Bless the Maker and His… Clams??
Our wannabe Arakeen in all its (very limited) glory.
Ryutaro Goto & Makoto Tanaka (2019)
So apparently there's a clam which lives its life riding a literal sandworm1.
Very un-clam-like galeommatid clam Chlamydoconcha orcutti.
Cricket Raspet (CC BY 4.0)
This clam, Montacutona sigalionidcola, belongs to an order of clams called the Galeommatida, who are some of the least clam-like clams out there right alongside things like shipworms (Teredinidae) and the watering-pot shells (Penicillidae).
Now, galeommatids are known for three things. For one, their shells are weak and thin, totally unsuited for protection2. For two, their feet (the tongue-like appendage seen at right) secrete a mucus on which they can glide around like snails2. For three, they frequently live in close contact with other marine animals2. While most galeommatids are content to live inside the burrows and hidey-holes of their unsuspecting hosts, a few get a bit more physical with their relationship3. Montacutona sigalionidcola is one of these clingy clams, being exclusively found attached to the head of the sigalionid scale worm Pelegenia zeylanica1. The clam gets a mobile home which flees from danger and shifting beach sand, while the worm gets not much of anything. These worm-riders are exclusively female, as male galeommatid clams are dwarfs who live parasitically inside the bodies of their mates2.
And Montacutona sigalionidcola is hardly the weirdest of its family. There are dozens of species riding all sorts of worms, each specialised for just a single host3. Some galeommatids get even more creative with their choice of real estate, hiding out in the shells of hermit crabs3, hanging in the throats of sea cucumbers3 or even sucking blood from the hearts of mole crabs4. Even the ones who choose to go it alone go all-in on weirdness, cloaking their thin shells in tentacled flesh to the point where you would be forgiven if you mistook them for snails and slugs. Why do they look like that? We don't really know.
And yet, despite all their weirdness, all galeommatids still make their living in the same way as 'normal' clams, filtering edible bits from the water. It seems that no matter how far you go, some things will never change.
References
1 Goto, R., & Tanaka, M. (2019). Worm-riding clam: Description of Montacutona sigalionidcola sp. nov. (Bivalvia: Heterodonta: Galeommatidae) from Japan and its phylogenetic position. Zootaxa, 4652(3), 473–486. https://doi.org/10.11646/zootaxa.4652.3.4
2 Mikkelsen, P. M., & Bieler, R. (1992). Biology and comparative anatomy of three new species of commensal Galeommatidae, with a possible case of mating behavior in bivalves. Malacologia, 34(1-2), 1-24.
3 Goto, R., Kawakita, A., Ishikawa, H., Hamamura, Y., & Kato, M. (2012). Molecular phylogeny of the bivalve superfamily Galeommatoidea (Heterodonta, Veneroida) reveals dynamic evolution of symbiotic lifestyle and interphylum host switching. BMC Evolutionary Biology, 12, 172. https://doi.org/10.1186/1471-2148-12-172
4 Bhaduri, R. N., Valentich-Scott, P., Hilgers, M. S., Singh, R., Hickman, M. E., & Lafferty, K. D. (2017). Facultative parasitism by the bivalve Kurtiella pedroana in the mole crab Emerita analoga. Journal of Parasitology, 103(6), 646–651. https://doi.org/10.1645/17-28
