Caledoniella montrouzieri

Mantis shrimps (Stomatopoda) are some of the most formidable crustaceans in the sea; armed with trinocular colour vision, and a pair of powerful raptorial limbs that punch so hard, it generates supercavitating bubbles which collapse with such energy, the vapour within them briefly turns into white-hot plasma. Arguably, one of the most impressive animals in the sea. But to Caledoniella montrouzieri - a mantis shrimp is simply a big juicy host. 

Underside of a parasitised mantis shrimp, showing the male, female, and egg capsules of Caledoniella montrouzieri. 
Photo from Fig. 1 of the paper, taken by Ryutaro Goto.

Most parasitic snails belong to the Eulimidae or Pyramidellidae family, and both of them parasitise slow-moving or sedentary invertebrates such as echinoderms, molluscs, and polychaete worms. But Caledoniella has taken a different, independent route down to parasitism town. It doesn't belong to either of those families, and instead of a slow life feeding on some barely mobile hosts, it lives life in the fast lane, clinging to the belly of a nimble, predatory crustacean.

Such a lifestyle requires some specialised anatomy. In most snails, the foot is a flat muscular organ that is used for crawling over various surfaces. But in Caledoniella, the foot has been transformed into a big suction disc that allows it to cling firmly onto its very agile host. During the course of its evolution, it has also lost one of the key diagnostic characteristics of molluscs - the rasp-like radula in the mouth which snails use to scrape bits of food, be they algae or the flesh of other animals, into their mouth. Instead, it has a mouth that is more suited for suction feeding, and has highly developed salivary glands to facilitate its liquid diet. This snail is a vampire of mantis shrimp, sucking on their host's gill filament for that sweet, sweet hemolymph.

Caledoniella has some noticeable sexual dimorphism with the female snail being much larger than the males. When these snails mature, they pair up as a monogamous couple, living out their lives together on the underside of a mantis shrimp. But this happy couple likes to keep themselves to seperate parts of the mantis shrimp, with the female living near the tail of the shrimp, and the male living near the middle of the abdomen. Sitting between them are all the egg capsules they have been busily making together. This gastropod couple takes their toll on the mantis shrimp, which experience stunted growth, reduced moulting, and infertility.

So how did Caledoniella ended up with its unique way of life? The closest living relatives of Caledoniella are snails that live as roommates with mantis shrimps, hanging on the walls of the crustacean's burrow. While these snails are frequently in the presence of the burrow's main tenant, that's as far as their relationship with the mantis shrimp goes. They are strictly commensals that never lay their foot on the mantis shrimp, and it is likely that was the lifestyle of Caledoniella's ancestors. But at some stage, after living in such close quarters with mantis shrimps for so long, some of those meek wallflower snails just couldn't resist getting more intimate and started taking a bite of its crustacean roomie, thus giving rise to the clingy blood-sucking Caledoniella. 

But when you trace its evolutionary history even further back, it seems those mantis shrimp roommates have themselves evolved from snails that originally lived in the burrows of an entirely different animal - spoon worms! So the ancestors of Caledoniella switched from sharing quarters with spoon worms, to living with mantis shrimp, to living on mantis shrimps

Perhaps somewhat surprisingly, Caledoniella is not the only mollusc that spend their lives clinging to the mantis shrimp - there are also a few species of tiny clams from the Galeommatoidea family called yoyo clams that live attached to the mantis shrimp's belly.  But unlike Caledoniella, they don't go as far as to feed on their host's blood. These clams receive protection from living on the belly of this heavily-armed crustacean, and the host's agile movements provide it with plenty of water flow for all their respiratory and filter-feeding needs. While they aren't blood-suckers, they seem to have followed the same evolutionary pathway as Caledoniella, evolving from ancestors that originally lived as commensals in the burrows of mantis shrimps.

For molluscs, it seems that sharing room with a marine benthic invertebrate is a surefire gateway to becoming a clingy parasite.

Reference:

Goto, R., Takano, T., Eernisse, D. J., Kato, M., & Kano, Y. (2021). Snails riding mantis shrimps: Ectoparasites evolved from ancestors living as commensals on the host’s burrow wall. Molecular Phylogenetics and Evolution, 163:107122.