The population structure of the brittle star Ophiura sarsi Lütken in the Gulf of Maine and its trophic relationship to American plaice ( Hippoglossoides platessoides Fabricius)

Abstract

This study examines the population structure of the ophiuroid Ophiura sarsi Lütken in an area southeast of Portland, Maine and its exploitation by its major predator, the flatfish, American plaice ( Hippoglossoides platessoides Fabricius). Ophiura sarsi and American plaice were sampled 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 were dominated by small individuals with disk diameters between 2 and 11 mm, and with estimated growth rates, based upon the modal progressions from the persistent 1985 year class, of ≈4 mm per year. Larger individuals (≈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 growth, 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 year. The strong presence of 1 mm or smaller O. sarsi in January, 1986 indicated a major recruitment event. A survey of the reproductive condition of the gonads showed evidence of continuous but seasonally variable reproduction, with the major spawning centered around January. O. sarsi was the major dietary constituent of the American plaice, occurring in at least 65% of the fish stomachs throughout the year. Plaice consumed the most abundant size classes of O. sarsi in the population, with disk sizes between ≈4 and 10 mm. These brittle stars were of low caloric value, yet the more nutritious, older brittle stars were not eaten in significant numbers by even the largest plaice. The 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.