The bright white and orange cratena on its host hydroid. Cratenas live on hydroids and feed on them too. Photo by Shaunak Modi.

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’.

Limpets
Limpets live on rocks in the tide pool and feed on algae that grow on it. Photo by Shaunak Modi.

Soldier crabs: an army of hustlers with 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.

Soldier-Crabs
Soldier crabs dash into their underground burrows in the sand as soon as they hear something approaching. They gobble up sand, sift it for organic matter and leave behind small pellets in circular patterns on the shores. Photo by Shaunak Modi.

Cone snails: the ones 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.

Cone-snails
Cone snails are venomous, nocturnal predators that spend their days buried under the sand or under rocks and corals. Photo by Abhishek Jamalabad.

Snapping shrimps: the 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.

A snapping shrimp creating a burrow with great speed. It lives in the crevices or under the rocks in a tide pool. Footage by Jessica Luis.

Blenny fish: the ones trying to colonise land

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.

The blenny fish stays amongst the rocks in the intertidal zone, sometimes even at low tide. It has also adapted to getting oxygen from air using its oesophagus. Photo by Shaunak Modi.

 

Banner image: Mumbai’s intertidal zone, an ecosystem under threat, inhabits many creatures with their own real estate stories similar or more exciting than those of the megapolis itself. Photo by Sejal Mehta.

Article published by Sandhya Sekar
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