64 DECEMBER 2017 • WORLD AQUACULTURE • WWW.WAS.ORG Maturation Reproductive patterns differ among species and geographic locations; nevertheless, gametogenesis is controlled by endogenous rhythms and hormonal cues that are stimulated by exogenous parameters, such as environmental factors. Among these, water temperature and photoperiod are the two main factors affecting initiation of gamete synthesis and breeding periods in many marine invertebrates. Environmental cues often act synergistically to elicit biological responses but the mechanisms by which individuals interpret them are not fully understood. When adults receive environmental stimuli, hormones are produced to regulate physiological changes in gonads. Serotonin and noradrenalin levels, for example, increase in adductor muscle and gonad tissues of scallops (Martinez and Mettifogo 1998) and glycoside level increases in gonads of the sea cucumber A. japonicus during gametogenesis (Aminin and Anisimov 1987). In crustaceans, gonad maturation is controlled by a complex of glands located in the eyestalk. Eyestalk ablation is a common procedure in crustacean hatcheries to promote frequent sexual maturation (Huberman 2000). Manipulation of environmental parameters has improved synchronized gametogenesis and allowed gamete production outside the normal reproductive season. Out-of-season gametogenesis has been instrumental to the year-round production of gametes because several broodstock groups can be conditioned at different environmental settings. Spawning Natural spawning of marine invertebrates can be achieved under controlled environmental conditions. However, the perception of environmental stimuli leading to gamete release is a complex pathway that cannot always be completed in captivity. Changes in water temperature, sperm from a conspecific, microalgae, food concentration, and lunar cycles have induced spawning in many marine invertebrates. Thermal shock is an efficient technique to induce mature bivalves to spawn in captivity (Helm et al. 2004). More recently, chemicals have been used to induce final oocyte maturation and spawning. Injections of serotonin in scallops (Martinez et al. 1996) and potassium chloride in sea urchins (Williamson and Steinberg 2002) have induced spawning. Compounds such as 1-methyladenine, dithiothereitol and 2,3-dimercapto-1-propanol have been tested in sea cucumbers but success rates remain variable (Léonet et al. 2009, Chen et al. 1991). Steroids, serotonin, and dopamine have also been tested in crustaceans as an alternative to eyestalk ablation (Alfaro et al. 2004) but results have been inconsistent and spawning irregular. Gamete and egg quality During gametogenesis, oocytes acquire nutrients directly from adult diets or by transfer from muscle reserves and digestive glands (Villalaz 1994, Utting and Doyou 1992), providing a constant allocation of amino acids, fatty acids, vitamins and minerals to gametes. Quality of gametes and fertilized eggs can indicate if optimal conditions were provided to broodstock during gametogenesis and spawning. Biochemical composition has been used to evaluate egg quality. Insufficient reserves of fatty acids and amino acids in eggs for embryonic development can result in low hatching rates, high important for species with non-feeding larvae that depend exclusively on oocyte nutrient reserves for development (lecithotrophs). High fecundity, egg quality, and hatching rates have been associated with high levels of DHA and a high DHA:EPA ratio in bivalve and crustacean diets (Berntsson et al. 1997, Palacios et al. 1999). Conditioning Great success of bivalve broodstock conditioning was achieved with laboratory production of unicellular algae. However, choosing appropriate phytoplankton species requires consideration of nutritional content (Helm et al. 2004). Generally, a combination of diatoms and flagellates is provided to broodstock to provide a wide spectrum of nutrients because the biochemical composition of microalgae varies among species and culture conditions (Brown and Blackburn 2003). Concerns have also been raised about the nutritional value of cultured microalgae due to frequent bacterial contamination (Volkman et al. 1989). The high cost of microalgae production and frequent bacterial contamination during culture have led to the development of prepared algal pastes that consist of a concentrated solution of a mix of microalgae. These pastes can be used as a supplementary diet or fully replace live algae. Additionally, alternative diets have been tested such as dry microalgae, yeasts, and microencapsulated diets that can potentially have more stable nutritional content than live algae (Helm et al. 2004). However, the reproductive output of individuals fed exclusively with these diets has been variable and research is still needed to explore their benefits. Crustacean broodstock are still fed with fresh fish and bivalves. Although fresh fish might be more palatable, it causes water quality deterioration and potentially transmits diseases. Recently, efforts have focused on the development of formulated feeds (Bray et al. 1990). FIGURE 2. Figure 2. Conditioning and spawning of domesticated marine invertebrate broodstock in captivity. (A) Conditioning of Eastern oysters Crassostrea virginica prior to spawning. (B) Sea scallops Placopecten magellanicus were induced to spawn with intergonadal injections of serotonin. (C) Green sea urchins Strongylocentrotus droebachiensis after injection of potassium chloride in the coelomic cavity. Individuals are upside down in beakers filled with seawater where gametes will be released and collected. (D) Conditioning of American lobster Homarus americanus prior to breeding. Photos: Bruno L. Gianasi.
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