Many chemical factors have been shown to influence oyster settlement and metamorphosis, including naturally occurring chemicals isolated from the environment or from oyster tissues themselves. The functional roles of these chemicals have usually not been determined. Our research has
been directed at elucidating the roles of certain of these chemicals and defining a model of settlement and metamorphosis which explains their interaction for oysters of the genus Crassostrea. A distinction can be made between factors which promote settlement (the reversible
behavioral phase) and metamorphosis (the irreversible morphogenetic phase). Control of settlement behavior appears to be via a dopaminergic receptor-mediated neural pathway, while control of morphogenesis is through an adrenergic receptor-mediated pathway. Exogenously applied L-DOPA,
which triggers a distinctive pattern of settlement behavior, is converted into dopamine within the larval body and acts through dopaminergic receptors. The behavioral response to L-DOPA can be blocked by using L-DOPA decarboxylase inhibitors to prevent the conversion of L-DOPA to dopamine.
Dopaminergic agonists will trigger the behavioral sequence while dopamine antagonists will block it. Bacterial supernatants which induce larval settlement behavior appear to do so through the action of bacterially produced ammonia (NH3), which triggers in some way (probably in a
non-specific manner) the behavioral pathway. The morphogenetic phase of metamorphosis is controlled by the catecholamines, norepinephrine (NE) or epinephrine (EPI). Pharmacological analysis has shown that these catecholamines act on alpha-1 adrenoceptors. We have demonstrated with HPLC analysis
that norepinephrine increases significantly through development. Our current model assumes NE is the catecholamine of most importance to the metamorphogenetic pathways. The ontogenetic appearance of competence (the ability of larvae to respond to behavioral or metamorphic stimuli) has also
been a major focus of investigation. The ability to behave in response to dopaminergic stimulation appears before the ability to metamorphose in response to epinephrine, suggesting the functional dopaminergic neural circuitry develops before the adrenergic neural circuitry.
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