Within the marine ecosystem there are well known observations of worms (often sipunculans) using small corals as protection from predators on the sea floor and the coral gaining benefits from the worm as the worm prevents the coral from being suffocated by sediment creating an obligate-symbiotic relationship (meaning that one organism cannot live without the other) offering each other protection and transport.

Over the last year a strange but extraordinary discovery was made in South Japan in regards to the world of crustaceans, a new species of Hermit crab (Diogenes heteropsammicola) has been observed using small corals as a home instead of their more common gastropod (snail) shells, which is thought to be due to the same reasons the worms previously mentioned had a relationship with the coral.

The scientific journal that documented this discovery was published in PLOS ONE on September 20th by Momoko Igawa and Makoto Kato from Kyoto university. The journal outlines and observes the new discovery and explains the exact science as to why the hermit crab might adopt this new form of accommodation.

Hermit crabs/Sipunculans and the walking coral :

The Hermit crab itself is only small at only a few millimeters in size and is colored with bright red legs and bleach white claws but despite its small size it is capable of carrying corals far larger and heavier than itself of which there two different species (Heterocyathus alternatus and Heteropsammia cochlea) which are both solitary non-reef forming species. It is shown in the previously mentioned literature that it is thought that the coral itself settles on the shells of the sipunculan worms initially where the coiling of the worm creates the cavity within the corals calcium structure which the hermit crab is then able to occupy, this was thought to be because there was no distinct morphological difference between the corals that were occupied by the sipunculan worms or those occupied by the hermit crabs, this was outlined by Momoko Igawa during observations made at the Amami and Okinawa Islands off southern Japan.

The hermit crab Diogenes heteropsammicola (Igawa, Kato)

Benefits of the coral to the hermit crab/worm:

Usually throughout a hermits crab life cycle it will continuously grow and outgrow gastropod shell and have to constantly find and switch to new shells to accommodate their larger size, but this is not the case with Diogenes heteropsammicola as symbiotic coral grows along with the hermit crab and allows it to have the one home for the entirety of its life, along with this benefit the hermit crab has the added benefit of extra protection from predators such as crabs and octopus due to the nematocysts (stinging cells) contained within the corals tentacles, it is known that the even though the worm does not need a hard gastropod shell like the majority of hermit crabs,  it still benefits greatly from the added protection from the coral.

Heteropsammia cochlea, with expanded tentacles, at Watson’s Bay, Lizard Island (Anne Hoggett)

Benefits to the coral?

Although it may seem that the majority of the benefits fall on the side of the hermit crab the coral does gain its own share of advantages as the symbiotic “relationship status” would suggest. the main benefit presented to the coral is from its transportation to new “the grass is greener” areas where it is able to feed on fresh supplies of food, another advantage gained by supplying the hermit crab with a home is that in general most coral species cannot tolerate sediment build up to a high level so being topside down in the sand wouldn’t place the coral in the best of positions, enter the heroic hermit crab or worm which can  flip the coral back to its right position which the stops the coral from suffocating.

The differences:

As I mentioned before both the new species of hermit crabs and the sipunculan worms have inhabited the two solitary coral species but what are the key differences?

Momoko Igawa’s and Makoto Kato’s work seems to suggest that the differences of the two key differences are the feeding mechanisms that the crab and the worm adopt as well as the differing habitat in which they occupy.

The Hermit crab is a suspension/filter feeder that inhabits sandy or gravelly sea bottoms whereas the sipunculan worms are detritus feeders living in more muddy silty environments suggesting that the micro habitat may differ, this may mean that the adaptation to use the coral as protection may not be due to an environmental pressures but leads more to a response to pressure from predators.

This fantastic discovery that has been made extremely recently just goes to show how much we still have to learn about our wonderful oceans and the amazing creatures that call it home.

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