. These cercariae have a stubby, sucker-like tail and while they superficially resemble the cercariae produced by the fluke which was found in glass limpets, DNA analysis shows that it does not genetically match with any of the other flukes in the samples. The life cycle and hosts of those peculiar cercariae are currently unknown, but their presence indicates that there many other infected hosts yet to be discovered at those hydrothermal vent communities.

So while this study has managed to fill some gaps in our knowledge about parasitism in the deep sea, there are still many mysteries. Digenean flukes need rich communities to complete their life cycles, and their presence at hydrothermal vents tells us that even though such vent sites are short-lived, these habitats can support a very rich community of different organisms. But many of these biodiverse habitats are under threat from a wide range of current and planned human activities. There is so much more that we need to learn about these biomes of the deep, and we also need to learn to value them, lest they become casualties in the face of ceaseless demands for minerals and other commodities.

Reference:

Dykman, L. N., Tepolt, C. K., Blend, C. K., & Mullineaux, L. S. (2025). Discovery of indirect parasite life cycles at deep-sea hydrothermal vents. Marine Ecology Progress Series 755:1-14.

Gibellula agroflorestalis

Imagine an infectious disease that turns you into an immobile white statue - while that sounds like something out of a dystopian science fiction story, this is the fate which awaits any spiders that encounter Gibellula, a genus of araneopathogenic (spider-killing) fungi. There are many species of this fungi found all over the world and their spores can infect all kinds of spiders. Gibellula is in the same broad group of fungi as the zombie ant fungi, but instead of producing a single or a few mushroom-like fruiting bodies that emerge from the carcass of their hosts, Gibellula smothers the infected spiders in a dense layer of white, fluffy mould.

Left: Examples of spiders infected with Gibellula agroflorestalis including orb-weavers (a-c) and jumping spider (d), Centre: numerous spore-bearing stalks (conidiophores), their mycelia twisted together, Right: Close-up of the head of a conidiophore.
Photos from Fig 3 and 4 of the paper

This post focuses on Gibellula agroflorestalis, a newly described fungus which was discovered by a group of researchers in the agroforestry systems located at the Abreu e Lima municipality of Brazil. 

Agroforestry is a system of agriculture where instead of cultivating a single type of crop, different crop plants are grown alongside trees and other plants, and these environments can provide habitats for a wider range of organisms. The forests that the researchers studied were composed of fruit plants such bananas, coconuts, limes, and mangoes, growing alongside various trees such as pink trumpet trees, pau brasil, and juazeiro.

So how does one find mouldy spiders amidst this semi-cultivated forest? Well, in short, with a lot of time and effort. The researchers hiked through multiple agroforestry properties while meticulously examining every plant they came across, carefully inspecting the underside of their leaves which is where many fungus-infected spiders end up. On the rare occasion when they come across an infected spider, it was collected intact along with the leaf it was sitting on, placed into a plastic cup, and later stored in silica gel to preserve the specimen. In total, the researchers found 17 infected spiders, ranging from jumping spiders to crab spiders to orb-weavers, alongside other spider hosts which could not be identified, because by that stage they have become more Gibellula than spider, twisted and fungal.

When the researchers examined the fungus under a microscope, they found that the structure and size of its teardrop-shaped spores were different from other known species of Gibellula. Furthermore, DNA analyses showed the fungus to be genetically distinct, and that as a species, G. agroflorestalis is fairly genetically diverse, which is probably why it is able to infect so many different types of spiders.

While there are other species of Gibellula fungi out there which have been recorded from spiders in natural forests, G. agroflorestalis was the first to be recorded in an agroforestry system, showing that these environment may in fact serve as habitats to some unique species. The discovery of G. agroflorestalis reminds us that a lot of unseen biodiversity can be found through careful observations. So it's good to stop and smell the roses, but you should also check underneath the leaves for mouldy spiders.

Reference:

da Rocha Alves, J.E., da Silva Santos, A.C., Pedroso, S.K.B., Melo, R.F.R., & Tiago, P.V. (2025). Untangling a web of spider fungi: Gibellula agroflorestalis (Hypocreales, Ascomycota), a new species of spider parasite from Brazil. Journal of Invertebrate Pathology 209:108278.

Lysiana exocarpi

Sometimes parasites get their own parasites too, and if you think that "enemy of my enemy is my friend", then you'd think this would be good news for the host. But that depends on the host-parasite pairings in question. This post is about a study on mistletoes, a plant that many people associate with Christmas celebrations, but they are also parasitic plants, specifically, they are "hemiparasites" - which are plants that can do their own photosynthesis, but they draw water and other nutrients from a host plant.

Left: A harlequin mistletoe attached to a box mistletoe (red arrow indicating attachment point), Right: Close-up of the attachment point (indicated by red arrow) between a harlequin mistletoe and box mistletoe.
From Fig. 1 of the paper

Mistletoes have varying degrees of host specificity, with some of them parasitising only a selected handful of trees and shrubs species, while others can infect a wide range of different plants. They parasitise their host using a modified root called haustorium, which bores into the host plant's stem, tapping into its flow of water and nutrients. But sometimes, mistletoes find themselves on the receiving end of a haustorium from another mistletoe. After all, mistletoes are just another type of plant. Parasitic plants that engage in such a lifestyle are called "epiparasites" by botanists, though they also fall under the larger umbrella of hyperparasites - parasites of parasites.

The Australian harlequin mistletoe (Lysiana exocarpi) is a very versatile hemiparasite - it can infect over a hundred different plant species and when the opportunity arises, it parasitises a fellow mistletoe, namely the box mistletoe (Amyema miquelii). One of the challenges for an epiparasite is maintaining a lower water potential than its host. Water has a tendency to move from areas of high concentration to lower concentration, and in plants, this is how water is transported from the roots to the shoots/leaves because the atmosphere (where the shoots/leaves are) have lower water concentration than the soil (where the roots are). As water diffuses into the atmosphere from the leaves, it draws more water from the roots to the shoots.

So in order to suck up water from its host, a mistletoe would need to maintain a lower water potential than the shoots of the host tree - this is why mistletoes are very thirsty plants. And an epiparasite parasitising another mistletoe would need to maintain an even lower water potential to ensure water would flow to it through both its host mistletoe as well as the tree that its host mistletoe is parasitising. So when a mistletoe is parasitising another parasitic plant, it would need to change certain aspects of its physiology.

This study took place at the Onkaparinga River National Park in South Australia, in a woodland composed mostly of pink gum (Eucalyptus fasciculosa). The researchers conducted a variety of measurements on both host trees and mistletoes, and collected samples of their leaves. What they found was that when the harlequin mistletoe is parasitising another mistletoe, it opened up more of the stomata on its leaves, so water is released into the atmosphere at a higher rate. At the same time, it also grew leaves with larger surface area, and had higher concentration of potassium and magnesium in them. All this decreases the mistletoe's water potential, which means the harlequin mistletoe gets more thirsty when it's parasitising another mistletoe. 

But what happens to its host mistletoe? Well, surprisingly enough, it seems that the box mistletoe doesn't suffer from being parasitised. It compensates for the cost of its thirsty epiparasite by simply drawing even more resources from its eucalyptus host, essentially outsourcing the cost of hosting a harlequin mistletoe to the tree. All this means that the host tree ends up taking the full brunt of BOTH parasites. Eucalyptus trees which are host to a parasitised box mistletoe have stiffer leaves than if it is parasitised by the box mistletoe alone. Among eucalyptus, growing stiffer leaves is often a symptom of nutrient and water deprivation, which is perhaps not surprising since the tree is hosting a pair of very thirsty plants, and this can have long term impacts on its growth and reproduction.

So at least when it comes to parasitic plants, the enemy of your enemy is not necessarily your friend, in fact, you might end up paying the price for their antagonistic relationship.

Reference:

Scalon, M. C., & Rossatto, D. R. (2024). Challenging the 'Immunity Hypothesis': Primary or Secondary Parasitism as Different Survival Strategies for the Harlequin Mistletoe Lysiana exocarpi (Behr) Tiegh. Flora 323:152662.

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.

Dicyema japonicum

On this blog, we like to revel in the obscure and bizarre, and it doesn't get much more obscure and bizarre than dicyemids - a group of parasites/symbionts which live exclusively in the kidneys of cephalopods like octopus, squid, and cuttlefish. But aside from its unusual habitat, the exact nature of dicyemids themselves has been a vexing mystery. Evolution has stripped them down to the bare minimum, they are bizarre animals with bodies that are composed only of 8-40 cells, with a significantly reduced genome. This also makes classifying them difficult, because they just don't have many of the usual features that scientists would use to work out an organism's evolutionary origin. There is some evidence to indicate that they might be relatives of flatworms (but perhaps not?), while other studies point to the so-called "jawed worms" as their relatives.

Left: Stained micrograph of Dicyema japnonicum, Right: East Asian Common octopus
Dicyema and Octopus photos taken by Dr. Hidedaka Furuya, used under Creative Commons (CC BY-SA 4.0 and 2.5) license

While dicyemids have very simple bodies, their life cycle is rather complicated. They have worm-like "vermiform" stages that live in the host's kidneys, and proliferate by giving birth to clones of themselves. Once their population reaches above a critical threshold, they get frisky and switch to sexual reproduction, producing stubby, ciliate-covered larvae called "infusoriform" that swim out to sea in search of a new host. Because of the different roles those stages play in the parasite's life cycle, they also exhibit very different behaviours.

A group of researchers in Japan investigated the behaviour of dicyemids from the East Asian common octopus (Octopus sinensis), a well-known and commercially-fished octopus species. Using octopuses obtained from commercial catches, the researchers were able to isolate both the vermiform and infusoriform stages of the parasite, and put them through a series of tests. The parasites were exposed to various different stimuli including light, drops of octopus fluids, pieces of coconut gel to mimic the interior of an octopus' kidneys, and flowing artificial sea water in a serological pippette to simulate the flow of urine through the kidneys.

They found that the wormy vermiform stages responded to flowing water by swimming against the current, like fish in a stream, and upon contact with the coconut gel, they stick their mushroom-shaped front end (called a "calotte") into the gel and hung on tight, as they would to the walls of an octopus' kidneys. In contrast, the infusoriform stage seems to swim around in a random manner, and tend to ignore or stayed away from the coconut gel. And even though they are supposed to go and infect a new host, they were not particularly attracted to any of the octopus fluids that the researchers presented them with, in fact, they were actually repelled by octopus blood.

The behavioural contrast between the two stages reflects their respective role in the parasite's life cycle. The vermiform is the reproductive stage that exists to populate the host's kidneys and give birth to the next generation. There is a lot of urine flowing through that part of the body, so if you actually want to stay there, you need to either keep swimming in place, or hang on tight. Because if you just go with the flow, you'll be on a one-way journey to the outside world. That's why in addition to swimming against the flow, when the vermiform stage touches something that feels like the interior of an octopus' kidney, it clings on for dear life. 

In contrast, the infusoriform is the dispersal stage which is meant to leave the octopus, so just going with the flow is a good way to achieve that end. But if their goal is to reach an octopus, why do they have an aversion to their host's blood? The researchers behind this study suggested that free flowing blood might be a sign of an injured or dying host which is not worth entering. This also tells us that these tiny parasites don't reach the kidneys through entering the host's circulatory system. 

But if that's the case, how do they reach the octopus' kidneys then? Also, if they are so indifferent to the scent of host material in the water, how do they even find their hosts? This is particularly puzzling as dicyemids are very picky about the host species they infect. But just as puzzling is how they are able to sense changes in their surrounding environment in the first place, since no sensory organs have been identified on these parasites. Dicyemids may seem like simple animals, but there's clearly a lot more to them than meets the eye.

Reference:

Hisayama, N., Takeuchi, Y., & Furuya, H. (2024). Taxes of Dicyemids (Phylum Dicyemida). Journal of Parasitology 110: 506-515.

Saccularina sp.

The bay scallop (Argopecten irradians) is a highly prized shellfish, but it has suffered through a rough history from overharvesting, habitat loss, and natural enemies. This culminated in a massive population decline in the 1980s that led to the closure of all its fisheries across certain regions along North Carolina. With the depletion of its wild populations, efforts are being made to raise the bay scallops in aquaculture to meet demands. But now a new woe has fallen upon this besieged bivalve with the appearance of a never-before-seen parasite.

Left: Bay scallop infected with Saccularina, the red arrows indicating the parasite's sporocysts in the gills. Right: the cercaria stage of Saccularina which is a "cystophorous"-type cercariae.
From Fig. 1 and 2 of the paper

In 2012, a researcher started noticing a gill-dwelling parasite in both caged and wild bay scallops along the coast of North Carolina, and later at the Gulf Coast of Florida. These parasites are readily visible in the shellfish’s gills as they become swollen with the parasite's presence. Examination under a microscope revealed the parasites to be a species of parasitic fluke which is using the scallop for the asexual stage of its life cycle. Essentially, these trematode flukes are converting the shellfish into a parasite clone factory that pumps out a stream of free-swimming larvae to infect the next host in the life cycle.

Given such an operation consumes a lot of the host’s resources, this can interfere with the scallop’s growth and survival, and thus it is a major concern to the scallop fisheries in North Carolina. The key to managing any parasitic infection is an understanding of its natural history and life cycle, and unfortunately, given its relatively recent discovery, the life cycle of this parasite is almost entirely unknown. However, related fluke species can give us some clues, and as it turns out, this bay scallop parasite is no ordinary fluke.

First of all, DNA analysis showed that one of this fluke's closest relatives is Saccularina magnacetabula, a trematode species found on the other side of the world in Australia. While it is genetically similar enough to the bay scallop parasite for them both to be in the same genus (Saccularina), there is enough geographical and genetic distance between them that they are clearly different species. Furthermore, instead of scallops, S. magnacetabula infects the Sydney cockle (Anadara trapezia) as the chosen bivalve for its asexual stage. As for the parasite's next stops, it's a multi-part journey involving tiny crustaceans, followed by a type of smallish Australian fish called whiting (Sillago sp.), and finally the adult fluke completes its life cycle nestled in the fin membranes of the giant herring (Elops hawaiensis).

Saccularina magnacetabula, and by extension, the bay scallop parasite, belongs to a family of flukes called Didymozoidae - a flukey group of flukes with some very unusual anatomy and habits. While the adult stages of most trematodes are generally leaf-shaped, didymozoids come in all kinds of shapes and sizes. And they are found in a wide range of different bony fishes all over the world, mostly marine species. Additionally, instead of living in the final host’s gut like most flukes do, didymozoids cram themselves into all kinds of nooks and crannies such as the muscles, the gills, or even the fin membranes as is the case for S. magnacetabula.

While there are many known species of didymozoids, the life cycles for most of them are a mystery, with the hosts for the asexual stage known only for a few species. But those handful of species alone showed didymozoids to have quite the eclectic range. Aside from bivalves like cockles and scallops, the asexual stage of other didymozoids infect snails, but not just any regular snails - one species is known to use pelagic sea snails (which are also called “sea elephants”), while another species infects worm snails which are peculiar sea snails with twirly shells that encrust on rocks and other hard surfaces.

So, based on the information above, we can make some inference about the likely source of the bay scallop parasite. It’ll have to be some kind of predatory sea-dwelling fish harbouring the adult stage of the fluke, and given S. magnacetabula completes its life cycle in the giant herring, the bay scallop parasite is most likely completing its life cycle in some kind of predatory herring-type fish in the region, which means ladyfish or Atlantic tarpon. 

While we may have some clues about the bay scallop parasite’s life cycle, how they might have gotten there is more of a mystery though. This parasite was first seen in bay scallops in 2012, but if the disseminator of this parasite is really a local fish species such as the tarpon, why has it only been noticed now? Whatever its origin turns out to be, it seems the bay scallop parasite is not ready to give up its (many) secrets.

Reference:

Boggess, H. F., Varney, R. L., Freshwater, D. W., Ben-Horin, T., Preister, C., McCurry, H., Wilbur, A. E. & Buck, J. C. (2024). A newly discovered trematode parasite infecting the bay scallop, Argopecten irradians. Aquaculture 589: 740960.

Nectonema sp

Sometimes a new scientific discovery comes about while one is doing the most mundane things, and it might not even be a scientist who happens to be doing it. Last year, an unusually high number of tanner crabs started showing up in the waters off the southern coast of Hokkaido. While these crabs are a bane for flounder fishermen as they have a habit of tearing up their nets, tanner crabs are easy to catch and they taste good, so numerous crabs have ended up in markets all over Japan, being sold for a relatively low price.

Top right: a cooked tanner crab with a coiled-up Nectonema worm inside of it. Top left: another cooked crab with a smaller Nectonema worm inside of it. Bottom: A Nectonema worm extracted and unraveled from the first tanner crab (scale bar = 2 cm). Photos from Figure 1 of the paper, taken by Rieko Yamamoto 

So what does this have to do with parasitology? Well, earlier this year a woman named Rieko Yamamoto had bought and boiled up some tanner crabs for a meal, but upon opening her would-be crab dinner, she discovered that one of the crabs came with an extra helping of worm, all coiled up like a bundle of cables. This worm was about 82 centimetres long and took up a lot of space in the crab's body. But instead of doing what some people might do, which is to toss the crab out the window in disgust, she calmly placed the parasitised crab in the freezer and contacted Dr. Keiichi Kakui, an invertebrate zoologist at Hokkaido university, who was able to identify the worm as Nectonema.

Nectonema is a genus of horsehair worm, and while horsehair worms are more commonly known from land-dwelling arthropods such as crickets and praying mantis, there is one offshoot lineage of horsehair worms that have taken up life within the denizens of the seas. Nectonema has previously been reported in many types of crustaceans, including rock crabs, shrimps, lobsters, squat lobsters, and even marine isopods, but this is the first time that it has been found in a tanner crab.

A week after this discovery, Ms Yamamoto bought another eight crabs and found one of those crabs also came with a worm, which means this parasite might not be all that uncommon among tanner crabs. Fortunately, Nectonema doesn't cause any harm to humans, so there are no public health issues here, though it might be an alarming sight to those who are unfamiliar with these worms.

The life cycle of these marine horsehair worms is a mystery, though if their more well-studied relatives is anything to go by, it might involve the larva infecting a smaller invertebrate first, before being eaten by the final host where it can grow to its full adult size. While horsehair worms in land-dwelling hosts are known for altering the behaviour of their hosts, such behavioural manipulation is due to the worm needing to move its terrestrial host into a water body to complete its life cycle. This is not necessary for Nectonema since it is already surrounded by water in the sea.

Nature is full of surprises, but if you are prepared and observant, you might come across a scientific discovery while having your next meal. So if you ever find a worm in your dinner - don't panic! It might turn out to be an important scientific discovery.

Reference:

Kakui, K. (2024). Nectonema horsehair worms (Nematomorpha) parasitic in the Tanner crab Chionoecetes bairdi, with a note on the relationship between host and parasite phylogeny. Diseases of Aquatic Organisms 159: 153-157.

Epifagus virginiana

Epifagus virginiana is a parasitic plant that grows on the roots of American beech trees. Also known as "beechdrops", clumps of their brown stems can be found protruding from the forest floor, reaching up to 30 cm tall and lined with purple-white flowers. To most people, they look like just another ordinary plant amidst the undergrowth. But Epifagus lacks a key component which is usually a defining characteristic of plants - chlorophyll, the pigment which allows plants to harness solar power. Instead, the way this plant obtains its nutrients is via an underground tuber attached to the roots of its host. The beechdrop is a very discerning parasite - as its name indicates, it usually goes after beech trees, but sometimes they switch up their target and end up engaging in botanical cannibalism.

Left: The stem and flowers of beechdrops, Epifagus virginiana, Centre: a beechdrops tuber, with adventitious root (ar) indicated, Right: two beechdrops tubers linked via a parasitic connection.
Photos from Fig. 1 and Fig. 2 of the paper.

A botanist named Dr. Luiza Teixeira-Costa was conducting a study on these parasites in the mixed forest at Powdermill Nature Reserve in Pennsylvania, where there is a dense population of beechdrops. After carefully excavating a dozen of those parasitic plants, she noticed one of the specimens was composed of two Epifagus plants locked in a peculiar pairing. When she looked at that specimen more closely, she found that the pair consisted of two beechdrops tubers clinging tightly to each other, as if one of the plant was parasitising its fellow parasite.

To see if there's an actual parasitic connection between the tubers and that this was not two plants that had wrapped around each other via happenstance, Dr. Teixeira-Costa tested whether fluid could be passed between the two plants by injecting one of them with a special tracking solution, and examining it with micro CT scan to create a 3D image of the plants' internal structure. The scanning revealed that there is continuity in the vascular tissue that connects the two plants. It would be like if you find a pair of animals that have joined together and are connected via their circulatory systems. But that was one specimen - perhaps this was just a freakish one-off occurrence?

In order to get a better picture of this phenomenon, Dr. Teixeira-Costa examined the collections of Epifagus specimens at the Meise Botanic Garden and the Harvard University Herbarium, to see if there are more of such pairings. Out of 150 Epifagus that were in those collections, four of them were composed of a pair of E. virginiana plants attached to each other, in a similar way to how these parasites would usually attach to their host. 

In addition she also searched through the online digital archive of herbarium specimen images from 233 herbaria via the SERNEC portal. Out of the 3097 Epifagus specimens in those collections, she found 52 specimens that showed potential signs of parasitising a fellow beechdrops. All those specimens had been collected from across the plant's distribution range in the United States. What this exhaustive search revealed is that while such cases of beechdrops-on-beechdrops is not a frequent occurrence, it is not negligibly rare either. Furthermore, it is widespread and not restricted to just one particular region.

Orobanchaceae plants such as Epifagus are usually able to recognise its own kind and avoid this kind of friendly misfire - so what made some beechdrops turn on one of their own? One possible scenario might involve an Epifagus plant attaching itself to a beech tree root, and was followed by the seed of another Epifagus which responded to the stimuli given off by the tree root. But instead of latching onto the beech tree root, it ended up parasitising one of its fellow Epifagus that was already there.

These examples of intraspecific parasitism have been described as "botanical version of cannibalism" and have been documented in a range of other parasitic plants. If anything, compared with the beechdrops, other parasitic plants seem to be far less discriminating about parasitising their own kind, particularly among mistletoes and dodders. In the world of botanical parasites, sometimes a parasitic plant's worst enemy is a fellow parasite.

Reference:

Teixeira-Costa, L. (2023). Cannibal plants: intraspecific autoparasitism among host-specific holoparasites. Botany 102: 176-183.