WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2017 31 In countries with a pearling tradition such as Japan, French Polynesia, Australia, and Indonesia, commercial pearl culture relied for years on wild spat collection (Monteforte et al. 1995). In recent years, overexploitation of natural beds has led to hatchery production of spat to ensure a continuous source of juveniles for pearl production (Southgate 2008). Despite this, vigor, health, and reliability of spat supplied through wild spatfall and hatchery production is yet to be determined. Pearl oysters have been cultivated in hatcheries since the 1970s. Thus, successful protocols are now well established for the three main pearl-producing species: Pinctada fucata, P. maxima, and P. margaritifera (Southgate 2008). In Mexico, there is limited information regarding optimum conditions to maintain larvae and spat of the mother-of-pearl oyster P. mazatlanica (Martínez-Fernández et al. 2003, Saucedo et al. 2005, 2007) and the winged pearl oyster Pteria sterna (McAnally and Valenzuela 1990, Araya-Nuñez et al. 1991, 1995, Serrano and Salinas 1993). With the winged pearl oyster in particular, these studies report slow larval growth, delayed settlement of spat, or failure to complete metamorphosis, suggesting that culturing conditions were not optimal. This study reports the first case of improved development and settlement of winged pearl oyster larvae, as well as successful massive production of spat in the hatchery. We also describe the main morphological changes of larvae and spat during development. Spawning and Larval Rearing Winged pearl oyster broodstock were collected from the wild in spring (April), when water temperature was 22-23 C and gonads were ripe (Saucedo and Monteforte 1997). They were transferred to the laboratory, cleaned, and induced to spawn by thermal stimulation (19 to 26 C; 30-45 min at each temperature). Larvae were raised according to Saucedo et al. (2005, 2007). Briefly, gametes were fertilized in 120-L bins and veliger larvae were raised in triplicate 1500-L white conical fiberglass tanks containing 1-µm filtered, gently aerated seawater. Stocking density started at 13-14 larvae/ mL to resemble the natural affinity of wild animals to aggregate and form clusters (Moneforte et al. 1995). Density gradually decreased and reached 1 larva/mL after day 15. Throughout the trial, water temperature was set to 24 ± 1 C and salinity to 37 g/L. Larvae were fed Isochrysis galbana (Clon T-ISO) at 10 × 103 cell/mL (days 1-8) Advances in Hatchery Production of the Winged Pearl Oyster Pedro E. Saucedo and a 1:1 mixture (cell count) of I. galbana and Chaetoceros muelleri at 30 × 103 cell/mL (day 9 to settlement). Culture tanks were drained, washed, and filled with clean seawater every three days. Pediveliger larvae were settled in identical, triplicate 1500-L tanks containing collectors made from plastic fiber bags (both the outer bags and inner substrates) (Saucedo et al. 2005). Tank water was renewed daily and the same microalga mix as that fed to larvae was provided at 60-80 × 103 cell/mL. Collectors were kept in settlement tanks for one month. After this period, spat were recovered and counted to determine total yield. This procedure was done for the outer bag, inner substrate, and tank walls and bottom. Larvae and spat samples were collected in triplicate from all sieves during tank draining. They were fixed, counted, and data used to determine cumulative survival. Larvae and spat were also photographed and the digital images were processed with Image Pro Plus 6.01 to determine absolute growth (µm) and describe anatomical changes during development. Early Larval Development and Settlement Survival and growth of larvae during development are shown in Figure 1. Two hours after fertilization, free-swimming trocophore larvae (42 ± 4 µm) without a visible shell and with a relatively round shape display a clear velum with cilia (Fig. 2a). After 18-22 h, the early veliger larvae (65 ± 7 µm shell height) have a concave, thin, semi-transparent prodisoconch with a distinctive velum with cilia (Fig. 2b); no developing organs are visible at this stage; the hinge is 68-70 µm long; survival was 70 percent. About day 8, the first umbo larvae measure ~100 µm shell height and show a very robust velum and cilia (Fig. 2c); the prodisoconch increases in thickness, the shell shape changes to roundly-oval, and the hinge smooths as the umbo develops as the most conspicuous shell structure; mean survival was 25-35 percent. Pediveliger larvae occurred on days 15-17 (Fig. 2d); they reached 198-205 µm shell height; the velum nearly disappears and the foot develops as larvae adopt the benthic stage; when completely distended, the foot may be 316 µm long; the eye spot is very small and not easily visible; mean survival was ~21 percent. Metamorphosis and settlement of pediveliger larvae started on day 19 and lasted 2-3 days, yielding a final survival rate of 17 percent (CONTINUED ON PAGE 32) FIGURE 1. Mean survival and growth of hatchery-reared winged pearl oyster larvae during development.
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