A newly discovered hermit crab species can live inside the cavity of a walking coral, exhibiting a symbiotic relationship.
Diogenes heteropsammicola with and without its coral house [Photos by Momoko Igawa]
Most hermit crabs live like kids newly out of college. Rent an apartment, move out at the end of the lease, look for newer, hopefully better accommodations because you can now hopefully afford it, rinse and repeat. Hermit crabs do something like this too. They find an empty snail shell, live in it and eat to their heart's content, grow bigger, then look for a somewhat larger shell. The crabs rinse and repeat as well, albeit until the end of their lives.
However, one species can forgo this entire process, which can get competitive. Instead of a shell, the new species Diogenes heteropsammicola lives inside a living walking coral that grows as the crab grows. It's like owning a small-ish starter home and having the resources to add extensions every few years.
The walking coral Heteropsammia cochlea [Photo by Anne Hoggett, CC BY-SA]
This strange new crab takes up residence inside two types of coral: the Heteropsammia and the Heterocyanthus. Usually, a marine worm called a spinculan lives in these corals, forming a symbiotic relationship. The spinculan gains shelter by living in a cavity at the base of the coral. The coral also has a sting, which protects its partner symbiont from predators. As the spinculan crawls along the ocean floor, it takes the coral along with it.
So how does this benefit the coral? The coral is something called a “solitary coral”, which means it doesn't live on a reef. They live on sandy seabeds in shallow waters, putting them at risk of getting buried under the sand or getting overturned by currents. The coral can't walk away from situations like this—at least, not by itself.
Thus, it's strange that a hermit crab species has been found to inhabit these corals as well. Researchers found the crab along the coast of the Amami Islands, a group of islands scattered from Japan to Taiwan. It's possible that the crab has completely replaced the spinculan in the symbiotic relationship with the walking coral. If this is the case, then how did the hermit crab evict the spinculan?
Photo via Wikimedia Commons
It's possible that the crab has evolved to live inside the walking coral, completely replacing the spinculan. Tellingly, the crab has physical features that seem to indicate that the crab is meant to live inside the coral. It has a spindly body and claws that fit right into the cavity. Researchers have also found that the crab is quite an excellent housekeeper, brushing debris off the coral they live in. The spinculan did this as well, so it seems that the coral is not missing out by taking on a new tenant.
Perhaps one of the most remarkable things about this new hermit crab species is that it was the third wheel in a symbiotic relationship. Usually, organisms in a relationship like this don't switch partners unless the new partner is from a closely related species. Thus, the relationship between the crab and the coral is quite rare.
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