Comparative branchial plume biometry between two extreme ecotypes of the hydrothermal vent tubeworm Ridgeia piscesae

Abstract

The vestimentiferan tubeworm Ridgeia piscesae Jones, 1985 from the Juan de Fuca Ridge is a morphologically plastic species with two contrasted ecotypes. The “chimney-dwelling” ecotype lives along the sides of active hydrothermal chimneys, where temperature and sulfide concentrations are high, but oxygen concentrations are low. Its morphology corresponds to the “short-fat” morphotype. The “basalt-dwelling” ecotype lives on rock outcrops in surrounding basaltic fields, where temperature and sulfide concentrations are low, but oxygen is readily available. Its morphology corresponds to the “long-skinny” morphotype. We compared the ultrastructure and biometry of their branchial plumes, the essential organ for respiration and nutrition in this symbiotic tubeworm. The branchial epidermis of both ecotypes contains electron-dense organelles, which may be related to the presence of sulfides in their environment. The diffusion distance across the gill epidermis is about 20% shorter in the short-fat R. piscesae. For tubeworms of the same mass, specific branchial surface areas (SBSA) are similar in both ecotypes (mean 24 cm2/g for worms of 1–2 g body mass), which is comparable with the SBSA in the vestimentiferan vent tubeworm Riftia pachyptila Jones, 1981 . The SBSA of R. piscesae follows a power curve that is negatively correlated with total body mass. The implications for gas uptake efficiency between these two ecotypes are discussed.