Moreton Bay Community Seagrass Watch monitoring is well underway for the March-April survey period and some interesting results have already coming in.
One would often associate the word hermit with being a loner but the Hermit Crabs found amongst the seagrass watch site OR4 are anything but that. A quick calculation shows each quadrat averages 13 hermit crabs. Over the entire site (50m * 50m) this equates to 130,000 hermit crabs per site. That’s a sizeable party!
Hermit Crabs belong to a diverse group of crustaceans scientifically known as the Anomura but they are not true crabs. The name Anomura literally means “differently-tailed” and refers to the unusual abdomens that tend to be somewhat reduced in size, and flattened compared to those of shrimps and lobsters (QLD Muesum, 2013). These abdomens are often carried as a broad flap tucked under the body, but in contrast, the hermit crabs have large fleshy soft abdomens that they protect by hiding them in discarded snail shells. Anomurans are found in a wide variety of habitats from the shoreline to abyssal depth.
Hermit Crabs differ in that they only use two pairs of legs as their main mode of locomotion and the fifth leg is a lot smaller than the legs used for walking. Of course the most obvious difference is that they live almost their entire lives in gastropod shells. This is because hermit crabs have a soft, twisted abdomen which needs to be protected and a shell offers just such protection while the opening of the shell can be closed off and defended by the crabs strong claws.
The only time, other than at the larval stage, when hermit crabs are not in a shell is when they swap it for a new, larger one (as they grow and moult). This can cause much fighting in areas with high crab populations as they are very particular about choosing a suitable shell and there can be a strain on the resource.
Hermit crabs, like true crabs, have compound eyes on stalks allowing some species to detect movement as far away as 20-30 metres. They also have different length bristles and hairs all over their bodies and more densely on their legs which work as touch receptors. The short hairs send signals on water currents and the longer bristles bend to surface contact
They have long antennae which can detect chemicals in food to stimulate the crab to search for it. They feed mainly on algae but will also scavenge, while some species can be predatory and others use their antennae to filter feed.
Hermits brood a mass of eggs inside the shell, the size of the egg mass depends on the size of the crab. The egg hatches into a “zoea” which swims around in plankton. It goes through several moults, growing and developing until in it’s final larval stage when it settles to the seabed and transforms into a juvenile crab. From that stage it will continue to moult, grow and change shells throughout it’s life.
There are around 1400 species currently known in the world and with their variety of beautiful colours, patterns and choice of shell homes they add a lot of character to the marine environment as well as playing a major part in natures clean-up cycle.