Epidermal nutrient absorption in marine invertebrates: A comparative analysis

Abstract

SYNOPSIS. This paper reviews present knowledge on the transport mechanisms responsible for integumentary uptake of exogenous monosaccharides and amino acids in marine invertebrates. The discussion is based on work in the author's laboratory, using the polychaete Nereis diversicolor Miiller as the main experimental model. Comparison is made with solute transport in other animal epithelia, especially those of vertebrate origin.Transport across the apical epidermal membrane via specific transport systems or by diffusion in the lipoid plasmalemma is described with emphasis on the trans-membrane concentration gradients maintained. The epidermal cells seem to be functionally asymmetric, favoring outflux across the basolateral membrane (into the extracellular fluid) over that across the apical one. The intercellular spaces provide a paracellular pathway for nutrient diffusion from the extracellular fluid to the exterior, but the quantitative importance of this route needs further investigation. Although a net uptake from low external concentrations into the epidermis is clearly established, there is insufficient evidence specifically related to a true trans-epidermal net flow, a problem of critical importance for evaluating the nutritional role of exogenous organic material. Accumulating transport systems in the apical membrane seem to be involved in solute recycling at the cuticular-epidermal interface, thereby decreasing the effective epidermal permeability to diffusional loss of valuable nutrients. © 1982 by the American Society of Zoologists.