Sympatric growth morphs and size bimodality in the green sea urchin (Strongylocentrotus droebachiensis)

Green sea urchins (Strongylocentrotus droebachiensis) have been a dominant structuring force in boreal. hard-bottom communities and recently have become an important fishery. Despite these roles, relatively little is known about the age, growth, and demographics of field populations. To begin to address these issues, we systematically sampled urchins during 1997 and 1998 at two sites (Allen Island [AI] and Schoodic Peninsula [SP]) in the Gulf of Maine, USA. Basic demographic information (density and size structure) was obtained and a subset of urchins aged by counting annual bands on the interambulacral plates. Size-at-age analyses showed that the population at AI consisted of two sympatric growth morphs, fast growing (fg) and slow growing (sg), each described by its own von Bertalanffy growth parameters for mean size and size variance. The two morphs also differed in maximum observed ages (sg, 6-11 yr, vs. fg, 16-18 yr) and in maximum test diameter sizes (sg, 30-35 mm, vs. fg, 50-70 mm). At SP, a single (fg) morph was identified which had growth characteristics similar to the fg morph at AI. An obviously bimodal size-frequency distribution at AI reflects the accumulation of urchins of each morph at asymptotic size. We advanced several hypotheses, including intrinsic factors (e.g., sex ratios, morphology, and genetics) and extrinsic factors (e.g., density, spatial distribution, settlement history, growth history, and migration) to explain the origin and coexistence of the two growth forms. Using primary and post hoc data analyses, we rejected sex ratio, density, spatial relations, and morphology. Analysis of annual growth and age structure indicated that the two morphs had different settlement histories, suggesting the mixing of two genetically distinct populations. Our post hoc analyses could not reject the genetics or growth history and migration hypotheses. Genetically different larvae, or else a phased process of differential settlement, migration, and growth (threshold migration hypothesis) are the most likely cause(s) for the two morphs. Our data indicate that field growth rates in green urchins are intrinsically highly variable but also vary interannually, probably in relation to their environment. Intrinsic variability provides a novel explanation for bimodality of size-frequency distributions in urchin populations. Interestingly, the sublegal size (<50 mm) and short life span of the sg morph preempts its recruitment into the fishery. Moreover, if the sg morph is widespread and genetically based, intensive harvesting may enhance selection for the sg morph. These findings may have serious implications for understanding the population structure and managing the green urchin fishery, and for assessing the grazing impact of these urchins in nearshore communities.