Functional metabolites in benthic invertebrates
There is ample evidence that chemical composition, concentration, flux, and hydrodynamic transport all have profound effects on chemically mediated ecological interactions. An example is the common usage of amino acids, sugars, and nucleotides as food cues. The production of dissolved organic matter (DOM) in certain microenvironments can locally elevate concentrations of compounds relative to surrounding habitats, creating stable chemical gradients. At spatial scales smaller than the tiniest turbulent eddies (roughly less than 1 mm), turbulent mixing is relatively unimportant and chemical transport is dominated by molecular diffusion and advection. It is likely that communication systems began with the evolution of specific meanings for pre-existing molecules. One class of molecules used in specific communication is peptides. Peptides are excellent signals in marine systems given their high solubility, short half lives due to rapid consumption by microbes, and correspondingly high signal to noise ratios. Information is specified by the sequence of amino acids, much as letters provide information within a word. The aproteinaceous barnacle settlement pheromone/kairomone, arthropodin, was the first peptidelike signal molecule reported in crustaceans. Arthropodin induces larval barnacles to temporarily attach to a surface, and then to permanently attach and metamorphose to the juvenile stage. Arthropodin functions as both an aggregation and a settlement pheromone. Small peptides with arginine or lysine at their carboxy termini induce ovigerous mud crabs (Rhithropanopeus harrisii) to release and disperse brooded embryos and induce oyster (Crassostrea virginica) larvae to settle near conspecific adults. Moreover, in contrast to the more typical sigmoidshaped dose/response curve, chemical induction in these marine organisms occurs only within a very narrow range in concentration of a molecule, spanning less than one order of magnitude.
Another well-known example of peptide-mediated signaling is found in the escape responses of the sea anemones Stomphia coccinea and S. didemon in reaction to contact with certain species of marine asteroids. Like other sea anemones, these organisms are sessile animals that respond to the presence of the tripeptide imbricatine released from the predator starfish Dermasterias imbricata by detachment and swimming behaviour. Another amazing example is provided by the avoidance reaction in the sea urchin Strongylocentrotus nudus induced by a steroid sulfate released from the starfish Plazaster borealis.
Chemical signals are known to regulate the trophic relationships of corals. All species of reef-building corals have mutualistic symbioses with unicellular algae, called zooxanthellae, which live within the coral cells and are abundant in tissues exposed to sunlight. Although reef corals are uniquely versatile in their ability to procure nutrients and energy, they principally depend on the translocation of carbon from their algal symbionts to meet their energy demands. The release of translocated materials from the algae is controlled by chemical communication with the coral host. Specifically, the chemical signal that induces carbon release is a mixture of free amino acids unique to the tissues of corals and other cnidarian species.
Considerable effort has been expended in identifying environmental signal molecules that induce marine larvae to settle and metamorphose. This research has met limited success because many of these morphogens are unstable, tightly complexed (adsorbed or bound) with other molecules, or present in only trace amounts. Neurotransmitters, such as gammaaminobutyric acid (GABA) and dihydroxyphenylalanine (DOPA), have been suggested to mimic the function of natural signal molecules, but the peripheral or central neural site (or sites) of action by these mimetics is still unclear. Remarkably few attempts have been made to characterize the structures of pheromones other than sperm attractants. Courtship and mating pheromones can be difficult to identify because breeding seasons are short and materials hard to obtain. Specific courtship behavioral acts are often troublesome to discriminate from other activities, thus making bioassays of active material impossible in some cases. Still, outstanding progress has been made towards elucidating the structures of mating pheromones in brown algae, and fishes. Whereas terpenes and other hydrocarbons appear to be the principal pheromones in worms and brown algae, steroid hormones and their metabolites produced by ovulating female fish are potent attractants to mature males in some species.
Additionally, chemical defenses produced by prey organisms (animals, plant, and microbes) render their tissues unpalatable or toxic to consumers. Despite the crucial ecological importance of such molecules, underlying mechanisms are lacking for most processes that structure communities.
Probably the most clear-cut examples of chemicals working as deterrent compounds in marine habitats are represented by molecules isolated in the molluscs of the sub-class Opisthobranchia. The opisthobranchs are marine slugs that are apparently unprotected, because the mechanical protection of their shell is either reduced or completely absent. However, chemical studies on these molluscs have accumulated evidence that they are well protected by chemical metabolites. These chemical weapons are obtained by bioaccumulation or biotransformation of dietary compounds or are synthesized de novo. In particular, current evolutionary theories suggest that opisthobranchs acquired through evolution the ability to produce de novo molecules present in their ancestral diet by horizontal genetic transmission, by a retro-synthetic mode or by a colossal gene loss. Since the first chemical study of the sea hare Aplysia kurodai in 1963, opisthobranchs have become the subjects of numerous studies on the defensive role of chemicals stored in their bodies. It seems that these organisms have elaborated very specialized and differentiated behaviours in order to optimize defense and reduce space competition. So, different genera of dorid nudibranchs are specialized predators of different sponges from which the molluscs obtain different chemicals that are committed to deter potential predators. Many opisthobranchs accumulate and concentrate the deterrent chemicals in outer structures of the body, more exposed to attack. Thus, the round-shaped vesicles displaced along the gills in many molluscs of the genus Hypselodoris contain pure forms of the defensive sesquiterpenes, and the coloured border of the mantle in Chromodoris is the source of deterrent diterpenes. Many other opisthobranchs compensate their physical vulnerability with chemical secretions. Opisthobranchs of the order Sacoglossa are one of the few groups of specialized herbivores in the marine environment. The presence of defense metabolites found in the secretion and mantle of these animals is due to the selective accumulation and in vivo chemical transformation of major metabolites acquired from the algae Caulerpales. A few sacoglossans also have the ability to “steal” and store chloroplasts (kleptoplasty) from algae. These molluscs have the unique ability to assimilate and maintain the photosynthetically active endosymbionts by synthesis of chloroplast proteins in the cytoplasmic ribosomes of the molluscs. These animals also biosythesize a rather uncommon class of polyketides that are suggested to serve as co-specific and intra-specific chemical signals; the same molecules also seem to act as physiological protectants against the deleterious effects of light in highly photophilic habitats.
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