NOTES ON THE BIOLOGY OF THE FIVE-LUNULED SAND DOLLAR

Abstract

The five-lunuled sand dollar, Mellita quinquiesperforata (Leske, 1778), is abundant on the sand flats at Beaufort, North Carolina, where the following ob servations were made during a short stay in August, 1957, at the Duke Uni versity Marine Laboratory. Burrowing. The animal lives in shallow water in a horizontal position, usually completely covered by a thin layer of sand although sometimes the posterior lunule remains visible. Specimens observed were motionless although the animal can progress under the sand. On being removed and placed on top of the sand under water it promptly burrows under again. The statements about its method of burrowing in Pearse, Humm and Wharton (1942) are totally at variance with my observations and appear wholly erroneous. They say that in burrowing the animal moves directly downward, waving its spines and tube feet so as to move sand from beneath the test to the upper surface, and does not progress anteriorly as it descends. My observations on animals in their natural habitat agree with those of Kenk (1944) on the closely related Mellita lata at Puerto Rico. Unlike regular urchins, sand dollars and other irregular urchins are polarized with definite anterior and posterior ends and can move only in the anterior direction, that opposite the posterior lunule in the present species. When placed on the sand under water in its natural habitat Mellita promptly begins to bury itself. It does this by simply advancing into the sand in a horizontal or slightly oblique orientation, using its ordinary method of locomotion by waves of movement along the spines of the oral surface. Kenk gives a suc cession of photographs showing Mellita advancing into the sand, which rapidly covers it. I did not time the reaction but only a couple of minutes is required for the five-lunuled species to cover itself with a thin layer of sand whereupon it comes to a halt. It positively does not move downward, does not remove sand from beneath it, and does progress forward. It is improbable that the tube feet play any role in locomotion as they are small and feeble and of limited distribution. Lunules. Contrary to the statement of the Berrills (1957), in Mellita sand positively does not pass through the lunules from the oral to the aboral surface during the burrowing process. To the contrary as the animal advances into the sand, sand falls through the lunules to the oral surface, thus if anything retarding the burying process. It is known for a Japanese species (Ikeda, 1941) that the lunules are important in burrowing and righting but this is definitely not the case in Mellita. The further suggestion of the Berrills that the lunules strengthen the test by fusing the oral and aboral surface is extremely im probable because the two surfaces of the test are so much connected by interior