Left: Diogenes heteropsammicola and its coral house, Right: the hermit crab without its coral house. Image: Momoko Igawa
The life of a hermit crab is one of repetition. Find an abandoned snail shell. Live in it. Nom on some flecks of detritus. Grow bigger. Find a slightly bigger shell. Repeat all steps for the rest of your crustacean life. The most onerous part is continually upgrading the shell, a process that can get pretty intensely competitive with other crabs around. However, a newly-discovered species of hermit crab avoids the shell renting game altogether, opting to reside in a living coral, one that grows alongside the crab, meaning no more relocating once the square footage gets a bit tight.
Meet Diogenes heteropsammicola, discovered by scientists at Japan's Kyoto University and described in a paper published today in the journal PLOS ONE. The researchers first encountered the species during surveys of shallow water habitats in the Amami Islands, a subtropical island chain north of Okinawa. The tiny new hermit crab is certainly unusual in appearance — red with white claws, and longer, scrawnier limbs than what is typical for a hermit crab — but it's what the crab carries on its back that's so odd. D. heteropsammicola appears to only use living, growing, solitary coral as a house. While some hermit crabs are known to have houses that grow with their bodies (like these recently discovered deep-sea dudes that carry around a stretchy colony of anemones), this new Japanese species is the first to use a live coral — a behaviour that's extra surprising considering these particular corals are famous for supporting a completely different tenant.
The two types of solitary corals that the new hermit crab has been found within (Heteropsammia and Heterocyanthus) are normally occupied by a symbiotic marine worm (a "sipunculan"). The sipunculan and the coral have evolved a pretty mutually sweet deal. The worm lives in a cavity at the base of the coral, gaining shelter and protection from the coral's stinging tentacles. As the sipunculan pokes its head out and pulls forward to forage for food in the soft seafloor, it drags the coral along with it. The coral polyp gains transport across soft bottom habitat as a result. Because these corals seem to slowly slide along like sailing stones, they are known as "walking corals." The coral's worm buddy provides some premium plan service, continually making sure the coral doesn't get buried in the seafloor sediment.
It's an attractive enough arrangement that the hermit crab has apparently decided it wants in. D. heteropsammicola seems to have evolved to act as a complete replacement for the sipunculan in the worm-coral relationship. The crab has several physical features that suggest a specialised existence in these corals. Its spindly body and claws are well-suited for folding up in the narrow coral cavity. The new species also has a weird "telson," which is the segment of the abdomen furthest from the head. For most hermit crabs, the front end can be adorably twitchy, but the butt end is decidedly not cute, looking instead like a sad, soggy cheese doodle. This squishy, corkscrew telson typically twists right-handedly, having evolved to match the usual direction for the spiraling of snail shells. But in D. heteropsammicola, the butt is symmetrical, likely an adaptation to living in the corals, which have cavities that can twist in both and left and right directions.
The scientists also observed the behaviour of hermit crabs collected from a trawl and placed in an aquarium and found that like the sipunculan, the crabs are meticulous about brushing debris and sediment off their gracious host. So, not only can the hermit crab steal the worm's house, but it can take its job too.
This crab's sustainable, low-waste approach to real estate is unconventional among hermit crabs, and the coral bungalow is unprecedented, but the true biological marvel is its third wheel involvement with an established symbiotic relationship. This is an animal that has evolved to cash in by seamlessly stepping into a highly-specific ecological role held by a completely unrelated creature. When species get locked in a situation of evolved, mutual codependence, any shifting of responsibility to other species is rare, and was only known to occur between very closely-related species. Thanks to this discovery, we now know that's not always the case.