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The hidden biodiversity on Mumbai’s sea shores: Meet the city’s crabs, barnacles, limpets and more

The hidden biodiversity on Mumbai’s sea shores: Meet the city’s crabs, barnacles, limpets and more

For a city that is fixated with its housing, real estate opportunities and acquiring homes with a view, finding a space to call your own in Mumbai carries its own form of trauma. If the rents and EMIs don’t knock the life out of you, the negotiations on the transactions will. In this milieu, the citizens that have a prime piece of the pie are shore creatures – and they navigate it with the expertise and eloquence that we humans regularly deny our land spaces.

Having said that, the threat of Mumbai’s upcoming coastal road, which involves reclamation across multiple shores along the city’s coast, no longer looms in the distance but is terrifyingly close, stomping the intertidal zone, rock by rock. The consequence is an erasure of a major swathe of an ecosystem.

The piece of land that reveals itself only at low tide is known as the intertidal zone. It’s where animals that can survive only in the conditions that are unique to this space (ebb and flow of saline water, varying temperatures) live and grow.

It would be easy for us to dismiss the intertidal as an empty piece of land, one that is washed clean over every few hours by the waves. But we’d be wrong.

The intertidal zone is a neighbourhood that criss-crosses with many lives at street-corner hangouts, room-sharing, private condos. The ever-changing shore is a dynamic habitat, adaptive, resilient and vulnerable all at once.

Here, we meet a few of the shore homeowners. This is by no means a comprehensive list – far from it – thankfully, the more we walk the intertidal, the more life we find living in it. For now, these are just a few animals whose houses you can stop by for a chat.

Hermit crabs, at home in their shells

If you watch a hermit crab trudge across the intertidal, you know that housing is a weight it literally carries on its shoulders. While these are crustaceans, under the phylum Arthropoda, these aren’t true crabs (the ones with short abdomens and hard shells). Hermits have long abdomens and soft bodies that make them vulnerable but are still designed to twist and fit into shells.

Which is why they carry around shells discarded by other animals (like snails) – as their homes. Their bodies have adapted to this form of life – their eyes rest on long stalks so they can take in their surroundings, their feet and abdomen have appendages that fix into the shell they choose (so don’t try and pry them out of their shells, you might hurt them, or cost them a limb).

Increasingly, because of the way our shores are changing, hermits have been known to make do even with the trash we leave behind.

When hermits outgrow their shell, they find larger ones, and in some cases, fight other hermits over truly remarkable real estate. Relentless strugglers, like us humans, hermit crabs will always have a bigger, better real estate opportunity on the horizon.

Barnacles, the sticky squatters

If you’ve walked on a rocky shore at low tide, and had the unfortunate experience of slipping on the moss-covered rocks, chances are your bleeding hands or legs have met acorn barnacles and their sharp-as-knives shells.

These animals attach themselves to rocks on the shore and are seen in abundant numbers – from the highest hide-tide zones downwards to the surf zone. They grow on almost everything near the shore including submerged jetties, pillars, even living organisms like oysters, gastropods and other marine animals. Weirdly enough, these aren’t gastropods, but crustaceans (same family as crabs!), part of a class called Cirripedia (meaning “hairy foot”).

As larvae, these creatures swim around freely but later attach themselves head-first to a substrate, developing the conical shell-like structure around them, which remains closed in the absence of water.

They’re active when submerged in water, so at low tide, one can see them open to feed inside tide pools. Specially-paired appendages called cirri reach out through the ‘mouth slit’ and the animal uses these to sieve food particles from water. There are 900 species of barnacles, including the goose barnacle that can be seen on Mumbai’s shores (called so because of the white colour and the shape of the covering).

These barnacles have come in from things floating in from the ocean to the shore and this can include driftwood to manmade cast-offs. In the deep ocean, you’ll see barnacles growing on whales and ships. They’re unpopular with sailors as they grow on the bottom of ships and slow down the speed, thus increasing fuel consumption. They also do the same to other animals in the water, like turtles and whales and on the shore, wreak significant havoc on piers, jetties, pipes, cables.

Cratena, the art of war

Certain sea slugs, like some species of Cratena, live on their food. This gorgeous white and orange little animal lives on creatures called hydroids.

Hydroids are often mistaken for plants due to their appearance – the ‘stalk’ is attached to the substrate and the rest of the animal seems to branch out. Part of the phylum Cnidaria (which also hosts jellyfish, sea anemone, corals), hydroids belong to the class Hydrozoa, which means ‘water animals’ in Greek. They possess polyps – individual sessile animals, with tentacles, mouth and a gut. They are colonial animals and like most cnidarians, possess stinging cells called nematocysts.

This doesn’t really worry the Cratena, in fact it uses its home in many different ways: it lays eggs on hydroids, feeds on the polyps and even stores its stinging cells to use as a defense against a predator. The cells are stored in tiny sacs on the tips of their cerata (outgrowths that appear like flames on the sea slug) and are discharged when the animal is attacked. According to a study, some species of sea slugs prefer to eat hydroids that have also just had their fill.

Study author Trevor Willis speculates that it’s a way for the animals to get calories from plankton while also not overeating hydroids, which they depend on for shelter, in addition to nourishment.

Limpets, the ones that come back home

The limpets are almost invisible in the tide pool neighbourhood. Most of us move past them to find more charismatic animals – not even noticing these little umbrella-like shells on the rocky substrate on which they live and eat.

Limpets are captivating creatures for many reasons, the primary one being how they feed. Like most gastropods (the group containing snails and their relatives), they scrape off and feed on algae and small animals that live on the rocks, with a bristly tongue called a radula — which has tiny teeth on it. According to a study, “the tensile strength of limpet teeth can reach values higher than spider silk, considered currently to be the strongest biological material, and only comparable to the strongest commercial carbon fibres.”

The strength in their tongue teeth helps avoid any damage to the limpet despite the abrasiveness of the rocky substrate. Limpets are also known to maintain the balance of algae and other organisms in their neighbourhood.

Certain species of limpets have strong homing instincts and will come back ‘home’ after grazing. They have a broad foot that allows them to move across the rocks and also clamps down when they’re stationary. They create mucus trails and are known to follow them back to their rocky home. They pick rocks, which have slight indentations, which match the outline of their shells, then rub their shells against the rock, until they fit perfectly, creating what is known as a “home scar”.

Soldier crabs and beach condos

You’ve (almost) seen these on every stroll on the beach. The soldier crabs (genus Dotilla), called so because of the large numbers in which they troop in at low tide and dash into their underground burrows as soon as they sense human footsteps on the soft sand.

This crab is made for hard work on the intertidal. It has elongated pincers that face each other to facilitate its meals. It gobbles up sand, sifts it for the thin coating of detritus (organic matter), and leaves behind the pellets in circular patterns resembling what could be described as crustacean art.

In terms of real estate, the soldier crab is a bit of a hustler. It has two kinds of homes – a vertical underground burrow that it lives in, and a temporary air chamber appropriately called an igloo. Why? This study found the need for a temporary structure. While building the igloo, the crab rotates like a corkscrew in wet sand, making circular walls and a roof out of sand pellets, resulting in an igloo that doubles as an air chamber. The crab can then burrow deeper down into the sand using this same action, away from the incoming tide and predators it may bring (the same function that its burrow performs when the sand is tough and dry), while still having access to a quicker vertical movement.

Cone snails, built for the kill

A formidable, largely nocturnal predator, the cone snail spends most of the day at home – buried in the sand, under rocks or coral (that is not to say you might not find the occasional cone snail out in the day). There are many different types of cone snails. Some live in the shallows, while others survive at depths of several hundred feet.

This animal is built for the kill. It smells its prey with its nose, the ‘osphradium’. A highly venomous creature, the cone snail hunts by harpooning venom into its prey with its proboscis (snout), paralysing it before it reels the prey in and eats it.

The venom is contained in tiny darts, 9 to 10 mm long, derived from the tongue or radula. Unlike other gastropods, the radula teeth are shaped like barbed harpoons. B.F. Chhapgar describes it best in his book Marine Life in India: “These teeth are manufactured in an L-shaped bag called the radular sac. When fully formed, they shift from the long arm of the sac to the short arm, which opens to the throat of the snail. By the time the dart reaches there, it is filled with venom.”

The tooth itself is replaced if it fails to hit the target. It pretty much grows its own bullets for an ever-replenishing arsenal.

Some cone snails eat up most of their neighbourhood without being picky – molluscs, worms, fish, the whole lot. While all cone snails are venomous, only the fish-eating ones are deadly to humans. However, since there’s no way of telling, it’s a good plan to keep away from the lot.

Snapping shrimps, neighbourhood loudmouths

The pistol shrimp (Family Alpheidae) or snapping shrimp live in tide pools, in crevices or under rocks. They are tremendous burrowers and create tunnels at great speeds.

These creatures are the noisemakers of the street – the token loud family that every neighbourhood has.

The pistol shrimp has one larger claw. According to B.F. Chhapgar’s book Marine Life in India, “A more careful examination shows a knob on the finger and a corresponding socket in the thumb. There are two small patches, one at the base of the finger and the other at its joint with the palm of the claw. When the finger is opened wide, these two smooth patches act as suckers and stick to each other. The animal has to exert great force to overcome this suction.”

This force is the snap we hear, which causes the animal to release a high-speed water jet that travels at around 100km/ hour, creating a ‘cavitation bubble’. This bubble collapses with heat and pressure, knocking out or stunning prey.

In some parts of the world, burrowing shrimps have been found living in a mutually beneficial relationship with certain kinds of goby fish. A study examined the association from the Indo-Pacific region: “The shrimp maintains a burrow while the goby stands watch, retreating quickly into the burrow when danger approaches and thereby warning the shrimp, which has poor vision compared to the goby.”

While this roommate behaviour hasn’t been officially documented from the shores of Mumbai, it is something worth researching to document.

The Blenny fish colonisers

These remarkable fish are adapting to life on the intertidal is a way that earns them a special mention in this list. It has been a theory for scientists that blennies leaped out of the water to avoid being eaten by predators. But incredibly, a study shows that blennies have increased their intertidal activity to remain above predators that come in with the high tide.

According to this piece: “It turns out the aquatic environment is a nasty place for blennies, full of enemies wanting to eat these small fish. But life is less hostile on the rocks, with birds their main worry,” said Terry Ord, the study’s lead author and evolutionary ecologist from the University of New South Wales, in Sydney, Australia.

Ord doesn’t think this is just a survival technique, like mountain lions and bears scrambling up trees when they sense danger. He thinks that blennies are “in the process of moving out of the sea and colonising land on a more permanent basis.”

To seal their ideas of colonisation, research also shows that blennies are highly territorial and sometimes remain holed up in crevices above water at low tide to guard their territories. They have adapted to getting oxygen from air using their oesophagus.

It’s true; Mumbaikars will really do anything for new housing.


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