Pinnotheres pholadis

Pea crabs are a type of tiny crabs that live mostly in bivalve shellfish such as mussels, oysters, and scallops. Scientifically known as pinnotherids, they hang out in the mantle cavity, a muscular bag which these molluscs use to pump water in and out of their body. Nestled in this flesh chamber, pea crabs get to enjoy a steady stream of aerated water and delicious phytoplankton, while protected by the bivalve's hard shell (although this does make hooking up a bit tricky). While some of these crabs can take up a fair bit of room in their host's body, they don't tend to feed on the shellfish's tissues. So are they just innocuous house guests hanging out with their bivalve homies?

Top left: A Pinnotheres pholadis pea crab in a scallop's mantle cavity, Top right: A female pea crab,
Bottom left: A male pea crab, Bottom right: A female pea crab with eggs.
Photos from Fig. 2 and 3 of the paper.

Well, the study featured in this post shows that at least when it comes to Pinnotheres pholadis, their presence can have a detrimental effect on their hosts, especially among juvenile shellfish. Researchers in Japan collected both wild and farmed Yesso Scallops from twelve different locations across Mutsu Bay in Japan and examined them for pea crabs. They looked through 881 scallops, and found that close to one-third of them carried pea crabs. When they compared the conditions of the scallops with and without crabs, they found that the crab-endowed juveniles tend to be skinnier and have smaller shells, especially if they housed more than one crab. 

So if the crabs aren't feeding on the scallop's tissue, why would they affect the host's condition? Like other bivalves, scallops feed on algae and other organic detritus in the water, which they suck up and trap in strings of mucus for easy swallowing. But this bundle of nutrient-rich mucus also happens to be the pea crab's favourite food, and the resident crab can intercept the mucus strings before they can be swallowed by the shellfish. So these crabs are literally snatching food out of the scallop's mouth, making them a kind of internal kelptoparasite.

While older and larger scallops might be able to offset their tenant's appetite due to their greater filtration capacity and larger reserves of energy,  juvenile scallops need all the nutrients they can get to fuel their growing body. So they are less able to afford the crab skimming off the top from their phytoplankton slime smoothee. The researchers also found that crabs living in larger scallops tend to be bigger, possibly due to a combination of having more room in the mantle cavity, and also because bigger hosts are able to gather more food from the surrounding water. In this system, the available space with the bivalve places an absolute limit on how big the crabs can get.

So while for humans, having crabs may simply be itchy and embarrassing, for scallops, having crabs when you are young can potentially ruin you for life.

Reference:

Yamazaki, T., Odani, K., Nakayama, R., Watanabe, T., & Kawai, S. (2026). Parasitism by pinnotherid crabs in the Japanese scallop Mizuhopecten yessoensis: first host record and quantitative assessment of host impacts. International Journal for Parasitology: Parasites and Wildlife 29: 101198.

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.

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.

Unikaryon panopei

Like any living things, parasites can themselves become host to other infectious agents as well, and parasites that specialise in parasitising other parasites are called hyperparasites.  The paper we will be looking at today is about some microsporidian parasites that have evolved to parasitise flukes. Microsporidian are a group of single-celled parasites which are somewhat related to fungi, and they infect a wide range of invertebrate animals - including many parasitic animals.

Left: Fluke metacercaria infected with Unikaryon panopei surrounded with smaller, uninfected flukes
Right: Swollen fluke cell, filled with spores. Photos from Fig. 1. of the paper.

The species featured in this post - Unikaryon panopei - infects flukes which parasitise crabs. More specifically, these flukes were found in black-clawed mud crabs from Tampa Bay, Florida. The researchers who conducted this study collected a relatively small number of crabs - fifteen in total - but that was more than enough to find some which were infected with flukes, because all of them were absolutely loaded, with some crabs harbouring up to 250 fluke larvae. 

The fluke's free-swimming larval stages are able to get through the crab's tough exterior with a microscopic, scalpel-like structure called a stylet, which they use to slice their way through the vulnerable parts of the crab's cuticle, such as the leg joints and gill filaments. Once inside, they crawl to the hepatopancreas (also known as the digestive glands), where they curl up and transform into spherical cysts called metacercariae, and wait for the crab to be eaten by a bird. The flukes essentially use the crab as a temporary stopover and transport to get a ride into shorebirds.

At least that was the plan - until Unikaryon came along to completely ruin their lives, and some unlucky flukes found themselves becoming incubators for microsporidian hyperparasites. Fluke larvae which are heavily infected with Unikaryon swells to twice their usual size, and become filled with spores which are packaged in brown ovoid throughout the fluke's body. While in moderately infected flukes, the spores are mostly concentrated in the intestine and the still developing reproductive organs, in heavily-infected flukes, the hyperparasite replaces all of the fluke's internal tissue and organs, turning it a spore-filled husk.

When the researchers examined the evolutionary lineage of U. panopei in relation to other microsporidian parasites, they found that these hyperparasites might have evolved from microsporidians that originally parasitised crustaceans. For whatever reason, over time, they switched to targeting the parasites of said crustaceans instead. In addition to U. panopei, a handful of other Unikaryon species have also been reported from various species of flukes, and even one species from fish tapeworms. 

In addition to infecting the metacercariae cyst stages as found in this study, Unikaryon has also been found infecting other life stages of flukes, including the asexual stages in snails, and the free-swiming stages which are produced by infected snails. Yet despite being present in those other life stages, Unikaryon has never been found to infect adult flukes.

Given how Unikaryon has been able to insinuate itself into different parts of the fluke life cycle, while remaining strangely absent in the adult stage, this raises the question of how the flukes even get infected with these hyperparasites in the first place. Do they pick it up from the environment? If so, how - given the fluke stages they infect are situated deep in their host's bodies? How do they get released into the surrounding environment, and how are they transmitted to new hosts? Or is the hyperparasite inherited at birth, and just gets passed down each subsequent generation? If so, how could that be possible since it is absent from the adult stage of the fluke's life cycle? 

There are so many questions relating to some of the most basic aspects of this hyperparasite's ecology. Since most groups of parasites are severely under-studied, it is not surprising that we know even less about some parasites' own hyperparasites. These microsporidians are single-celled mysteries, packed in the bodies of animals, which themselves dwell in the armoured bodies of unassuming crustaceans.

Reference:

Sokolova, Y. Y., Overstreet, R. M., Heard, R. W., & Isakova, N. P. (2021). Two new species of Unikaryon (Microsporidia) hyperparasitic in microphallid metacercariae (Digenea) from Florida intertidal crabs. Journal of Invertebrate Pathology, 182, 107582.

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.

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.

Nepinnotheres novaezelandiae

Life as a pea crab seems pretty sweet, you spend most of your time sitting snug and protected within the armoured shelter of a shellfish, while your host's filtration current bring you a constant stream of oxygen and food - everything that a pea crab needs for a good life. Well, almost everything - because there's more to life than just being protected and fed. Much like other organisms pea crabs need to reproduce - that's how evolution works, and unlike many other living things, a pea crab cannot just clone itself.

Male Nepinnotheres novaezelandiae squeezing in between
the valves of a mussel. From video here.

So when it comes to reproduction, the balance of living the pea crab life tips from "pretty sweet" to "absolutely terrible" - especially if you are a male pea crab. For them, trying to find a mate is a harrowing challenge than none of us can possibly imagine. First of all, to reach a potential mate, you have to leave your host, which means you have to pass the gates that are the valves of the host mussel, without being caught in between them. At that stage, those valves that had offer such formidable protection for the pea crabs then become death traps, with about 13% of male crabs meeting their end at this molluscan gate - their bodies litter the mussel bed.


Once outside, the male pea crab faces even more challenges. These tiny crustaceans, which are more accustom to a cosy life inside a shellfish, have to cross the treacherous, open areas of the mussel bed, filled with horrible monsters (in the form of predators like fish, octopus, and larger crustaceans) for which an exposed pea crab is just a convenient snack. Furthermore, male crabs only make up 20% of the population despite the more or less equal sex ratio of immature pea crabs. The length that they have to go to just to find a mate probably has something to do with that...

Despite the odds, almost 90% of all female crabs in the population carry fertilised eggs, so some male crabs must be having successes - but how?

The researchers who conducted this study noticed that the male pea crabs always set out under the cover of darkness when they will be less likely to be spotted by predators, and also because mussels are more relaxed at night. From the researchers' perspective, this also means that all the experiments and observation of pea crab behaviour had to be done in the dark. So in addition to sea water tanks, they set up some infra-red cameras to capture footages of all this activity - like some kind of voyeuristic shellfish reality TV show.

So what would coax a male crab out of his cosy home? To find out, the researchers constructed a flow-through observation chamber lined with PVC tubes in which they placed pea crab-infected mussels. When they placed a mussel with a female crab upstream of one with a male pea crab, the male crab would exit their host 60% of the time, roused into action by something which seem to secreted by the mussel (or the female crab in the mussel) upstream.

Male Nepinnotheres novaezelandiae tickling the mantle edge
of a mussel. From videos here.

The crab then makes its way to the mussel where the female crab resides. Once there, the pea crab patiently tickles the mussel's mantle fringe with its legs to try and convince the bivalve to let it enter. This is also the reason why the male crab only do this at night, because a mussel's response to such tickling can be very different in daylight. Try the same trick during the day and the bivalves would slam shut, crushing the amorous crab between its valves. On average, the crab will spend over three hours fiddling away at the mussel to coax the shellfish into opening up.

Additionally, in a different flow-through seawater tank where the crabs were given more freedom to roam from one host to another, the researchers recorded how long it took for the male pea crab to leave its host and reach a mussel containing a female crab. The entire journey from exiting the original host mussel to reaching their final destination took seven hours on average, though this varies from a quick hour-and-a-half jolt, to an eighteen-and-a-half hour-long trek for one particularly unfortunate individual.

So when love (or at least lust) is in the water, the pea crab will give up the easy life, and risk life and limbs for an evening rendezvous.

Reference:
Trottier, O., & Jeffs, A. G. (2015). Mate locating and access behaviour of the parasitic pea crab, Nepinnotheres novaezelandiae, an important parasite of the mussel Perna canaliculus. Parasite, 22: 13.

Calyptraeotheres garthi

There are many parasites that castrate their hosts - the parasitic barnacle that feed on the velvet belly lantern shark, the nightmarish Sacculina that takes over the body of a crab and turns it into a baby-sitting zombie, or the nematode that sterilise queen hornets and turn them into mobile nurseries.

Limpet without (left) and with (right) C. garthi
Image from Fig. 2 of the paper

There are two main ways that a parasite might stop its host from having babies. It can manipulate host physiology and suck up resources that would have otherwise gone into growing and maintaining the host's reproductive organs, which then simply shrivel up from being starved of nutrients. Alternatively, a parasite might actively occupying the space where the reproductive organs or resulting broods would normally be, displacing any would-be eggs and/or offspring.

Pea crabs are tiny crabs that specialise in making a living inside the body of marine invertebrates like various molluscs and echinoderms. The species featured in the study we are looking at today is Calyptraeotheres garthi, which lives inside limpets on the coast of Argentina. The researchers that conducted this study started noticing that most limpets that have crabs tend not to produce any egg sacs during the reproductive seasons, so they tried to find why that is by examining limpets from the field and by raising both crab-free and crab-infected limpets in the lab to compare their reproductive output.

Out in the wild, about a third of the crab-free limpets carried eggs during the breeding season, and a few of the limpets infected with smaller male or juvenile crabs managed to produce at least some eggs. But the limpets that were harbouring fully-mature female crabs had no eggs at all. This was similar to what they observed in their captive limpet population - while half of the uninfected limpets could spawn and produce a brood, none of the crab-laden limpets managed to do so.

Even though the crab-infected limpets did not produce any eggs, they had intact ovaries which were filled with oocytes (egg cells) ready to go. And when the researchers they remove crabs from infected limpets, they quickly recovered. Within a week or two after crab removal, those limpets started producing eggs again. So how was C. garthi stopping the limpet from producing a brood? Is the crab hogging all the nutrients and leave none to the developing eggs?

These limpets feed by collecting phytoplankton (floating, single-celled algae) into a mucus string around the gill fringe, and that is what C. garthi feeds on - pilfered strings of algae-loaded slime from its host. While you might think this free-loading would be taking a major toll on the limpets, they do not seem to be too affected by this. Crab-infected limpets carried on feeding and digesting at the same rate as crab-free ones, so the little crustaceans was not affecting the limpet's usual energy intake - at least not to a level that the host cannot compensate.

Calyptraeotheres garthi with a stomach full of phytoplankton
Image from Fig. 4 of the paper

So C. garthi is not stealing much nutrient from the hosts, but is it actually adding caviar to its green salad and treating itself to the limpet's eggs? After all, it is in the perfect position to snack on a serving of freshly-produced eggs. But when the researchers examined field-collected limpets that harboured crabs but still managed to squeeze out a few eggs, none of their egg sacs showed no signs of damage by the crab, which means C. garthi were only interested in one thing - sweet green slime strings.

Despite not being a severe physiological drain, their physical presence occupy the spot where the limpet would carry its brood of eggs. So while the limpet can still carry on feeding and digesting as normal, it gets brood-blocked by the crab.

The relationship between the limpet and the crab is made even more complicated by seasonal changes. During summer, the larger limpets that are infected with C. garthi are healthier than crab-free limpets, but in winter the situation is reversed. However, on top of that, during winter, only the larger limpets with crabs suffer a decline in health, while those below a certain size threshold gets away with carrying around a food-stealing crab without any severe consequences.

From our perspective, under certain circumstances, it might actually seem beneficial to have a pea crab, seeing as crab-harbouring limpets seems to be healthier during certain times of year. But from an evolutionary perspective, this pea crab is extremely harmful - by preventing its host from reproducing, it is effectively terminating that individual limpet's genetic lineage - all just for a mouthful of green slime.

Reference:
Ocampo, E. H., Nuñez, J. D., Cledón, M., & Baeza, J. A. (2014). Parasitic castration in slipper limpets infested by the symbiotic crab Calyptraeotheres garthi. Marine Biology, 161: 2107-2120.

Hematodinium sp.

Today's parasite, Hematodinium sp., infects blue crabs and causes a disease known as "bitter crab". While the name may sound just slightly nauseating for your palate, for the afflicted crabs, its symptom is down right horrific. The parasite causes the crab's hepatopancreas (equivalent of our liver and pancreas) to malfunction, it starts suffocating, and its muscles eventually dissolve within its exoskeleton. Crabs that are experimentally infected start dying about 2 weeks after initial exposure, and this deadly parasite may have even contributed to the recent decline of blue crabs in Chesapeake Bay.

Hematodinium and related species are dinoflagellates, and while most dinoflagellate are free-living, this species belongs to a group which have evolved to be parasites, with many different species infecting a wide variety of hosts. While several different stages of the parasite have been isolated from the blood of infected crabs, little is known about how they are transmitted between hosts, nor the inner life of those different stages in the hosts. Because many parasites live enclosed within the body of their hosts, it is almost impossible to directly observe how they live and grow the way you might be able to observe a fish or a bird. Ideally, if you can isolate a parasite out of its host, put in it a clear container which closely mimics the conditions found within its host, and still have it complete its life-cycle, then you can find out a lot more about how it lives.

Recently, a group of researchers from Virginia were able to successfully complete the life-cycle of Hematodinium - in vitro - which means they were able to grow it in a culture of chemical broth that sustained the parasite's every need, without any host animals involved. This was accomplished through a painstaking series of transfers, starting with isolating the parasite from infected crabs, then moving each stage into different culture mixes as it grew, all while keeping the conditions as sterile as possible. Out of the 10 isolates they attempted to grow, only 4 successfully completed their life-cycle in vitro. The researchers also found out that the parasite grows best in the dark, and indeed light exposure kills them within weeks, which makes sense given that it is pretty dark inside a crab (a variation on the Marx Brother joke).

Through this in vitro technique, they were able observe the different parasitic stages of Hematodinium directly, and view them as they would have been while floating in the blood and organs of a blue crab. They noted that when Hematodinium cells first enter the crab as "dinospores," they turn into a worm-shaped form called a "filamentous trophont" (see the accompanying photo which was from a figure in the paper). About a month after that, the cells begin transforming into clumps that are composed of multiple clones of the original infection stage. These clumps then grow into a stage called an "arachnoid trophont," which resembles a blob with numerous tendrils around its fringe (which would be embedded in the hepatopancreas of the crab). These clumps tend to merge and form larger blobs as they come into contact with each other. When those "arachnoid trophonts" fully develop, the cells in the middle of the blob start producing spores that eventually turn into the infective dinospores that escape from the crab to infect new hosts, starting the life-cycle anew.

Reference:
Li, C., Miller, T.L., Small, H.J. and Shields, J.D. (2011) In vitro culture and developmental cycle of the parasitic dinoflagellate Hematodinium sp. from the blue crab Callinectes sapidus. Parasitology 138:1924-1934.

Postscript: Three days after this post went up, I was contacted by Peter Coffey, who used to work on this species of parasite with a bit of additional information/correction: I just have one quick comment on the first sentence in your post. In blue crabs we don't see the same bitter flavor that we do in Alaskan Tanner and Snow Crabs, so we haven't been calling infections in blue crabs BCD.
Thanks Peter!

October 4 -Gynaecotyla longiintestinata

Gynaecotyla longiintestinata is a marine trematode from the Aveiro estuary of Potugal. It is in the same family as Maritrema novaezealandensis- the Microphallidae. G. longiintestinata has a typical life-cycle for a trematode from the microphallid family - its uses a snail, in this case the intertidal whelk, Nassarius reticulatus, as a first intermediate host. From there, free-swimming larval stages called cercariae are produced and released into the environment, where they infect the second intermediate host, in this case the crab, Carcinus maenas. They occupy a very specific and peculiar microhabitat within the crab, namely its antennal glands. The definitive hosts of this parasite are shorebirds that feed on crabs.

Photo from:
Russell-Pinto, F. and Bartoli, P. (2002) Cercaria sevillana n. sp., a new cercaria (Digenea: Microphallidae) from Nassarius reticulatus (L.) (Mollusca: Prosobranchia) in Portugal. Systematic Parasitology 53: 175-182.

Contributed by Tommy Leung.

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 18 - Profilicollis altmani

Parasites that have complex life cycles involving marine creatures really baffle me - the odds of them completing their life cycle just seems so unlikely - and yet they do. Profilicollis altmani is a species of acanthocephalan (thorny-headed worm) that uses mole crabs (Emerita spp.) as its intermediate hosts and then infects shore birds like Herring Gulls as the definitive host. The adult parasite attaches to the intestines of the bird and then will release eggs into its feces where they somehow make their way to new foraging crabs. This parasite is also of recent interest because it appears to have jumped hosts into sea otters, where it can cause fatality. The otters are not normally hosts of these parasites, but perhaps are becoming infected as a result of eating prey that they normally do not.

Photo by Tricia Goulding, Romberg Tiburon Center for Environmental Studies, San Francisco State University.

June 24 - Zaops ostreum

How many kind of food can you name comes with its own side dish? Well, the eastern oyster (Crassostrea virginica) should be on that list. This week. we've already seen how a trematode infection can improve the taste of oysters, but it seems that oyster also comes with another gastronomic treat in the form of the pea crab Zaops ostreum. Pea crabs (family Pinnotheridae) are small soft bodied crabs which live inside a variety of marine invertebrates, with most species living in bivalves. Zaops ostreum infect the oyster as a tiny first stage larvae, and grow to maturity within the bivalve's mantle cavity, feeding upon food-laden mucus strings produced by its host's filtering action. It is a true parasite in that it causes harm to its host. Not only does it steal food from the oyster, it also forms an obstruction within the body cavity and erode the gill tissue. From a culinary perspective, there are many serving suggestions available for pea crabs - they can be served raw, deep fried, or sautéed, and can be eaten either as a side dish to oysters, or even on their own (if you can get enough of them to make a meal!).

Photo and contribution by Tommy Leung.

June 12 - Paramoeba perniciosa

This amoeba is responsible for causing disease in several species of crabs and lobster. It is a feared parasite amongst crab fishermen and Maryland foodies as epizootics can cause very high mortality of crabs such as blue crabs and gray crabs, by destroying their connective tissue.

Image from micro*scope.

May 27 - Pinnotheres pisum

Some crabs, like hermit crabs, live in the discarded shells of mollusks, but some, like the pea crab Pinnotheres pisum, just can't wait for the resident to move out first and move in as a roommate -- but the kind of bad roommate that steals your food and damages your house. P. pisum, which is at most about half and inch wide, lives in the mantle cavities of bivalves such as mussels and clams where it picks the food off their gills and can severely damage them in the process (kind of like you might imagine if something with 10 legs lived in your fridge full-time.) I chose this photo from wikipedia (click on it for a blown-up view), which shows a pea crab that has fallen out of the clam that an otter is eating. Better go find another landlord, little crab!

May 21 - Paragonimus westermanni

Yesterday was an instance of an animal parasite that was recently discovered in a human. Today's parasite, Paragonimus westermanni, is another fluke that can infect both humans and animals. Commonly known as the oriental lung fluke, humans become infected when they eat undercooked crab or crayfish, which are serving as intermediate hosts. Before the crustacean, the parasite was in a snail (oh how trematodes love their snails!) and how did they get there, you ask. The eggs of the flukes are coughed up by humans -- or by cats, it turns out. The parasites are very prevalent in many parts of Asia and as many as 80% of crabs can be infected. Common preparation methods such as pickling or salting will not kill the metacercariae and thus are easy routes to vertebrate hosts. Another popular Chinese dish - "drunken crab" - made by dousing crabs in wine is another means of infection. The species name is an honorific of a zookeeper who also observed the flukes in Bengal tigers.

April 8 - Maritrema novaezealandensis

The parasite Maritrema novaezealandensis is commonly found on the coast of South Island, New Zealand. It is a trematode with a typical three host life-cycle, using a snail as a first host intermediate host where clonal multiplication occurs, a crustacean as a second intermediate host where it form a cyst-like waiting stage, and gulls as the definitive host where it matures into a hermaphroditic adults and sexually reproduces.

This trematode parasite uses the New Zealand mudsnail (Zeacumantus subcarinatus) to asexually multiply, and in certain areas, more than 60% of the snails found are infected with this parasite. The parasite takes over the most of the innard of the snail, filling it up with clones of itself and diverting resources from its reproductive organs, thus castrating it. The snail is essentially a zombie under the control of the parasite. Maritrema then use the snail as a kind of "parasite factory" producing free-living swimming larval stages (also clones) call cercariae (pictured) which are released into the environment to infect the next host in the cycle which are crustaceans such small crabs and amphipods (tiny beachhopper-type animals). The cercariae penerate weak spots in the cuticle using a specialised structure known as a stylet which functions rather like a cross between a scalpel and a saw. The little cercariae (which are less than 0.15 mm long) use the stylet to cut their way into the crustacean. Once inside, they grow over the course of a few weeks and develop into a cyst. There, they wait to be eaten by a sea gull to complete their life-cycle.

The cue for the infected snails to release the free-swimming cercariae stage is an increase in temperature, and when that happens, hundreds of cercariae swarm out of an infected snail. The hotter it gets, the more parasites are released. In the summer during low tide, pools or puddles gathered on the mudflat can get quite warm in the glare of the afternoon sun. Trapped within those tide pools are various crustaceans and many, many infected snails. Triggered by the heat, each infected snail releases hundreds of cercariae into the water, turning the water into a swarming "cercariae soup" and rendering any crustaceans in the area into hapless parasite pin-cushions. For tiny crustaceans like amphipods, the experience of being penetrated simultaneously by multiple cercariae can be quite traumatic (imagine being stabbed multiple times by scalpels) and the experience can often be lethal. While killing the intermediate host before it can be passed on to the next host is not good for the parasite either, it demonstrates one of the ways that parasites can regulate the population of its host.


Some relevant papers on this parasite are:

Fredensborg, B.L., K.N. Mouritsen, and R. Poulin. 2004. Intensity-dependent mortality of Paracalliope novizealandiae (Amphipoda: Crustacea) infected by a trematode: experimental infections and field observations. Journal of Experimental Marine Biology and Ecology 311: 253-265.

Fredensborg, B.L., K.N. Mouritsen, and R. Poulin. 2005. Impact of trematodes on host survival and population density in the intertidal gastropod Zeacumantus subcarinatus. Marine Ecology Progress Series 290: 109-117.

Keeney, D.B., J.M. Waters, and R. Poulin. 2007. Clonal diversity of the marine trematode Maritrema novaezealandensis within intermediate hosts: the molecular ecology of parasite life cycles. Molecular Ecology 16: 431-439.

Martorelli, S.R., B.L. Fredensborg, K.N. Mouritsen, and R. Poulin. 2004. Description and proposed life cycle of Maritrema novaezealandensis n.sp. (Microphallidae) parasitic in red-billed gulls Larus novaehollandiae scopulinus from Otago Harbor, South Island, New Zealand. Journal of Parasitology 90: 272-277.

Contributed by Tommy Leung.

January 7 - Sacculina carcini

A great example of a crustacean that parasitizes another crustacean is the barnacle, Sacculina carcini, which is a parasite of crabs. These parasites are favorites of professors as they represent a great example of host manipulation. Sacculina mimics the broods of female crabs, causing her to groom the parasite sac and help the eggs disperse into the water. And if the Sacculina finds itself in a male crab - it just sterilizes it and causes it to act like a female!