Morphogen-Based Chemical Flypaper for Agaricia humilis Coral Larvae

Larvae of the scleractinian coral Agaricia humilis settle and metamorphose in response to chemosensory recognition of a morphogen on the surfaces of Hydrolithon boergesenii and certain other crustose coralline red algae. The requirement of the larva for this inducer apparently helps to determine the spatial pattern of recruitment in the natural environment. Previous research showed that the inducer is associated with the insoluble cell wall fraction of the recruiting algae or their microbial epibionts, and that a soluble but unstable fragment of the inducing molecule can be liberated by limited hydrolysis, either with alkali or with enzymes specific for cell wall polysaccharides. We now show that the parent morphogen can be solubilized by gentle decalcification of the algal cell walls with the chelators EGTA or EDTA, suggesting that the morphogen may be a component of the calcified recruiting alga itself, rather than a product of any noncalcified microbial epibionts. The solubilized inducer is subsequently purified by hydrophobic-interaction and DEAE chromatography. The purified, amphipathic morphogen retains activity when tightly bound to beads of a hydrophobic-interaction chromatography resin, and this activity (tested with laboratory-reared larvae) is identical in the ocean and the laboratory. We have attached the purified, resin-bound inducer to surfaces coated with a silicone adhesive and thus produced a potent artificial recruiting substratum--i.e., a morphogen-based chemical "flypaper" for A. humilis larvae. This material should prove useful in resolving the role of chemosensory recognition of morphogens in the control of substratum-specific settlement, metamorphosis, and recruitment and in the maintenance of species isolation mechanisms in the natural environment.

Adult Plasticity and Rapid Larval Evolution in a Recently Isolated Barnacle Population

Balanus amphitrite, a common barnacle species, was introduced into the landlocked Salton Sea in 1943 or 1944. In 1949, Balanus amphitrite from the Salton Sea was classified as the subspecies, Balanus amphitrite saltonensis, based upon morphological differences between Salton Sea and coastal individuals. This classification was maintained following an investigation of the Balanus amphitrite complex in 1975. Such a designation implies that the morphological divergence is underlain by genetic differences. Using field and laboratory transplantations, I tested the alternative hypothesis that the observed morphological divergence in the adult stage of Balanus amphitrite was the result of phenotypic plasticity. The results show that the divergence in the examined adult characters is in fact due to environmentally induced phenotypic plasticity. There were also phenotypic differences between larvae from the Salton Sea and those from coastal habitats that only became apparent during experimentation with the adult stage. Here, however, experimental results suggest that the divergence was due to an evolutionary process, probably selection. These results also provide the basis for two slightly precautionary conclusions: (1) the observation that individuals living in typical and novel habitats differ cannot even weakly indicate a cause for the difference, and (2) a consideration of the divergence of populations is incomplete if all of the life history stages of the organism are not studied.

Desiccation, predation, and mussel-barnacle interactions in the northern Gulf of California

Field experiments were conducted in order to determine the potential for desiccation and predation to mediate the effect of mussels (Brachidontes semilaevis) on barnacles (Chthamalus anisopoma) in the highly seasonal northern Gulf of California. We did this by removing both mussels and a common mussel predator (Morula ferruginosa: Gastropoda) and by spraying selected sites with sea water during summertime spring low tides. We also determined the effect of crowding on resistance to desiccation in barnacles, and the effect of barnacles on colonization by mussels. The mussel-barnacle community was not affected by keeping experimental quadrats damp during daytime low tides throughout the summer. Exposure to summertime low tides, however, did affect the survivorship of isolated, but not crowded, barnacles; and barnacle clumps enhanced the recruitment of mussels. Hence crowding in barnacles had a positive effect on both barnacle survivorship and mussel recruitment. Morula had a negative effect on mussel density, and mussels had a negative effect on barnacle density. The effect of Morula on barnacle density was positive, presumably due to its selective removal of mussels. These results suggest an indirect mutualism between barnacles and the gastropod predator, because barnacles attract settlement or enhance the survival of mussels, and the predator reduces the competitive effect of mussels on barnacles.