THE POPULATION-STRUCTURE OF THE BRITTLE STAR OPHIURA-SARSI LUTKEN IN THE GULF OF MAINE AND ITS TROPHIC RELATIONSHIP TO AMERICAN PLAICE (HIPPOGLOSSOIDES-PLATESSOIDES FABRICIUS)

This study examines the population structure of the ophiuroid Ophiura sarsi Lutken in an area southeast of Portland, Maine and its exploitat ion by its major predator, the flatfish, American plaice (Hippoglossoi des platessoides Fabricius). Ophiura sarsi and American plaice were sa mpled during a 1-yr period (1985-1986) from a site of 148-156 m depth. The bimodal size-frequency distributions of the O. sarsi population w ere dominated by small individuals with disk diameters between 2 and 1 1 mm, and with estimated growth rates, based upon the modal progressio ns from the persistent 1985 year class, of almost-equal-to 4 mm per ye ar. Larger individuals (almost-equal-to 15-25 mm) were generally less numerous. These older brittle stars, which formed the right-hand peaks of the size-frequency distributions, displayed little noticeable grow th, which suggests that these peaks were comprised of multiple cohorts . Recruitment to the population appeared to be continuous, due to the occurrence of 1 mm or smaller brittle stars throughout most of the yea r. The strong presence of 1 mm or smaller O. sarsi in January, 1986 in dicated a major recruitment event. A survey of the reproductive condit ion of the gonads showed evidence of continuous but seasonally variabl e reproduction, with the major spawning centered around January. O. sa rsi was the major dietary constituent of the American plaice, occurrin g in at least 65% of the fish stomachs throughout the year. Plaice con sumed the most abundant size classes of O. sarsi in the population, wi th disk sizes between almost-equal-to 4 and 10 mm. These brittle stars were of low caloric value, yet the more nutritious, older brittle sta rs were not eaten in significant numbers by even the largest plaice. T he high proportion of small, energy-poor O. sarsi found in the plaices ' diet might be the result of their greater abundance or accessibility at this site.