PHYLUM ECHINODERMATA

The spiny skinned animals known as echinodermata are sea stars, serpent or brittle stars, urchins, cucumbers, and sea lilies. There are a little over 1000 species of sea stars. Rather than using the terms dorsal and ventral to describe locations of echinoderm features, the terms used are oral for the side with the mouth and aboral for the side opposite the mouth.

FORBES SEA STAR: Asterias Forbesi

Of the many varieties of stars, only one, the Forbes sea star is found locally. It normally has five arms, but may have fewer if it has been damaged or if it has separated an arm by itself (autotomy) in response to a predator or other danger.

Behavior:
Sea stars do not have an internal skeleton. Instead their arms contain calcareous plates called ossicles interconnected by cartilage. The joints between ossicles allow considerable flexibility so that the sea stars can bend and contort to conform to whatever substrate they are lying on. They use their tube feet to hold tightly. In the aquarium they appear to be motionless, but in the ocean they move about searching for prey. Occasionally one in the Touch Tank will cruise about, and it is surprising to watch how fast it is able to go. When a sea star moves, it stays attached to the bottom, crawling down and up rather than bridging a gap.

Respiration:
Sea stars breathe through skin gills, called papules, that ring the bases of the protective hard white spines on the upper surface of the body. Cilia around the openings of the gills bring fresh water and help oxygen and carbon dioxide to diffuse in and out. Respiration also takes place through the surfaces of the tube feet. Between the spines on the aboral surface are pedicellarias, projections that resemble tube feet. Pincers on the ends can remove debris and small animals or plants that might interfere with breathing. Cilia on the skin also keep water moving to keep debris, dirt and microscopic organisms off the surface. Irritating toxins can also be secreted to discourage visitors. [Invert a sea star on a hairy forearm for a minute or two while the tube feet are being extended on the oral surface and the pedicells will grasp the hairs].

Locomotion:
On the under or oral side of each arm is the ambulacral groove. It is not prominent when the animal is relaxed, but as the muscular plates protecting the groove are withdrawn sideways, the line becomes darker and the tube feet can protrude. Tube feet are hollow muscular elements inside the body connected to bulbous containers that hold sea water. When the containers, (ampullae) are squeezed, the tube feet extend. When the bulb at the end of a tube foot touches something hard, a muscle inside can be withdrawn to form a sucker or suction cup. Depending on the substrate, specialized cells called duocells on the tube feet secrete an underwater glue (and respectively a glue release) and with these suction cups the sea star can hold tightly enough to prevent being dislodged by wave action. Besides their holding action, the tube feet are for locomotion. Each tube foot can be moved independently by means of muscles on its outside surface. Coordinated movement and alternate suction and withdrawal of the tube feet allow the sea star to move slowly towards food or into safe resting places. The arm that points in the direction of movement is the head. If there is a sideways shift of direction, the newly pointing arm acts as the head. The orange spot (madreporite) on the upper surface of the sea star's central disk is part of the ambulacral system. It is a fine sieve or filter comprised of a porous calcareous plate that keeps solids out when more water is needed in the vascular system.

Food:
Sea stars move around to find food: fish, oysters, mussels, barnacles, clams, snails, worms, crustacea and carrion. They can take small bivalves inside through the mouth and spit out the empty shells. Barnacles can be eaten if the extended stomach is placed against the shell openings to pre-digest the flesh. When a sea star fastens to a bivalve it may, but does not have to, exert force to pull open the shells. The holding force (not the pulling force) available has been measured up to 1350 grams. Sometimes clams gape rapidly. In fact, gape is not always necessary as there may be small openings along the edges of the shells through which digestive juices or the stomach itself may be inserted. Even without force being applied, bivalves gape when the internal concentration of carbon dioxide gets too high. A sea star that has just eaten may have an extended central area, as if it had swallowed a marble.

Vision:
A pigmented mass of cells at the end of each arm is known as the eye. It is a light perceiving organ that does not make images. Small tentacles surrounding the eye mass, sensitive to chemicals (taste), to touch and to vibrations are used when the sea star is cruising in search of food. The more important sensory organs, however, are the tube feet. The papules may also be sensitive, particularly to chemicals.

Reproduction:
Sea stars may be male or female, but this is not detectable from outside. Rising spring temperatures seem to induce spawning accompanied by pheromone production that encourages others to join in the process. Females may produce 2,500,000 eggs during the season. Untold quantities of gametes (male and female components) are, therefore, expelled at the same time and the chances for fertilization in the open water column are increased. Fertile eggs that survive eventually develop into larvae and then into infant sea stars.

Enemies: Sea stars appear to have few enemies. A few fish can crunch the arms. Spider crabs are fond of sea stars, but usually must be satisfied with one arm because, when the sea star is seized, it can separate the arm that is being attacked (autotomy) and thereby escape.