Transport and metabolism of alanine and palmitic acid by field-collected larvae of Tedania ignis : estimated conseqeunces of limited label translocation

The epidermis of larvae of Tedania ignis (Porifera, Demospongiae) is uniformly ciliated except for the posterior pole. The epidermal cells are long, columnar, and monociliate; each cilium arises from an epidermal crypt; symbiotic bacteria were not observed in larval cells. These lecithotrophic ("nonfeeding") larvae can feed by assimilating dissolved organic materials (DOM) from seawater. Larvae transported both the amino acid alanine (mean = 2.73 pmol [larva.sup.-1] [h.sup.-1]; [S] = 1 [Mu]M) and the fatty acid palmitic acid (mean = 16.27 pmol [larva.sup.-1] [h.sup.-1]; [S] = 1 [Mu]M) from seawater. Following assimilation, the label from alanine was recovered primarily in small molecular weight compounds; the label from palmitic acid was localized chiefly in the lipid fraction. Estimates of the contribution of transport to metabolism (mean respiration rate = 940.7 pmol [O.sub.2] [larva.sup.-1] [h.sup.-1]) reveal that alanine transport is energetically insignificant. Palmitic acid transport, in contrast, could account for 21%-55% of larval metabolism. Autoradiographic analysis of the distribution of the label in larvae suggests that epidermal cells are the chief recipients of the assimilated materials. Thus, the contribution of transport to whole-larva metabolism may underestimate the tissue-specific value. At palmitic acid concentrations of 1 and 0.25 [Mu]M, the contribution of transport to the estimated metabolism of the epidermis would be 131% and 33% of energy requirements. Thus, the potential benefits of DOM to larvae are dependent not only on the nature of the epidermal transporters and the solute concentration, but also the degree to which materials are distributed among tissues.