Sea urchins belong to the phylum Echinodermata – (spine bearing) comprising a spherical or flattened test (body encasing) and covered with mobile spines all over. They are also called as ‘sea hedgehogs’. Sea urchins are close relatives of sand dollars and cake urchins. More than 900 species of sea urchins have been recorded in the world oceans. Sea urchins exhibit radial symmetry (Radial symmetry is the regular arrangement of body parts around a central axis. When an organism is radially symmetrical, it can be cut from one side of the organism through the center, to the other side, anywhere on the organism, and this cut would produce two equal halves) which is a characteristic feature for this exclusively marine phylum.
Regular sea urchin, Salmacis virgulata is a common species available in the Indian coastal waters. They are often wrongly identified as purple sea urchin (this is a different species – Strongylocentrotus purpuratus) because of the purple spine all over its body. In Greek mythology, Salmacis was a nymph who fell in love with Hermaphroditus and became united with him in one body. There is still a knowledge gap on why this regular sea urchin was named so.
The mouth of Salmacis virgulata is located on the underside possessing calcareous jaws called as Aristotle’s lantern (a complete chewing organ). This is a complex arrangement of muscles, calcareous teeth and plates forming an eversible organ, common in most echinoids, functioning in ‘mastication’ (biting and grinding food in mouth in order to make it soft enough to swallow). Recently scientists have revealed that the name Aristotle’s lantern was given for the entire animal and not just for the jaws.
Regular sea urchins move slowly, feeding mostly on algae with a centrally located mouth. They graze in aggregations along sea stars, predators and scavengers, consuming incidentally the left over. They have tube feet that arise from within the test, connected to a water vascular system, allowing the sea urchin to pump water in and out of the tube feet for locomotion. The gonads (eggs and roe) of sea urchins are considered as a delicacy in many countries.
The purple spines of Salmacis virgulata is sharp and vary in size from species to species. Some spines are also capable of injecting poison into the predator’s body as an action of defence. There are two types of spines namely primary and smaller secondary spines (this is banded). Sea urchins possess a peculiar behaviour of attaching materials like broken shells or sand particles in between the spines as an act of protection against sunlight.
Salmacis virgulata have been recorded from east Africa, India, China and South Japan, along the coast of Asia and Philippines. They prefer rocky substrata and reef areas as their habitats. Regular sea urchins can be collected from shallow waters to great depths.
Species: S. virgulata
Author:L. Agassiz in L. Agassiz & Desor, 1846
Special Thanks to Mr. NITHYANANDAN MANICKAM, Echinoderm Taxonomist, Kuwait, for identifying the specimen.
© Deepak Samuel July, 2013.