Biochemical and genetic engineering for improved production of abalones and other valuable molluscs

Abstract Intensification of production, modernization of technique, and improvement of cultivars are needed to increase the economic efficiency of molluscan cultivation industries in the United States and certain other countries. To help achieve such improvements, modern methods of biochemical and genetic engineering can be adapted to control biological processes which intrinsically limit the efficiency and yield of molluscan production. Critical life-cycle stages of several species of abalones and other commercially valuable molluscs which thus far have proved amenable to improved control by these means include reproduction, larval settlement, metamorphosis, and acceleration of early growth. Analysis of the physiological and molecular mechanisms that control reproduction in molluscs reveals a prostaglandin-dependent regulation of spawning in abalones and certain other species. Spawning of gravid adults can be induced by the addition of prostaglandins to the surrounding seawater, or, more reliably and inexpensively, by activation of the endogenous enzymatic synthesis of prostaglandin-related spawning triggers in response to added hydrogen peroxide. Peroxide activation of the prostaglandin-dependent spawning reaction has been found widely useful for obtaining synchronous and copious release of fully competent gametes in a large number of species of abalones, oysters, scallops, mussels, clams, and other valuable molluses; a number of these species had not been successfully spawned by other methods. Settlement and metamorphosis of many species can be brought under similar biochemical control. The efficient induction of settlement and metamorphosis of the larvae of many Haliotis (abalone) species normally requires contact with any of several crustose red algae usually associated with natural recruiting substrates. This requirement can be met by providing competent larvae with the essential algae or with specific proteins purified from these algae, although production of these inducers is both time consuming and costly. Most conveniently and inexpensively, this natural requirement can be met by providing the larvae with certain unique amino acid constituents normally associated with the inducing proteins. Thus, γ-aminobutyric acid (GABA) can be used simply, safely, and inexpensively to induce complete and rapid larval settlement and metamorphosis — with minimal mortality — in many commercially important abalone species. This and similar neurotransmitter-related, amino acid-derived compounds are proving comparably effective for the reliable induction of settlement and metamorphosis in a number of other valuable molluscan species. Recent analyses of the requirements for rapid early juvenile growth in abalones reveals a mechanism of growth control which is susceptible to exogenous hormonal acceleration. Present experiments aimed at genetic cloning and amplification of recombinant-DNA templates for the essential growth-regulating hormones, progress in the establishment of useful molluscan gene banks, and potential uses of this technology for molluscan aquaculture are described.