The star and the urchin

Tales from the tropical seas continue with the deceptively sedentary life forms from the ocean beds – the starfish, the sea urchins, the sea cucumbers, and other charismatic echinoderms.

Many who have travelled to India’s expansive coastline have chanced upon starfishes on the beach. Those of us who have had the opportunity to explore rocky intertidal areas or clear lagoons have squirmed at the large worm-like sea cucumbers in the sand. Since many of these animals lie seemingly motionless at the bottom, they are not as fascinating to us as the fish, eels, stingray, and other more charismatic or entertaining creatures of the Indian Ocean. And yet, they play a role that is of exceptional importance to the health of our coral reef and intertidal ecosystems.

 

Linckia laevigata, or the Blue Sea Star, is the most commonly observed member in the genus. It is found on shallow reefs and sand flats that experience some amount of wave action.

Sea cucumbers, starfish, feather stars, and sea urchins are Echinoderms. The term effectively translates to ‘spiny skin’ and is a reference to their tough, often spiny exteriors. These creatures are exclusively benthic ocean dwellers and are not found in freshwater. Echinoderms have a five-part radial symmetry with an internal skeleton made of calcium carbonate plates or ossicles. Their vascular system is a network of canals that radiate through the body and end in tube feet below the animal. Their body fluids circulate through this vascular system effectively providing them with hydraulic control over their tube feet. These flexible, cylindrical feet serve in locomotion, feeding, and even respiration.

 

Sand urchins (Genus Maretia), also commonly known as heart urchins, emerge at night to feed and return to the safety of the sand during the day

Since most people dive or snorkel during the day, sea cucumbers are most often seen as thick, worm-like animals, resting on the sand or moving slowly along the reef. Most starfish and feather stars too, seem to remain fixed to one place and sea urchins are tucked into crevices with only a few of their spines visible. But take a torch and visit the reef at night and the world of echinoderms comes alive! Delicate brittle stars of every colour and size materialise on the reef. Shrimp and crabs that live in symbiotic relationships with sea cucumbers and starfish emerge from beneath their hosts under the cover of darkness to feed and mate. Sea urchins appear from the coral crevices they hide in during the day. Feather stars sometimes swim past with their awkward Mexican-wave movement. And sea cucumbers with feeding arms that look like trees out of a fairytale can be seen half-buried in the sand, their branching arms catching plankton that is sucked off by a central mouth.

 

Godeffroy’s Sea Cucumber Worm (Euapta godeffroyi) can be seen here in the shallow tide-pools at Neil Island.

This bizarre world of creeping movement and deceptively sedentary lifestyles is not to be overlooked. Sea urchins and certain reef-dwelling sea cucumbers are major grazers on the reef and help to keep algae in check. Without these animals, the reef would be a significantly different space, with algae dominating the substrate and allowing very little room for coral growth. On the flip side, a large number of crown-of-thorns starfish can destroy a coral reef in a matter of days, and the Indian Ocean has been a witness to many of these ‘plagues’ over the last 30 years. Many sea cucumbers that feed on organic matter in the sediment ingest large quantities of sand in the process, passing this excreted sand back to the benthos. Like earthworms on the forest floor, they help to turn over reef sediment. Many of these animals have developed intimate relationships with fish, shrimp, crabs, and worms, playing host to an astonishing diversity of creatures.

 

The crown-of-thorns starfish (Acanthaster planci) is one of the most important echinoderms influencing the nature of reef communities in the Indian Ocean. Plagues of these starfish can destroy reef systems in a matter of days

 

This squat lobster has taken up residence at the foot of a feather star. A healthy feather star can play host to a number of crustacean symbionts.

 

This photo of the Echinothrix sea urchin clearly displays its 5-part radial symmetry

 

From the perspective of human use, a number of fisheries thrive on the collection of sea urchins and sea cucumbers. The gonads of some sea urchins and the dried body wall of certain types of sea cucumber are highly valued in food markets in parts of Southeast Asia. However, the unsustainable collection of these creatures has severely degraded the reefs off Sri Lanka and this trade is on the rise in the Andaman Islands and India. So the next time you are walking the shoreline or diving a shallow reef, stop and spend a few moments with a starfish, urchin, or sea cucumber. Marvel at some of their exceptional colours and patterns, and appreciate these creatures for their silent but crucial role in the ecosystem. And if you are lucky, you may get a glimpse of the Periclemens shrimp, commensal crab, or shy clingfish nestled in the folds or spines of these amazing echinoderm hosts.

 

This article was first published in the May 2015 edition of Saevus Magazine