Adaptations by Invertebrate Larvae for Nutrient Acquisition from Seawater

Abstract

Unlike life on land, animals that live in seawater are surrounded by a medium that contains organic nutrients in dilute solution. Larval forms of soft-bodied marine invertebrates are adapted to take advantage of the fact that most of the organic carbon in their environment is in solution as dissolved organic material (DOM). New evidence for the importance of DOM to metazoans is presented by showing that larval forms can increase in biomass, even in the absence of paniculate foods. Such increases occurred only in those species capable of transporting DOM. The physiological basis for using DOM as an energy source is dependent upon an increased transport capacity for DOM as growth proceeds. Using bivalve larvae, mass coefficients and exponents were determined for (i) alanine transport rates and (ii) metabolic rates. These coefficients were not statistically different when determined over the life span of a larva. Thus, as growth proceeds, these larvae increase their ability to obtain a potential supply of metabolic fuel (DOM) in direct proportion to the increase in their metabolic demand. The percent of this increased transport capacity that larvae could actually utilize in nature will depend upon the substrate concentrations in their environment. Current views on what these concentrations are in seawater may be altered as more attention is given to the fine scale distributions of organic chemicals in the ocean. After DOM has been transported by the animal, its metabolic fate can now be rigorously studied using bacteria-free larvae. Measurements of amino acid synthesis in larvae cultured under axenic conditions suggest that a much greater plasticity may exist in the biochemical requirements of larvae for dietary amino acids.