Norman J. Blake

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.

Development of an Argopecten-Specific 18S rRNA Targeted Genetic Probe.

Abstract: Comparison of 18S ribosomal RNA gene sequences between diverse bivalve species, including eight scallop species, allowed the design of an 18S rRNA targeted oligonucleotide probe (BS-1364) that was specific for scallops belonging to the genus Argopecten (bay and calico scallops). The high sequence similarity of the 18S rRNA gene between Argopecten irradians and Argopecten gibbus (98.8%) prevented the design of an A. irradians species-specific probe. Hybridization studies using amplified 18S rDNA from a diverse collection of bivalve species demonstrated that the specificity of the digoxygenin-labeled probe was consistent with the predicted specificity indicated by sequence comparison. Hybridization studies using laboratory-spawned bay scallop veligers indicated that a single veliger could be detected by probe hybridization in a blot format, and that probe hybridization signal was proportional (r2= .99) to the abundance of veligers. Methods for rRNA extraction and blotting were developed that allowed bay scallop veligers to be specifically and quantitatively identified in natural plankton samples. Preliminary studies conducted in Tampa Bay, Florida, suggest that introduced scallops can successfully spawn and produce veligers under in situ conditions. The Argopecten-specific probe and methods developed in this study provide the means to study the production and fate of bay scallop larvae in nature and provide evidence that scallops introduced into Tampa Bay have the potential for successful reproduction and enhancement of scallop stocks.

RESTORATION OF BAY SCALLOP (ARGOPECTEN IRRADIANS (LAMARCK)) POPULATIONS IN FLORIDA COASTAL WATERS: PLANTING TECHNIQUES AND THE GROWTH, MORTALITY AND REPRODUCTIVE DEVELOPMENT OF PLANTED SCALLOPS

Abstract Bay scallops (Argopecten irradians [Lamarck]) are a culturally and economically important component of Floridas nearshore marine community. However, many of the local populations that compose the bay scallop metapopulation in Florida have virtually disappeared since the 1950s. This study reports the results of a 3-year effort to restore bay scallop populations at 4 sites along the west central coast of the state (Tampa Bay, Anclote, Homosassa and Crystal River). During late summer of 1997, 1998 and 1999, wild adult scallops were retrieved from each of those four target sites and induced to spawn in the laboratory. The resultant offspring were grown to at least 20-mm shell height in a nursery setting and then transplanted to cages deployed at the site where their parents were originally harvested. The growth, survival and reproductive development of the planted scallops were recorded on an approximate 6-week schedule. Results suggest that caged scallops generally grew more slowly than their wild counterparts and that at most of the planting sites mortality was high, especially during late summer. Reproductive development and spawning, although delayed in the caged scallops relative to their wild conspecifics, appears to have proceeded in an otherwise normal fashion. Approximately 1,100 scallops survived and spawned during the first year of the project, whereas ~4,700 and 12,000 scallops survived to spawn in the second and third years of the project, respectively. Studies were also conducted to determine the optimal stocking density and the best placement of the cages. Results of the density study indicate that planting at lower densities increased growth and survival but did not necessarily result in more live scallops at the time of spawning. Results of the cage-placement study, which compared scallops planted in cages either inside or outside of a seagrass bed and either mounted on legs or placed directly on the sediment, revealed that scallops planted directly on the substrate within a seagrass bed suffered higher mortality and slower growth than did scallops planted in the other three treatment combinations. Overall results of this 3-year project suggest that planting cultured scallops in cages can be a successful strategy for increasing the local spawner stock density of bay scallops in depleted populations and, ultimately, for increasing larval supply to the metapopulation.

THE REPRODUCTIVE CYCLE OF THE FLAME SCALLOP, CTENOIDES SCABER (BORN 1778), FROM THE LOWER FLORIDA KEYS AND ITS RELATIONSHIP WITH ENVIRONMENTAL CONDITIONS

Abstract The reproductive cycle, sex distribution, and gonadal characteristics of the flame scallop, Ctenoides scaber, formerly Lima scabra scabra (Born 1778), collected from Boca Chica Key, FL, were investigated over the 21-mo period from January 1998 through September 1999. Gametogenic cycles were examined using qualitative and quantitative methods, and the relationships between those observations and environmental conditions (e.g., water temperature, salinity and phytoplankton concentrations) were analyzed. The relationships between sex, gonad color and shell height were also examined. Gamete development in both sexes was initiated in winter and was associated with small oocytes and follicles, cool water temperatures and moderate concentrations of food. Growth of gametes occurred throughout spring, as temperature and chlorophyll-a concentrations increased. A partial synchronous spawn occurred in early summer but did not seem to be related to environmental conditions. Maximum gamete ripeness and size occurred in late summer, when water temperatures were at maximum values and food densities were increasing. Decreases in female gamete and follicle sizes and increases in occurrence of partially spawned, spent and early growth gonads in autumn were suggestive of synchronous spawning, which coincided with a rapid decrease in water temperature and maximum measured chlorophyll-a concentrations. Decreases in oocyte size in February coincided with annual water temperature minimums and may represent the conversion of energy from reproduction to survival, and not spawning. The presence throughout the year of juveniles, ripe and partially spawned flame scallops and chlorophyll-a concentrations sufficient to support gamete development suggest a reproductive strategy of continuous spawning, common in tropical marine invertebrates. No relationship was detected between salinity and gonad condition. Flame scallops collected for this study ranged in size from 21–68 mm shell height (SH) and those >25 mm SH had gamete development. The smallest animals were found principally in summer, which suggests a massive synchronous spawning event. Analyzing sex distribution by SH showed that flame scallops are protandric sequential hermaphrodites. Flame scallops <40 mm SH were predominantly male (83%), those ≥40 mm SH were mostly female (71%), and 4% were in sexual transition near 40 mm SH. The sex ratio for the sampled population was 0.63M:1F. Histologic examination of fresh gonadal tissues revealed that female gonads were predominantly purple (96%) and male gonads were predominantly cream-colored (91%). Other gonad colors observed were not reliable indicators of sex nor was there a clearly defined association between color and those animals in sexual transition. Documenting the reproductive cycle of Ctenoides scaber in the existing Florida fishery is the first logical step to understanding its life history. Because flame scallops are important to the marine aquarium industry in the United States and are a potential food source for humans, it is important that we understand the reproductive cycle of this species to ensure proper management policies. This study provides basic information applicable to mariculture of flame scallops for commercial production to supplement the harvest of wild stock for the marine aquarium industry.

The giant Malaysian prawn, Macrobrachium rosenbergii, a potental predator for controlling the spread of schistosome vector snails in fish ponds

Abstract Under laboratory conditions the giant Malaysian prawn, Macrobrachium rosenbergii , was found to prey upon two major South American species of schistosome vector snails, Biomphalaria glabrata and B. tenagophila . Juvenile prawns fed on the snails with a consumption rate of 2.65–3.5% of prawn body weight per day, whether they were given an alternative food source (5% of body weight per day in marine ration) or not. Postlarval prawn were fed at a rate of 25% of prawn body weight per day in addition to the marine ration. The potential of M. rosenbergii to serve in the biological control of schistosome vector snails in fish ponds as well as its commercial value as an aquaculture crop is discussed.

Oxygen consumption of the deep-sea crabs Chaceon fenneri and C. quinquedens (Brachyura: Geryonidae)

1. 1. Oxygen consumption rates (VO2) were determined for the deep-sea crabs Chaceon fenneri and C. quinquedens, two important members of the continental slope megafauna in the eastern Gulf of Mexico. 2. 2. The VO2 of C. fenneri declined from 0.014 ml O2/g/hr at 12°C to 0.010 ml O2/g/hr at 6°C; VO2 of C. quinquedens showed a decline from 0.012 ml O2/g/hr to 0.008 ml O2/g/hr over the same temperature range. 3. 3. The VO2 of C. fermeri and C. quinquedens are comparable to those of similar size shallow water decapod crustaceans that inhabit equivalent temperatures. 4. 4. The oxygen consumption rates of C. fenneri and C. quinquedens decline with increasing depth of occurrence purely as a function of temperature.