Population characteristics of the estuarine isopod Gnorimosphaeroma insulare in three contrasting habitats: sedge marsh, algal bed, and wood debris

Abstract

Population characteristics of Gnorimosphaeroma insulare (Isopoda: Sphaeromatidae) were investigated in three habitats within the Squamish estuary, British Columbia: a log debris area, an embankment along the perimeter of a Carex lyngbyei marsh, and a Fucus distichus community. Gnorimosphaeroma insulare annual production was highest in the Fucus habitat (39.54–48.05 g dry wt∙m−2∙year−1) followed by "bank" (22.97–27.74 g∙m−2∙year−1) and then wood debris habitats (4.72–6.43 g∙m−2∙year−1). Cohort figures indicated that male productivity was greater than female productivity in all locations. Production:biomass ratios were similar in all three habitats and ranged between 2.47 and 3.17. Mortality rates were greatest in Fucus; those in wood debris and bank habitats were similar. Growth rates were highest in the bank habitat; those in wood debris and Fucus were similar. Growth and mortality rates of males were greater than those of females. Gnorimosphaeroma insulare is an annual semelparous species. Males died soon after mating, females soon after brood release. Fecundity was linearly related to body size but differed between locations (females from wood debris produced more eggs). Brood mortality was dependent on habitat: females from the bank habitat possessed the highest brood mortality followed by those from wood debris; females from Fucus did not possess any significant brood mortality. Egg development time was estimated at 120 days. The reduced carrying capacity of wood debris substrate is thought to be due to a paucity of microbial epiphytes, particularly fungi.