Phylum: Echinodermata Subphylum: Echinozoa Class: Echinoidea Morphology Sea urchins or urchins, archaically called sea hedgehogs, are small, spiny, globular animals that, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They have five-fold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive "tube feet". sea urchins are radially symmetrical, that is, built on a circular plan, rather than a bilateral one. Adaptions Sea urchins have several adaptations to help them survive. To protect themselves from predators, sea urchins will react immediately if something sharp touches their shell and they will point all of their spines towards the area being poked. They are also light-sensitive. This is why they are nocturnal.
Location and importance
Sea urchins can be found in all climates, from warm seas to polar oceans. Despite their presence in nearly all the marine ecosystems, most species are encountered on temperate and tropical coasts, between the surface and some tens of meters deep, close to photosynthetic food sources. Some common places they live are in rock pools and mud, on wave-exposed rocks, on coral reefs in kelp forests and in sea grass beds. Importance Ecological role of purple sea urchins. Sea urchins are major components of marine communities. Their grazing limits algal biomass, and they are preyed upon by many predators. Purple sea urchins (Strongylocentrotus purpuratus) are among the best studied species. They are actually used in many seafood recipes. Sea urchin is commonly found in sushi known as “uni”. It is also considered a delicacy in some countries, especially Japan. The primary urchin harvesting company in California sends 75% of the harvest to Japan. The market value for urchins in Japan ranges from $2.20 per tray to $43.00 per tray depending on the quality (taste, texture, freshness, age) of the urchin. In 1994, Japan imported 6, 130 metric tons of sea urchins at a total value of 251 million dollars. Sea urchin harvesting has become one of the highest valued fisheries in California, bringing 80 million dollars in export value per year. (Calvin et al., 1985; Sea Urchin Harvesters Association, 2000). In their feeding, sea urchins can destroy entire forests of kelp which are commercially and ecologically important for fisheries. They are even more important in that the blades of the kelp can be harvested for algin.
fun facts
Size of sea urchins depends on the species. They usually have 1.2 to 3.9 inches in diameter.
Sea urchins have globe-like shape of the body that is covered with large number of long spines. Bony plates form shell that provides protection for the soft inner parts.
Body of sea urchins has radial symmetry. That means that each sea urchin can be divided in five equal parts.
Color of sea urchins depends on the species. Majority of species are black, brown, purple, red or green in color.
Sea urchins have 5 rows of paired tube feet on the bottom side of the body. They end with suckers which facilitate adhesion to rocks, hunt and movement on the ocean floor.
Fun Vocab
symmetry:the quality of being made up of exactly similar parts facing each other or around an axis
nocturnal: done, occurring, or active at night.
facilitate: make (an action or process) easy or easier.
Uni:the Japanese name for the edible part of the Sea Urchin. While colloquially referred to as the roe (eggs), uni is actually the animal's gonads (which produce the milt or roe).
Gonads: an organ that produces gametes; a testis or ovary.