Marine Life


Echinoderms as larvae are bilaterally symmetrical. As they mature, they develop a pentagonal symmetry.  They are only found in the sea (there are none on land or in fresh water). Echinoderms mean "spiny skin" in Greek, but not all, echinoderms have spiny skin. There are over 6,000 species that can be organized in three groups:

Sea Stars: (group name Stelleroidea) or Starfish, divided in two sub-types:

  • Asteroideas are the true sea stars and sun stars.

  • Ophiuroideas are brittle stars and basket stars.

The differences between the two sub-types lies in how the arms connect to the central disk. Ophiuroideas (like OPHIODERMA LONGICAUDUM) have arms that do not connect with each other. There is a distinct boundary between arm and central disk. Asteroideas have arms that are connected to each other. Also, it is harder to tell with asteroids where the central disk ends and the arms begin.

The Sea Star have dermal branchiae on their skin used to absorb oxygen as well as Pedicellaria pincher-like organs used to clean the surface of the skin. Barnacle larvae could land on a sea star and start growing if it were not for these organs. Each ray of a sea star has a light sensitive organ called an eyespot. Though it can not see nearly as well as we do, sea stars can detect light and its general direction.


Photo by Publio Attard - Subway Dive Centre

Given enough time, sea stars can grow back arms that have been damaged or removed. For a few species, the severed arm can grow back into a complete sea star! For most sea stars, however, a severed limb dies. Sea stars eat many things.

A sea star's diet can include: barnacles, snails, sea urchins, clams, and mussels. A few species, such as the spiny star of the North Atlantic, eat other sea stars! They surround the shell and use the suckers on their feet to pull the two shells (or valves) apart. This creates an opening between the two shells that is only .01 inches wide. Using this tiny gap, the sea star puts its stomach into the clam's shell and eats its insides. When it is done, nothing is left but an empty shell.


Sea Urchins

Sea urchins, heart urchins, cake urchins, and sand dollars belong to a sub-group of echinoderms called enchinoidea. These creatures have many sharp spines pointing out in all directions that offer protection from predators.

The spines are able to swivel towards a predator because they are connected to muscles. These animals have five paired rows of tube feet with suckers. The mouth is on the bottom, and the anus on top. The mouth contains five teeth that point towards the center. The tubed feet are used to move along the bottom of the sea, where these organisms use their teeth to eat algae.


Photo by Publio Attard - Subway Dive Centre


Sea Cucumbers

Sea Cucumbers belong to a sub-category of echinoderms called Holothuroidea. Some members of the group look like cucumbers you find in the supermarket. The similarity ends there. Sea cucumbers lay on their side at the bottom of the ocean. They have five rows of tube feet running lengthwise. Its mouth is surrounded by tentacles. Unlike sea stars, the vascular system is not filled with sea water. Instead, sea cucumbers use a special body fluid.


Photo by Publio Attard - Subway Dive Centre

Sea cucumbers eat plankton and other organic matter. Some position themselves in a current that brings a steady supply of food, and spend hours there. The tentacles open up and collect food in the current. The sea cucumber brings each tentacle to its mouth to eat. Other sea cucumbers feed by sifting through sand. Many sea cucumbers are poisonous (though not for humans). If injured, sea cucumbers can kill the fish in the same aquarium as them. The sea cucumber has can expel all of its internal organs! This either scares off or satisfies predators. The sea cucumber can then grow another set of internal organs. Some other sea cucumbers secrete a verysticky substance as a defense mechanism. If you get this glue on your body, you will not be able to remove it without shaving your skin!

Divesubway.com 2007