Mellita isometra - Keyhole sand dollar
Habitat This species, as with the other two species from this genus, bury themselves into the sandy seabed of below-tide shallow seawater (intertidal or littoral). They can be more commonly found in waters near inlets. Keyhole sand dollars prefer to live in waters that have salinities that are greater than 23 parts per thousand.
Range Along the East coast of North America; from Massachusetts as far south as the Bahamas
Description When alive these sand dollars have velvety skin that can be dark green, grey, tan or brown. They are slightly pentangular in shape with a distinctive keyhole-shaped perforation toward the rear of the test. They have front-to-back bilateral symmetry and have their thickest portion slightly anterior to the center of the test.
Ecological Notes They live in groups for easier mating and for safety in numbers. When buried in the sand these omnivores use their tube feet to pull food particles from the sand, which can include algae, marine plants, shellfish and microorganisms. They use cilia and mucous to then get the food to their mouths. The small, sharp spines they have on them while living are passive defense against predators but they will also burrow into the sand for protection. They are important to their ecosystem because they control microorganism populations and shift the sand around when they burrow through it so that it is not stagnant.
Personal Information The specimen was found by the sand dunes at Oregon Inlet beach among many other types of shells.
George, Sophie B., and Shaun Boone. “The ectosymbiont crab Dissodactylus mellitae–sand dollar Mellita isometra relationship.” Journal of Experimental Marine Biology and Ecology, v. 294 issue 2, 2003, p. 235-255.
This paper looked at whether or not the number of crabs affected how many eggs the sand dollars produced and it looked to see if the size of the sand dollar affects how many crabs are attracted to it. The smaller a sand dollar is the more crabs it attracts, probably due to the fact that smaller sand dollars don’t bury as deep into the sand. Due to large variation in egg number production, the authors concluded that food availability, the sand dollar’s reproductive cycle as well as crab burden have an effect on how many eggs a sand dollar produces.
Cowan, William M.; Croom, Caroline R.; Zigler, Kirk S. “Observations of juvenile Anadara (Scapharca) Brasiliana (Bivalvia: Arcidae) on Mellita isometra (Echinoidea: Mellitidae) at St Catherines Island, GA, USA.” Journal of Molluscan Studies, issue 78, 2012, p. 225-226.
These researchers found that there were some juvenile clams on sand dollars, which is something they haven’t seen before. They wanted to know if it was common for juvenile clams to attach themselves to sand dollars and they wanted to know what species of clam they were finding on the sand dollars. They found that out of 300 sand dollars there were a total of 142 clams attached to them and that the species of clam was Anadara (Scapharca) brasiliana. They determined that it was a common thing for juvenile clams to be attached on the fringe and in the lunules of this species of sand dollar.
Contributed by Krista Lewis - 2016