ECHINOTHERMATA

The Biogeography of the (Dendraster excentricus)
by Karen Vitulano, student in Geography 316
Kingdom: Animalia Phylum: Echinodermata Class: Echinoidea Order: Clypeasteroida Family: Dendrasteridae Genus: Dendraster Species: Dendraster excentricus


Description of Species:
The Sand Dollar Dendraster excentricus is a marine invertebrate that lives in the sandy bottoms of sheltered bays and open coastal areas. They form dense beds in the low intertidal and subtidal zones of sheltered bays, and in the subtidal zone just beyond the break zone of coastal areas.
The body of Dendraster excentricus consists of a rigid test (hard external covering) covered with movable spines. It commonly reaches a size of 75mm in diameter although this is highly location-specific. Although the common conception of the Sand Dollar is the smooth white empty test that washes ashore on North American west coast beaches, the living animal is colored from a pale gray-lavender to a dark purplish black, its aboral (top) spine tips sometimes much lighter. It has a water-vascular system – a system of water-filled canals derived from the internal cavity or coelom that connect with tube feet. The tube feet are arranged in five paired rows that extend from the mouth on underside to the top (aboral) pole. The tube feet are protruded and withdrawn from the ambulacra – the five radial areas on the undersurface of the animal, and serve a locomotion, feeding, and possibly a respiratory function. The five ambulacral rows alternate with five interambulacral areas, where calcareous plates occur in patterns and are bound into the rigid test.


A flower-like pattern is seen on the aboral side of the body composed of pore pairs where specialized tube feet perform gas exchange. At the center is the madreporite - a perforated platelike structure that forms the intake for their water-vascular system, and adjacent to this on the interambulacra, the genital pores. Radiating out from the genital pores are the five flower petals, which represents the ambulacral radii. The flower pattern in this species is off-center, thus the meaning of the name excentricus.


Natural History:
Feeding
Dendraster excentricus is considered a suspension feeder and feeds on suspended organic particulate matter in the water currents. Various specialized spines, tube feet, mucus-secreting glands and the pedicellariae – small pincher-like organs with moveable jaws, are involved in feeding (Jangoux and Lawrence 1983). The spines on the aboral side are club-shaped and covered with cilia. When small organic particles and organisms flow onto this surface, little eddies are created and particles become trapped by mucus secreted on the spines. They are directed by these club-shaped spines and cilia to the margins and around to the oral side to the food grooves, where they move in mucus streams that lead to the mouth in the center (Morris 1980).
Larger suspended particles may be held by sucker-tipped podia and transported to the food grooves. For living prey such as crustacean larvae and small copepods, the spines on the oral side converge and enclose it into a little tent-like cone, where it is passed to a food groove by the jaws of the pedicellariae (Morris 1980). Sand is ingested with the food and it is thought that this is deliberately done by juveniles to act as a “weight belt” for stability in the sand.

The Western Sand Dollar is able to feed in this manner because of its eccentricity, which allows it to bury itself “standing up” obliquely in the sand. This is their feeding position, parallel to the surge current with the anterior end buried (Lawrence 1987). By positioning themselves closely together, they may exploit their hydrodynamic shape and influence the current flow past their bodies. It is likely that this is the reason they form densely packed beds of several hundred individuals per square meter. In light currents, they may stand perpendicular, lay flat, or even bury themselves (Jangoux and Lawrence 1983).




Dendraster excentricus at Monterey Bay Aquarium, Monterey, California