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.