Information on Sea Urchins

Sea urchins are spiny marine creatures. They are members of the phylum Echinodermata. This phylum includes sea stars, sea cucumbers, brittle stars and crinoids. The scientific classification of sea urchin is as follows:

Kingdom: Animalia
Phylum: Echinodermata
Subphylum: Echinozoa
Class: Echinoidea

The body wall of the sea urchin is a firm, globose shell which is made of fused skeletal plates. There are regularly arranged tubercles to which the movable spines are attached. Their skeletal plate is pierced by pores for tube feet. The mouth is placed centrally on the lower side of the body. In many species, the mouth is surrounded by a whorl of gills. Aristotle's lantern, a complex jaw and tooth apparatus, situated in the mouth is used for food fragmentation.

Sea urchins have long, sharp spines which is used for protection. In some species, these spines contain poison which is capable of causing injury. The Asteroidea, Echinoidea, and Holothuroidea classes can cause serious injuries with their venom. Sea urchins can cause injuries with their long or short specialized spines. A few also use pedicellaria, that is, the delicate organs which are equipped with pincer like jaws, to deliver the poison. In the United States, Echinoderm envenomations are not serious health issues. On the other hand, nonvenomous injuries are quite common in the American coastal areas.

Sea urchins have appeared on the earth around 500 million years ago. The male and female species of sea urchins is difficult to identify. Fertilization in sea urchins occur externally. The female releases millions of eggs and the male billions of sperms into the sea. The embryos of sea urchins are nearly transparent and ideal for observing the early stages of development.

When the egg of a sea urchin is fertilized by a sperm, the number of chromosomes are split into half, by the process of meiosis. With the formation of gametes, the chromosome number is restored. The following divisions that occur in the cell is carried out by mitosis. The cell number reaches around 64 ,after 2-3 hours and 6 cell divisions later. A swimming ciliated larva is formed, which hatches from the egg membrane. The second most important stage of cell migration, gastrulation, begins soon after. The descendants of the micromeres crawl into the hollow cavity inside the blastula and become mesenchyme cells. The cells on the sides of the embryo travel by forcing the ingression of the primitive gut called archenteron. The archenteron moves to the opposite side of the blastula, bends and contacts the side of the embryo. Here, the cell-cell interactions induce the formation of the mouth.

As the formation of gut is carried out, the mesenchyme cells move inside the blastocoel. They secrete the calcium-phosphatase skeleton of the sea urchin called the spicules. With the growth of the skeleton, the shape of the embryo takes the larval form, called the pluteus. The resulting organism is nothing like its parents, but it is a free-swimming pluteus larva which forms a part of the ocean plankton. With the growth in its size, it becomes ready to settle down on the ocean floor. When metamorphosis occurs, a small adult sea urchin grows on one side of the pluteus' gut. The tiny adult hatches and crawls out from the cast-away pluteus skeleton.

Sea urchins feed on plant and animal materials. A few species eat sand or mud, and digest the organic material present in it. Algae is a source of food for some species. Some also feed on kelp and other seaweeds.

The eggs and reproductive organs of sea urchins, known as 'roe' are the favorite gourmet dishes in Japan and Chile. In Japan, roe is served as 'sushi' or 'sashimi', with soy sauce and wasabi.

The next time when you walk down the beach, sea water kissing your feet and suddenly your toe is pierced by an ugly, spiny thing, do not be angry. It's just a sea urchin trying to protect itself from your menacing toe.