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Larvae of the common Caribbean scleractinian coral, Agaricia humilis, are induced to settle and metamorphose by contact with specific crustose (nongeniculate) coralline red algae. This requirement for an exogenous trigger of settlement and metamorphosis has been shown to control the distribution of recruits of this coral in the natural environment. Results reported here demonstrate that the stringency and specificity of this larval requirement persist for at least 30 days following the planktonic release of the brooded larvae, thus enhancing both the capacity for dispersal of the larvae and the substratum specificity of their metamorphosis and recruitment. The inducer of metamorphosis is shown to be associated with an insoluble macromolecular carbohydrate. This molecule is found with the partially purified cell walls obtained from a morphogenetic crustose red alga, Hydrolithon boergesenii, or its associated microflora. Because two non-inductive crustose red algal species also lack the cell wall-associated inducer, the substratum specificity of metamorphosis is probably the result of larval recognition of this molecule. In procedures that should prove widely applicable to other systems, purified and highly specific enzymes were used to cleave the inductive cell wall-associated polysaccharides and to solubilize the active morphogen. Enzymes were also used as probes with which to identify essential structural features required for the morphogenetic activity. These enzymatic and related biochemical studies show that the morphogen is associated with, and may itself contain, a sulfated glycosaminoglycan that includes multiple N-acetylglucosamine and galactose residues. The larval receptors that recognize this complex carbohydrate cue may thus be related to lectins. The solubilized morphogen induces normal settlement, attachment, and the metamorphosis of A. humilis and A. tenuifolia larvae on clean polystyrene surfaces, and the larvae seem to have no other requirement. This effect is apparently specific for larvae of species induced to settle by the intact alga; larvae of the sympatric coral Tubastraea aurea are not induced by this chemical, or by the intact algal surface. A wide variety of other natural and synthetic sulfated polysaccharides and related polymers have little or no inductive effect on the A. humilis larvae, suggesting that the larval receptors involved in substratum recognition are highly specific. A similar high specificity of lectin- and sulfated polysaccharide-mediated recognition, and the resulting control of differentiation, has been observed in a wide variety of biological systems.

Enzymatic Characterization of the Morphogen Recognized by Agaricia humilis (Scleractinian Coral) Larvae