A Kid’s Best Friend; the Hermit Crab
For many young Americans, their first encounter with marine life is with the hermit crab. Along the shores of the northern Gulf of Mexico, this animal is very common. It can be found day or night, high or low tide. Having a hard exterior shell, they are safe enough for kids to pick up and observe. Many are collected in beach buckets, or placed in make shift castles made from beach sand – with a bridge and moat of course – and the hermit crabs inside the palace reigning over all they can see. I am not sure how the hermit crabs feel about this, but the kids love it. And, usually… hopefully… they are returned to the bay when the day is over.
But who is this common crab from our shorelines?
Other than entertainment for children, what do we know about this interesting animal?
Well… we know they are crabs – which places them into the phylum Arthropoda and the subphylum Crustacea. Crustaceans have 10 legs, as opposed to eight for Arachnids and six for insects. They have an external skeleton – or shell – that covers their whole body and must be shed periodically to allow growth to occur. They have two sets of antenna – one pair is short, the other long – as opposed to one pair for the insects and no antenna for the arachnids. We know that crustaceans usually have a pair of chelipeds (claws) they use for manipulating food and defending against predators. And, all of this is true for our friend the hermit crab as well… but there are some differences.
First, they live in a mollusk shell. Most crustaceans do not do this – it is a little unusual. Why?
Well, as with all crustaceans, an external shell covers the body. However, the shell covering the abdomen (tail) of the hermit crab is very thin – too thin to protect it. So, it must do so by covering its abdomen with a mollusk shell.
How do they select their mollusk shell?
We actually played with this question when I was an instructor at Dauphin Island Sea Lab. We carefully removed the hermit crab from its shell (not easy to do) – placed its shell, along with many other empty ones at the far end of an aquarium – the shells were of different colors and shapes – and then released the hermit crab to see which it would select. The idea was to repeat this many times to see if there was a pattern. The crab would approach each shell and examine using their claws and antenna. More often than not – they would select their original shell. When we ran the test without placing their original shell in there – they generally selected the first one that fit – they did not seem to have a preference for shape or color. They just wanted a shell.
What about in the wild? How do they choose a shell?
Most research suggest they do not kill snails to get access to a shell – they search for empties. However, there is evidence they will fight with another hermit crab for one.
How do they hold on to the shell so they can carry it around?
Unlike their crustacean cousins, the abdomen is slightly curved to the right. Thus, they typically select what we call “right handed” shells. Right-handed shells are those whose aperture (opening) are on the right side of the columnella (spiral that the shell coils around). They extend their abdomen into the shell around the columnella and using the uropod (the last extension of the abdomen) they grab hold of the columnella – grab it very hard. The uropod also has small tubercles with which they can use to grab as well. When the shell gets to small, time to look for another.
Hermit crabs are basically scavengers, feeding on a variety of organic debris they find on the bottom. Their mollusk shell protects them from most predators, but they do have a few. The common octopus is one. These guys can easily pull a hermit from its shell. However, the hermit has an answer for this. Octopus do not like being stung by jellyfish any more than we do. Sea anemones are actually jellyfish. Instead of a bell with tentacles hanging down, they are stalks, with tentacles extending upwards – resembling a flower. Hermit crabs are known to grab sea anemones from rocks where they are attached and attach them to their shells – the octopus will leave them alone – I have seen one hermit with five anemones on its back, and I have seen them steal anemones from other hermit crabs.
Many times, we will find literally hundreds of hermit crabs up on the beach – out of the water – how long can they remain here? Do they need to be saved?
Gills function fine as long as they remain wet. Enclosed within this mollusk shell, they can hold seawater for quite some time – sometimes for days.
As far as needing to be saved, I am going to say no… they move onto the dry beach a lot. Many of them do this. They can easily crawl back if they need but do not. I cannot say why they do this but…
Another interesting question is how do they lay eggs living in a mollusk shell?
The males will provide their sperm to the female in a sac called a spermatophore. They female can store this spermatophore inside of her shell fertilizing her eggs when she feels is a good time. The fertilized eggs develop into a planktonic larva, which are released into the water.
There are times when we see hermit crabs everywhere, and times when there is none to be seen… do they migrate?
In a way, yes. They really like warm water. When the temperatures begin to drop, they move into deeper water. They have been found as deep as 72 feet. Salinity does not seem to be a problem for them. They are found in the Gulf, in the bays, and high up into bayous near where creeks enter.
Though there are different kinds of hermit crabs in our area, the Striped Hermit Crab (Clibanarius vittatus) is the most common. They are pretty neat animals, and as I have said – an exploring kids best friend.
Barnes, R.D. 1980. Invertebrate Zoology. Saunders College Press. Philadelphia PA. pp. 1089.
Clibanarius vittatus. 2010. http://www.sms.si.edu/IRLSpec/Cliban_vittat.htm.
Perry, H. 2004. The Striped Hermit Crab. University of Southern Mississippi.
Thinstriped Hermit Crab. Wikipedia. https://en.wikipedia.org/wiki/Thinstripe_hermit_crab.