Many soft-bodied creatures need some kind of close-fitting dwellings (home) to protect themselves. Some even lug their homes, wherever they go. While home-luggers such as snails develop shells that are permanently attached to their soft bodies, there are others like the larvae of caddisflies, bag worms and nymphs of certain species of predatory bugs, that construct makeshift homes to protect themselves.

Caddisfly larvae are aquatic. They build tiny homes for themselves using debris including particles of sand and other organic matter. Larvae of some caddisfly species drag their homes along as they forage. Bag worms, also known as case worms, are larvae of moths and are terrestrial. They can be commonly seen in our gardens. Different species of bag worms can be identified by the variations in the types of homes that they build and lug. Homes of nymphs of some predatory bugs, made with the remains of insects that they had fed on, thorns, bristles and other organic wastes, that they lug, make them appear like little temple chariots. The most interesting of the home-luggers are however hermit crabs.

Hermit crabs are crustaceans as are other species of crabs, prawns and lobsters, yet different in a number of ways. They have soft abdomens that they need to protect. For this, hermit crabs have to occupy the empty shells of dead marine gastropods (snails) or less commonly, something else that is hollow within and light enough for them to lug. And as they grow bigger, hermit crabs periodically discard one home before occupying a larger one. They may have to do this throughout their life which may be anywhere between 15 and 45 years depending on the species.

Various suggest that more than 800 species of hermit crabs are found in the world. Majority of the species live underwater in the shallow seas. However, hermit crabs in the family Coenobitidae are primarily beach dwellers. They enter the sea only to breed.

The largest species of beach hermit crabs is the coconut or robber crab. This large hermit crab can weigh up to four kilograms and may have a span of around one metre when the legs are fully stretched. Coconut crabs are found in many oceanic islands that lie within the Indo-Pacific region. In India, it is best-known in the Great Nicobar Island. Coconut crabs spend their early lives much like other hermit crabs occupying and lugging empty gastropod shells. But, as they grow older, they discard the shell and become free-living.

Hermit crabs and their little-known habits

Beach hermit crabs are scavengers and play important roles in environmental hygiene. They are nocturnal and spend most of the day resting inside their home-shells. In exceptional cases, as that in the Andaman and Nicobar Islands, beach hermit crabs enter dense littoral forests where they can be seen foraging even during the daytime.

Although beach hermit crabs tend to wander in search of food and to satisfy their other biological needs, being slow movers, they do not cover a large distance. Unlike other crabs that walk sideways (cancerous walk), hermit crabs walk forward and straight. Many individuals may sometimes come together and form clusters as they rest in secluded locations, or when attracted by a carcass washed on to the beach.

Discarding a gastropod shell and moving into another is a ceremonious affair when more than one beach hermit crab can be seen doing it simultaneously. Since the new home has to allow for further growth and increase in body size, hermit crabs tend to prefer over-sized gastropod shells, some of which can be ridiculously large when compared with the size of the owner. Therefore, when a tiny beach hermit crab withdraws itself fully inside, it is impossible to tell if a gastropod shell lying in the beach is occupied or not. While this habit may help to protect the beach hermit crab from predators and the heat radiating from the sand, there are unforeseen risks too.

Humans and beach hermit crabs

Beach hermit crabs do not seem to have a preference for any specific type of gastropod shells. It is more likely that they look for shells that fit them best and may even prefer those that have been already used. An already used gastropod shell is smoother inside, more worn out and does not hurt the tender abdomen of the hermit crabs. As a result, it may be that almost all species of gastropod shells (except cones and others with limited space inside) found strewn in the beach can potentially become homes to hermit crabs. Locally along the Coromandel coast, between Chennai and Puducherry, nearly sixty per cent of the gastropod shells collected had live beach hermit crabs inside.

Beach-goers who pick gastropod shells do so without realising that they have been occupied by hermit crabs. It is almost impossible to find out if there is one inside. The presence of a live hermit crab inside a gastropod shell is often noticed only when the ‘dead’ shells start moving on their own. Unfortunately, this happens many hours later and when the shell-collector is quite far from the beach making it difficult to return the hermit crabs to their habitats.

Moreover, dearth of gastropod shells is driving beach hermit crabs, worldwide, into occupying plastic containers and other discarded synthetic materials including tubes and small glass bottles which makes them vulnerable to predation and trampling. Beach cleaners are bound to remove these ‘artificial shells’ and discard them far away from the hermit crabs’ habitats. It is therefore likely that humans have unwittingly been responsible for the apparent local absence of beach hermit crabs.

Conservation of beach hermit crabs

There are many other indirect threats that beach hermit crabs face. Plastic pollution, shoreline fortification and oil spills can be potentially harmful to these lowly and often inconspicuous creatures. As many are not aware of these tiny home-luggers, there is a greater need for creating awareness especially among beach-goers. Wherever beach hermit crabs are still found it would be better if some boards or panels are placed with appropriate illustrations, texts and signs requesting people not to collect gastropod shells. Beach-cleaners (both volunteers and professional alike) should be trained to selectively pick plastic and other unnatural material lying in the beaches rather than glean or use machinery that removes whatever lies in the sand including live hermit crabs and their potential future homes.

Diversity of marine gastropod species along the Coromandel coast can be surprisingly high. During a post-tsunami survey of marine gastropods discarded as bycatch in a fishing harbor in Chennai, researchers of Care Earth reported at least seventy species from a sample of close to 900 shells. These included many species that structurally resembled conches with plenty of hollow space within. Shells belonging to around 11% of the species were occupied by sea-dwelling hermit crabs. Interestingly, of the shells occupied by the sea-dwelling hermit crabs, more than 40% belonged to the genus Babylonia. It is therefore entirely possible that shells of such species, when washed ashore, can become homes of beach hermit crabs too.

In India, just 23 species of marine gastropods, find a place in the Wildlife (Protection) Act 1972 (as amended in 2022). Of these eight are listed under Schedule I and the rest are in Schedule II. Also, the only species of protected hermit crab is the coconut crab (Birgus latro). It finds a place in Schedule I of the Act. Needless to reiterate the fact that the coconut crab too requires gastropod shells to stay safe during the early parts of its life history. Tiny beach hermit crabs found in the Great Nicobar Island and elsewhere in the Indo-Pacific region may well belong to this species.

In the absence of legal protection and public awareness, beach hermit crabs can silently disappear due to inadvertent gastropod shell collection and beach restoration activities. Unlike other beach crabs such as ghost crabs and mole crabs, beach hermit crabs remain fully exposed. They can neither sprint and disappear into holes like ghost crabs nor burrow deep down like mole crabs. It is therefore important to highlight the inherent vulnerability of beach hermit crabs and spread conservation awareness more widely among people especially at a time when beach-tourism is being aggressively promoted in the country.

R.J. Ranjit Daniels is an ecologist with the Care Earth Trust, a Chennai-based biodiversity and conservation organisation.

Read more: Horseshoe crab decline sparks urgent conservation plea

CITATION:

Daniels, R.J.R. (2007) Gastropod Diversity in the Chennai Coast. In: Editor-Director, National Symposium on Conservation and Valuation of Marine Biodiversity, Zoological Survey of India, Kolkata, pp379-386.

Anonymous (2023) The Wildlife (Protection) Act, 1972 as amended by the Wildlife (Protection) Amendment Act, 2022. LexisNexis, New Delhi.

Banner image: Coenobitidae hermit crabs are largely beach dwellers that enter the sea only to breed. Image by Hartmann Linge via Wikimedia Commons (CC BY-SA 3.0).

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Aditi Tandon Senior Production Editor

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