A Chemical Engineering View of Cnidarian Symbioses

Abstract

Chemical engineering theory proves useful in predicting selective constraints on algal-cnidarian symbioses. Scleractinian corals are used as a model for understanding how the architecture of polyps and colonies affects the transport of material between the host and symbiotic algae, and the environment. Transport properties of the symbiosis are described by mathematical functions involving molecular diffusion and forced convection (water motion). Metabolic rates (photosynthesis and aerobic respiration) measured under manipulated regimes of water motion provide evidence that the physical state of the boundary layer (laminar or turbulent) surrounding the symbiosis directly affects both symbiotic algae and coral. However, the algae's photosynthetic rate is affected by changes in the ambient flow regime to a lesser degree than the symbiotic association's aerobic respiration rate, indicating a buffering effect of the host tissue. Two mathematical models presented explore the relationship of size and shape of contracted and expanded polyps on maximal rates of gas or nutrient exchange: the size/shape spectrum of scleractinian polyps is understandable in terms of how diffusion limits delivery of metabolites to coral and algae. Polyps of differing size are not geometrically similar; the shape changes observed are consistent with keeping fluxes of dissolved substances to these symbiotic associations “diffusionally similar.” Expanded polyps possess diffusive boundary layers of considerable depth which limit delivery of metabolites to the algae. The mass transfer characteristics and size range of scleractinian polyps lie within the range where theory predicts an optimal polyp wall thickness should occur.