Watch out for scrambling shells

Geetha Iyer

If you happen to be spending an evening near the ocean, close to estuaries or mud flats, or if you are in one of the islands that are such favourite tourist destinations, look out for shells that seem to be walking or scrambling about in the sands in shallow waters. This amusing spectacle will be that of the hermit crabs going about their life! They carry their ‘home’ with them, a shell into which they can quickly withdraw when in danger.

Not true crabs
The hermit crab is a crustacean like the true crabs but is only distantly related to them. True crabs have a thick calcified carapace to cover and protect their bodies. Hermit crabs lack this covering over their entire abdomen. So to protect their tender abdomen which holds all the organs necessary for their survival, they seek refuge inside the empty shells of Gastropods¹. Although they live inside these gastropod shells they are neither related to Mollusca nor do they make these shells.

Hermit crab diversity
Worldwide there are close to 1100 species of hermit crabs. They come in different sizes from about a quarter of an inch to the size of a soft ball; but the largest among them is the coconut crab, which also holds the distinction of being the largest terrestrial invertebrate. They also vary in colour, from red to brown to purple with dots, stripes, and patterns over their body. Some are terrestrial while more numbers of them are found in ocean waters, in the intertidal regions. The terrestrial ones will not survive long if left in water, just as the marine ones will die out of water. Whether terrestrial or aquatic they need water to breed. So the female terrestrial hermit crab moves into water to lay her eggs.

It’s called a hermit crab because of this habit of carrying its home on its back, in which it resides. Despite its name, the hermit crab is not a hermit but a very social creature. They can be seen climbing over one another, resting together in piles. Studies have shown that there exists a network among them for the most important job of their life, which is finding new shells as they grow and moult². They will resort to even stealing, a not-so-hermit behaviour, when it comes to finding new homes.

Behaviour and biology
The hermit crab’s body is asymmetrically structured to live inside the gastropod shell. The abdomen is curved to the right to fit into the curved space of the shell. They have 10 jointed legs and the legs on the left side are larger than the ones on the right. The chelipads or pincers are the front two legs with grasping claws. To accommodate its body and 10 legs into the shell, the right chelipad is smaller in size, while the left is large and seals the entrance to the shell. The second and third pair of legs are used for walking. Locomotion in the hermit crab makes for an interesting study. While a true crab uses all eight legs and move sideways, the hermit crab uses only six and walks forward.

There are two small appendages at the tip of the abdomen. These help the animals to attach firmly to the shell.

Hermit crabs are very active at night feeding, searching for new shells, etc. They are omnivores while many are scavengers as well. They also feed on worms, planktons and organic debris.

Finding new shells
One of the hermit crabs major activities during its entire life is looking for shells to live in since it is essential for their growth, protection, reproduction, and survival. Like other arthropods, the hermit crabs too moult. Just before moulting they get out of their shell, dig themselves deep into the soil or the sand in the intertidal area and remain there till they regain their full muscular activity post moulting. They then need a new shell and start searching for it.

Hermit crabs look for new shells for a variety of reasons – because of a broken shell, or because the shell becomes a tight squeeze due to growth, foul smells, undesirable chemicals, or simply because they want a more spacious home. It seems there are shell market places for these creatures which are mostly sites where snails die. In the search for a new shell, might is often right, stealing is permissible and occasional swapping are all part and parcel of their lifestyle. Sometimes live snails are predated upon and removed for occupation. When snails die, there is a competition for occupancy,as demand outstrips supply. Therefore there is a constant monitoring of snail supply sites. When an empty shell is discovered, the first feature scrutinized is the opening of the shell; the large claw is inserted inside to inspect the shell; if suitable then the shell is turned over to remove debris if any, before occupying the new home. The empty shell that it leaves may be then taken up by another hermit crab.

Coenobita violascens – Terrestrial hermit crab This violet to blue coloured terrestrial hermit crab is a favourite as a pet and is commonly called Viola, Sunset Viola, or Chocolate Sunset Viola. When young they are red, orange, or sometimes brown in colour with cream colour on the legs and claws. As they grow older these colours fade and they slowly become the blue and violet colour for which they get their name. They are found along the entire Asian coastline from Bangladesh to China, from the southernmost islands of Japan down to Indonesia and Andaman and Nicobar Islands. Although hermit crabs are omnivorous, Violas prefer meats and seafood to plant material.

Hermit crabs breathe through gills, but this speciesis modified for breathing air. Additionally, they have two more organs, which can be loosely termed as lungs, to aid in breathing. However, moisture is essential for them to breathe. They may be found occupying a wide variety of shells, any type of shell they can fit in. Thus you can see them inside whelks, all types of babylonia, turbos, murex, frog shells, and bursa. Cone shaped shells like turritella are preferred by small violas.

Birgus latro – Coconut crab
Also known as robber crab, this is the largest terrestrial invertebrate and in India can be found only on the islands of Andaman and Nicobar. It’s closely related to Coenobita but does not live inside a shell. This crab can climb coconut trees, cut and drop down the coconuts to eat. It climbs down, de-husks the fruits to feed on the soft inner part. The fibres from the husk are sometimes used to line its burrows. But coconut is not their staple diet; they feed on a wide variety of nuts, fruits, soft parts of stems of fallen trees, carrion, and other organic matter. Like other hermit crabs they must return to the sea to breed. The juvenile crabs will live inside shells but as they grow into adults their exoskeleton hardens and they abandon the shell. This species of crabs are hunted by humans.Large scale habitat destruction especially after the tsunami has severely affected the population of this crab on the island and efforts are afoot to save the species.

References
1. ‘Nature by the Ocean’, Geetha Iyer; Teacher Plus; April 2012; pp 48-52.
2. http://www.scientificamerican.com/article/vacancy-hermitcrab-social-networks/

The author is a consultant for science and environment education. She can be reached at scopsowl@gmail.com.