Known in English as eccentric sand dollar, Dendraster excentricus is undoubtedly a very peculiar species of sea urchin. It is not the typical sea urchin that may come to your mind, but a species with an extremely flattened body, and very short, thin claws. The body of this species has adapted to bury and burrow in the sand. This species is found in the waters of the northeastern Pacific Ocean, from Alaska to Baja California, where its skeletons (called test) are frequently found on beaches. Juveniles are found closer to shore, but move offshore with age. The highest population density and the largest individuals are found in waters from 4 to 12 meters deep. There are records of specimens up to 90 meters depth, although these are sporadic.
The body of Dendraster excentricus, as mentioned above, is very flattened and circular in shape. It can reach up to 12 cm in diameter (usually 7-8 cm) and a height of 1 cm. The entire body is covered with small reddish-purple spikes. Although it does not appear externally, Dendraster excentricus has the typical 5-radial symmetry characteristic of echinoderms. This symmetry is more evident if we observe the skeletons of these specimens. In it we will find, on the dorsal side and not centered, a five-armed star-shaped structure. The fact that this flower-shaped structure is not centered is the feature by which its specific epithet ‘excentricus‘ was determined.
The diet of Dendraster excentricus is based on suspended particles, crustacean larvae, small copepods, diatoms, and plankton in general. When feeding, they tend to position themselves laterally, partially burying the anterior part of the body, so that they are perpendicular to the surface of the seabed. In this way, and thanks to a multitude of small pincers (called pedicellariae), ambulacral tubes and small spines, they are able to filter the water current and catch their food. The food is transported to the mouth by means of the ambulacral feet. In the mouth we find a structure unique to echinoids, called Aristotle’s lantern, with which they crush the food.
The behavior of Dendraster excentricus is mainly burrowing and gregarious. These animals bury themselves partially or completely in the sand to protect themselves from predators and strong currents. To do this, they use their spines and ambulacral feet to create a depression in the substrate and then slide downward. They can also move slowly over the bottom using their oral ambulacral feet. They usually form dense aggregations that can occupy large areas of the seafloor.
Reproduction of Dendraster excentricus is sexual and external. Individuals are dioecious, that is, there are separate males and females, although there are no noticeable external differences between the two sexes. They reach sexual maturity at ages ranging from 1 to 4 years. During the breeding season, they release their gametes into the water, where fertilization takes place. Eggs and sperm are released synchronously, usually in late spring or early summer. The eggs hatch into planktonic larvae called echinopluteus, which feed on phytoplankton and have bilateral symmetry. The larvae undergo metamorphosis to become benthic juveniles, which have radial symmetry and resemble adults. The life cycle of this species can last between 8 and 13 years.
Species that may prey on Dendraster excentricus include the starfish Pisaster brevispinus and the sole Platichthys stellatus. Occasionally some crabs and seabirds may also feed on them.
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