Temporal and spatial variation in egg size of 13 Panamanian echinoids

Abstract

Though egg size of marine invertebrates has received a great deal of theoretical attention, and though sea urchin eggs have been studied for nearly a century, practically no data on intraspecific variation of echinoid egg size exist. This study examines temporal variation in egg size of 10 species of regular and three species of irregular echinoids from either side of the Isthmus of Panama. Data on spatial variation in egg volume are also presented for eight of these species. In all species mean size of eggs from different females collected from the same locality on the same day were significantly different. Daily (within-month) variation in egg size was significant in three out of the five species for which sufficiently large samples were collected: Eucidaris tribuloides (Lamarck), Diadema antillarum Philippi, and Lytechinus variegatus (Lamarck). No significant differences between daily means of egg volume were evident in D. mexicanum A. Agassiz and in Echinometra viridis A. Agassiz. Monthly means of egg volume were significantly different in seven out of the 13 species studied: Lytechinus variegatus, L. williamsi Chesher, Tripneustes ventricosus (Lamarck), Echinometra viridis, E. vanbrunti A. Agassiz, Clypeaster rosaceus (Linnaeus), and C. subdepressus (Gray). Between-year variation was also significant in all of these species, except Lytechinus variegatus. All Caribbean species with significant monthly variation show a decline in egg size after September, but no obvious environmental fluctuation can be linked to this change. Echinometra lucunter (Linnaeus) showed no significant variation between months of the same year, but exhibited differences between years. No significant monthly or annual variation was observed in Eucidaris tribuloides, E. thouarsi (Valenciennes), Diadema antillarum, D. mexicanum, or Leodia sexiesperforata (Leske). When temporal variation at each locality was taken into account, only one species out of eight, Clypeaster subdepressus, showed spatial variation. Correlations between size of eggs collected at a particular time and the intensity of spawning by the population at that time were generally not significant, suggesting that size of mature eggs is not determined by reproductive state of the parental population. Correlations were also attempted (in three species of Echinometra) between egg size and body weight, gonadal content, and number of eggs carried by the mother. None of these variables was a good predictor of egg size. Only in E. viridis was there a significant correlation between egg size and gonadal content. Though its causes remain obscure, intraspecific variation in egg size found in these echinoids could be exploited to study the possible effects of this parameter on larval life history.