We are all familiar with hermit crabs occurring commonly along the shores of the UK. However, it is far less known that this group of crabs, that is rich in species in temperate (for example UK) to tropical waters only occurs as a few species in the deep ocean. But why is this? The answer still remains as unknown as the hidden lives of hermit crabs in the deep sea. During JC36, we’ve regularly come across a species of hermit crab of the genus Parapagurus, which was found from the continental shelf at 500m to abyssal 3600m water depths. Strikingly, when in shallow waters of the continental fringe of Whittard Canyon, many of the hermits where typically carrying a shell, whilst others appeared to have a shell and an anemone on top it; sometimes, the shell wasn’t visible at all given the size of the anemone being carried around. When going into deeper waters, only hermit crabs visibly carrying an anemone of the species Epizoanthus paguriphilus were found. An often-emphasised hypothesis for the absence of hermit crabs at greater depth is the lack or scarcity of shells that they need to protect their soft part of the body, the abdomen. It is not only that they need such a home, but also many different sizes of them in order to match their body size, as they grow bigger. No home = prey to others, a risk a hermit can’t afford to take!
Few species, such as our Parapagurus (see photos below) have overcome this problem (also often called “bottleneck” in ecology or evolution, when a constraint hinders adaptation/progression). Parapagurus has larvae floating in the water column; once they settle to the seafloor as juveniles, they grab an empty shell of adequate size to hide. Then they put an anemone on top, which yields extra protection. These two species have undergone a particular agreement (symbiosis) in which the anemone dissolves and replaces the shell and protects the hermit. As the crab grows, the anemone will adjust its shape to harbour the hermit. Because of this adaptation, Parapagurus is able to colonise depth without losing protection. It is still unknown whether larvae of this species only recruit to the seafloor in shallow waters and then make their way into the abyss as adults only. I personally believe this to be very likely, but much more research is needed.
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