SYMPATRIC GROWTH MORPHS AND SIZE BIMODALITY IN THE GREEN SEA URCHIN (STRONGYLOCENTROTUS DROEBACHIENSIS)

Abstract

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 sizefrequency 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 (Ͻ50 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.

Growth rates are related to production efficiencies in juveniles of the sea urchinLytechinus variegatus

“…Genetic analysis was not performed on urchins used in this study or in the Pawson & Miller (1982), Grosjean et al . (1996) or Vadas et al . (2002) studies.…”

Daily movement of the sea urchin Strongylocentrotus droebachiensis in different subtidal habitats in eastern Canada

“…Several workers hypothesize that migration explains the difference in the size structure of Strongylocentrotus droebachiensis between barrens and kelp bed habitats (Propp 1977, Meidel & Scheibling 1998, Vadas et al 2002. Since urchins on barrens can move up to 5 m per day, and 11 m in 3 d, they should easily be able to cover the distance between barrens and shallow water kelp beds.…”

Sea urchin grazing and kelp re-vegetation in the NE Atlantic

“…The kelp habitat and, thus, most of the sea urchin predators that live associated with kelp have disappeared, leaving the sea urchin populations more or less undisturbed. Sea urchins may live for many years (Russell 1998;Vadas et al 2002;Ebert & Southon 2003), but the fact that the barren ground has lasted for almost 40 years in Norwegian waters indicates that recruitment to the adult population is needed in order to sustain the high population density. However, the survival of post-settlement juveniles is probably sufficient for maintaining high population density on the barren grounds (Christie & Rueness 1998;Hereu et al 2004).…”

“…Consequently, the condition of adult urchins is sub-optimal and mortality tends to be high (Sivertsen 1997a;Christie & Rueness 1998). Christie & Rueness (1998) suggested, therefore, that barren ground populations depend on regular recruitment to prevent a population crash, but this idea is being debated by other authors, who suggest that urchin populations on barrens grounds only need occasional recruitments due to a long life span (Sivertsen 1997b;Vadas et al 2002). Sivertsen (2006) suggested that recruitment strategies may differ with latitudes, but this has not been studied by repeated sampling of recruitment and mortality between years.…”