Green Sea Urchin, Strongylocentrotus droebachiensis

The green sea urchin (Strongylocentrotus droebachiensis) is a species that can be found in northern waters in the Atlantic and Pacific Oceans. It can be found at depths of up to 3,770 feet in burrows dug using its spiny body. First, it will dig down to a desired layer of sediment, and then widen a den. Sometimes it will leave openings that are large enough for it to leave the den and return, but it is common for it to dig so deep that it becomes stuck. Its range extends to Puget Sound in Washington State, and area where waters contain a low amount of salinity, but this species is able to thrive in these conditions.

The green sea urchin can reach an average diameter of about 3.4 inches, a measurement which is taken by calculating the size of the test, or shell that is underneath the spines. Its body is mostly spherical, with the top and bottom sides being slightly flattened. It has five rows of tube feet, which enable it to move around, that extend from the anus to the mouth. Its spines are also used for movement, as well as defense, and are not thought to be toxic. These spines are attached to the test by ball and socket joints and are ridged on the outside. The outside of these ridges hold a pattern that resembles a fan and is made of calcium carbonate. The spines are able to regenerate if broken or removed.

Like the spines, the test is made of calcium carbonate and is comprised of twenty sections that form an exoskeleton. There are rows that the tube can extend through, and if any area of the shell is cracked or broken, it will slowly be reformed by more calcium carbonate until the fracture is gone.

The tube feet of the green sea urchin are used for movement, attachment, and for gathering food. The tube feet act as strong muscles that can stretch past the length of the spines, allowing it the maximum amount of movement in many directions. The tube feet are made of two parts that perform different tasks. The ampulla, which is hollow, raises the sea urchin while the podia are used for suction and stability.

Like other species within the Echinoidea family, the green sea urchin has three pedicellariae that can be found below the spines at the base of the body. These are pincher like in appearance and motion and can respond to outside stimuli. It was once thought that these jaws were a parasite or larvae that became attached to the sea urchins, but recent studies have shown that they are simply part of the animal. These teeth are not only used for consuming algae and other foods, but are also used for gathering seaweed to use as shade and possibly even defense. One of the pedicellariae on this sea urchin species is toxic and is used for poisoning fish, which are eaten as a supplement, or for defending against predators.

The internal systems of the green sea urchin include a water vascular system, which aids the species in movement. Water will travel through canals within the urchins body, reaching the podia that help stabilize it. The content of this water is similar to that of the ocean around it, but it also contains proteins and potassium ions, as well as wandering cells. This fluid is constantly being directed by the cilia that are located in the canal.

Both male and female green sea urchins hold five gonads located under the test, which can be found near the anus and are protected by plates. One of the plates, known as a madreporite, is open and allows water to move into the urchin. Eggs and sperm are released into the water column at the same time, typically in late spring, although it has not been confirmed that temperature patterns cause this to happen. Once the egg becomes fertilized, it turns into a gamete which undergoes mitosis several times until it is a larvae that is big enough to perform simple swimming motions. Once this complex action is complete, the larvae will attach itself to a level of sediment where it will be able to develop. The structure of the larvae changes to its appropriate form after this, allowing the sea urchin to grow at a normal rate for sea dwelling creatures.

This species of sea urchin is home to snail species that are classified within the Stiliferidae and Melanellidae families. These snails will lay their eggs among the sea urchins’ spines as a means of protection.

Image Caption: Strongylocentrotus droebachiensis. Credit: Hannah K R/Wikipedia