TRANSPORT AND METABOLISM OF ALANINE AND PALMITIC ACID BY FIELD-COLLECTED LARVAE OF TEDANIA-IGNIS (PORIFERA, DEMOSPONGIAE) - ESTIMATED CONSEQUENCES OF LIMITED LABEL TRANSLOCATION

The epidermis of larvae of Tedania ignis (Porifera, Demospongiae) is u niformly ciliated except for the posterior pole. The epidermal cells a re long, columnar, and monociliate; each cilium arises from an epiderm al crypt; symbiotic bacteria were not observed in larval cells. These lecithotrophic (''nonfeeding'') larvae can feed by assimilating dissol ved organic materials (DOM) from seawater. Larvae transported both the amino acid alanine (mean = 2.73 pmol larva(-1) h(-1); [S] = 1 mu M) a nd the fatty acid palmitic acid (mean = 16.27 pmol larva(-1) h(-1); [S ] = 1 mu M) from seawater. Following assimilation, the label from alan ine was recovered primarily in small molecular weight compounds; the l abel from palmitic acid was localized chiefly in the lipid fraction. E stimates of the contribution of transport to metabolism (mean respirat ion rate = 940.7 pmol O-2 larva(-1) h(-1)) reveal that alanine transpo rt is energetically insignificant. Palmitic acid transport, in contras t, could account for 21%-55% of larval metabolism, Autoradiographic an alysis of the distribution of the label in larvae suggests that epider mal cells are the chief recipients of the assimilated materials. Thus, the contribution of transport to whole-larva metabolism may underesti mate the tissue-specific value, At palmitic acid concentrations of 1 a nd 0.25 mu M, the contribution of transport to the estimated metabolis m 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, bu t also the degree to which materials are distributed among tissues.