Immigration as a factor in maintaining populations of the sea urchinLytechinus variegates(Echinodermata: Echinoidea) in seagrass beds on the southwest coast of Puerto Rico

Abstract

Abstract The abundance and size frequency distribution of the regular sea urchin Lytechinus variegatus in an extensive Thalassia testudinum bed on the southwest coast of Puerto Rico were studies for two years. Urchins were collected and measured at “deep”; (3 m) and “shallow”; (1–1.5 m) stations at an eastern site (near Isla Guayacan) and a site 3.8 km to the west (Punta Pitahaya) in the same seagrass bed. There was little larval recruitment to the L. variegatus populations and their size frequency distributions remained relatively uniform both seasonally and annually. The L. variegatus at Pitahaya were consistently larger (median test diameter ∼3.6 cm vs. 2.8 cm) but less abundant (∼4/m2vs. ∼ 14/m2) than those at Guayacan. Comparisons of the size frequency data do not demonstrate growth by the L. variegatus. Prédation by the gastropod Cassis tuberosa was a significant source of urchin mortality at Pitahaya (minimum estimate 0.8 L. variegatus eaten/m2/yr.) but the urchin population there did not decline during the study. Larval settlement and juvenile recruitment at Pitahaya were negligible therefore the population must have been maintained by immigration. This study suggests that settlement of L. variegatus may be highly localized and that populations of this important grazer in seagrass beds can be dependent upon immigration.