"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

March 19, 2020

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?

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.

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