Chapter 6 Reproductive Physiology

Summary 6.6.1 Gametogenic Cycles Gametogenic cycles in scallops include periods of inactivity (vegetative periods), cytoplasmic growth, vitellogenesis, spawning, and resorption of unspawned gametes. Several means of assessing gametogenesis in scallops exist, including visual observation of the gonad (bothgross and microscopic examination of gonadal smears), gonad weights and indices, histology (including oocyte diameter and stereology), and indirect means (larval and spat abundances). The most complete definition of a gametogenic cycle would include both quantitative estimates of fecundity and qualitative assessment of gametogenesis. Considerable variation exists in pectinid gametogenic cycles. Intra-specific variations in the timing of spawning periods and the amount of gametogenic material produced occur between years and sites (location and depth), probably due to variations in environmental conditions that influence the gametogenic process. Aspects of gametogenesis vary along geographic ranges either as the result of environmental or genetic variability. Inter-specific differences in reproductive cycles also exist; the relative influence of genetic and environmental factors is unclear. 6.6.2 Regulation of Gametogenesis Gametogenesis in scallops is regulated internally by neurosecretions and age and externally by environmental factors (primarily food and temperature), which synchronise the cycle. Regulation of gametogenesis is ultimately under genetic influence. Thus the factors regulating gametogenesis may vary between species or even within a species at different locations, resulting in variation in gametogenic cycles. The regulation of gametogenesis is most completely understood for the bay scallop, Argopecten irradians. Once a minimum age is attained, a change in neurosecretory cycle stage (perhaps involked by increased temperature) occur. Scallops then become responsive to environmental stimuli for either initiation or delay of gonad growth. If minimum threshold day length, food, and temperature criteria are met, growth of oocytes is initiated in conjunction with the transfer of nutrients from the digestive gland to the gonad. After a certain amount of gametogenic materials is accumulated in the gonad. development of oocytes to maturation is independent of food supply, providing minimal temperatures are maintained. This transition is marked by another change in neurosecretory cycle stage and effectively removes the gametogenic process from an uncertain environmental food supply. Fecundity, or the amount of gametogenic material produced, is determined by food availability above minimum threshold levels. Spawning of gametes appears to rely on a combination of exogenous and endogenous factors. Once a state of gametogenic maturity is reached (through environmental and neurosecretory control), a variety of environmental stimuli can initiate spawning. The most universal of these is temperature. A rapid change in temperature appears to be more important than an absolute temperature or the direction of change. There is no critical, species-specific temperature for spawning. Physical disturbance (wind, tides) also stimulates spawning, as does the presence of gametes of the opposite sex (ectocrines). The influence of lunar periodicity on spawning is apparently indirect, resulting from temperature changes associated with spring tides. The effects of salinity and day length, are poorly understood. Changes in dissolved oxygen and pH are not relevant in seawater. Neurosecretory product such as serotonin and dopamine appear to be important in the spawning process. These chemicals or similar ones, released into the water with the gametes stimulate spawning in other members of the population, helping to synchronise the spawning process. Duration of spawning is limited by temperature and food supply and a return of the neurosecretory cycle to the stage coinciding with the resting stage. Year class success might depend to a large extent on spawning intensity and synchrony, which in turn depends on gametogenic synchrony within the population and the presence of an effective spawning stimulus. The timing of spawning with respect to conditions suitable for larval growth and survival undoubtedly also contributes to recruitment success. 6.6.3 Energy Metabolism The study of reproductive energy metabolism is concerned with the ways in which energy acquired from food is ultimately incorporated into gametogenic material. In scallops, nutrient reserves are stored primarily in digestive gland and adductor muscle body components during periods of somatic growth when food is abundant. Carbohydrate (mainly adductor muscle glycogen) appears to be of primary importance, but lipid stored in the digestive gland and protein stored in the adductor muscle also play important roles. These reserves may subsequently be catabolised to support maintenance metabolism when food is scarce and/or the production of gametes, depending on the timing of gametogenesis in relation to seasonal food supply. Barber (1984) and Barber and Blake, 1981 , Barber and Blake, 1983 , Barber and Blake, 1985a , Barber and Blake, 1985b defined the gametogenic energy metabolism of Argopecten irradians concentricus. A period of growth and energy storage in which the adductor muscle increases in size and glycogen level occurs in the spring when food is abundant, prior to the initiation of gametogenesis. Metabolism is supported primarily by digestive gland lipid, which is declining in level at this time. Early in the gametogenic cycle maximum O/NH3 and CO2/O2 (RQ) ratios indicate a shift to carbohydrate as the primary respiratory substrate, as the adductor muscle reaches maximal weight and its glycogen reserves begin to be utilised. As these glycogen stores are depleted in conjunction with gamete development, the adductor muscle decreases in weight and protein content. Gametes are mature by the end of the summer, as evidenced by maximum gonad weight and mean oocyte diameter. Metabolism becomes primarily protein based as the adductor muscle reserves continue to be catabolised. After spawning commences in the fall (when food energy is less available), energy reserves are depleted and scallop physiological condition is poor, as indicated by rapid carbon turnover and increasing senescence. This cycle of energy storage and utilisation is an integral part of the reproductive process, acting as a buffer between the regulating factors and the actual production of gametes. The energy cycle itself is undoubtedly controlled by neuroendocrine and environmental factors but is critical to reproductive success in that it provides the energy and material required for gamete synthesis. Several mechanisms for the transfer of energy to developing oocytes have been proposed. These include: 1) direct transfer (via the hemolymph) of fatty acids from the digestive gland to developing ova; 2) the conversion of carbohydrate (stored in the adductor muscle) to fatty acids which are incorporated into developing ova; 3) the breakdown of protein (Primarily adductor muscle tissue) to support energetic demands after other reserves are exhausted; 4) the recycling of energy from oocyte atresis; and, 5) intestinal loop transfer. 6.6.4 Applications to Aquaculture Compared to oysters, clams, and mussels, the commercial culture of scallops is relatively new. A greater understanding of the factors regulating gametogenesis and spawning of scallops will lead to a more consistent supply of seed organisms (Devauchelle and Mingant 1991) . Like other bivalves, scallops can be conditioned out of season with a microalgal diet, although more research is needed in order to optimise specific dietary requirements and delivery systems. This will lead to improvements in conditioning time and subsequent health of larvae. Once scallops are fully ripe, spawning can be induced using thermal or mechanical stimulation, injection of serotonin, exposure to sexual products, or some combination.