Nutritional Strategies of Cnidarians: The Absorption, Translocation and Utilization of Dissolved Nutrients byHeteroxenia fuscescens

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Abstract
SYNOPSIS. Cnidarians posses polytrophic feeding capacities including: 1) endocytosis ofpaniculate food, 2) absorption of dissolved organic material, 3) utilization of assimilates from zooxanthellae. Heteroxeniafuscescens seems to be specialized to fuel metabolism only by epidermal uptake of organic compounds and by material originating from the cytosymbionts. Therefore, these soft corals are appropriate organisms to study the contribution of both substrates to nutrition quantitatively. By the uptake of only 12 amino acids and glucose, experimentally determined, approximately 80% of the energy demand can be covered, the contribution of algal products not included. Autoradiographic studies show that absorption occurs through epidermal membranes and that compounds are transported through the mesogloea in both directions at low rate. Furthermore, adaptations can be shown which intensify absorption of organic compounds and which reduce the loss of energy-rich substances from the coelenteron. For example, material originating from zooxanthellae which would wash out into the sea due to the polyfunctionality of the coelenteron, is partly retained by pharyngial tissue and the epidermis. Thus the loss of useful compounds is reduced.

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