Sea urchins are nature’s aquatic marine version of hedgehogs, although there is no close evolutionary relationship between the two. They simply share a defensive strategy – an outer coat of sharp spines.
Belonging to a group of animals that have a spiny skin, sea urchins are but one of several variations on a theme. Other relatives include sea stars, sea cucumbers, sand dollars, and sea lilies, or crinoids. All species within this group lack any sort of an internal skeleton, but instead have hardened plates, or ossicles, embedded in the outer wall of their bodies. Urchins are the most extreme of these, having a shell, or test, composed of calcium carbonate enclosing the soft parts of their bodies, and to which each of the protective spines is attached. Each spine is connected to the test via a ball and socket joint at the base that allows them to move in various directions. It is this ability that leads to the touch tank interaction of getting a sea urchin “hug.” As a means of capturing material landing on its upper surfaces, sea urchins can swing their spines together to trap it. When a finger is gentle slid in between the spines, the urchin brings its spines together, seemingly hugging it.
Another characteristic shared among these animals with spiny skins is the presence of five-part symmetry. This five-part nature is most clearly seen among the sea stars, which clearly present five distinct arms. While obscured by the spines of the living sea urchin, upon death and the loss of the spines, the sea urchin’s test can clearly be seen to exhibit this five-part symmetry. Pores in the test are clearly arranged in five distinct sectors clearly placing sea urchins within this group.
Besides the movable spines, urchins also have suction cups and small pincer-like appendages that can reach out from among the spines to move food items from the upper to the lower surface of their body where their mouth is located. Being herbivores, urchins dine on algae, and prefer large algae like giant kelp. In a well-balanced kelp forest, urchins usually stay hidden among the rocks and eat kelp that has fallen to the bottom. Unfortunately, when urchin predators are eliminated, and kelp becomes less abundant, urchins emerge from their holes and began eating any kelp they can find, eventually creating what is known as an urchin barrens.
This photo was taken at Santa Cruz in the northern Channel Islands on the California coast with a Canon EF100 mm f/2.8 Macro USM lens on a Canon EOS 5D Mk III in an Ikelite underwater housing. Lighting was provided by an Ikelite DS161 strobe set to eTTL exposure mode. The exposure was set to 1/180 sec at f/11 and ISO 400.