ABSTRACT
The infaunal ophiuroid Acrocnida brachiata (Montagu) (Echinodermata: Ophiuroidea) buries 10 to 20 cm in fine sand substrata, and is known from several locations in European coastal waters. A comparison of an intertidal and a subtidal population has been in progress since January 1984 in the Bay of Douarnenez (Brittany, France). The population structures of Acrocnida brachiata are assessed by measuring the disc diameter, defined as the distance from the outer edge of the radial shields to the disc perimeter on the opposite side. In both the intertidal and subtidal populations, three well marked peaks constitute the size-frequency histograms, suggesting discrete year classes. For the same month, the mean disc sizes of the 2nd and 3rd peaks of each histogram are lower in the intertidal than those in the subtidal population.
