Wassnig, Matthew (2011) Developing hatchery culture techniques for the winged pearl oyster, Pteria penguin (Röding, 1798). PhD thesis, James Cook University.

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Abstract

Pearl culture has traditionally relied on collecting juvenile pearl oysters (family: Pteriidae) from the wild and growing them to an appropriate size to be used in pearl production; a method that has become increasingly less viable due to a corresponding depletion in wild populations. Hatchery propagation of juvenile pearl oysters is now a necessity in regions where collection from the wild can no longer sustain commercial pearl production. The cultured pearl industry in the Indo-Pacific includes production of half-pearls or 'mabè' from Pteria penguin (Röding 1798). Pearl production from P. penguin has become progressively more reliant on hatchery culture of oysters; however, efficient production is constrained by a lack of knowledge regarding optimal culture techniques. This study aimed to develop hatchery culture techniques that could be implemented by industry to improve the prospects for pearl production from P. penguin and decrease the fishing pressure currently placed on wild populations.

The natural spawning season for P. penguin typically spans only a few months of each year, providing a short window for hatchery production of juveniles. A lack of knowledge regarding the diet and conditions required to optimise energy uptake by adult P. penguin has impeded the development of brood-stock conditioning programs that could be used to encourage gametogenesis outside of the natural spawning season. This study examined the pattern of suspension feeding by P. penguin in response to variations in microalgae diet, food concentration and water temperature. Brood-stock were placed in temperature controlled flow-though chambers that supplied individual oysters with a constant concentration of suspended microalgae. Feeding behaviour was quantified as the rate at which water was cleared of algae cells (clearance rate; CR) and the fraction of organic carbon absorbed during digestion (absorption efficiency; AE). The results showed that CR was greater when feeding on the two flagellate species Isochrysis sp. Tahitian (T-Iso) and Pavlova sp. (mean = 32 L h⁻¹ oyster⁻¹) when compared to the diatom Chaetoceros muelleri (27 L h⁻¹ oyster⁻¹). At temperatures of 24-28°CP. penguin maintained a stable CR with increasing food availability up to the maximum concentration tested (50 x 10³ cells mL⁻¹). Mean AE was highest for T-Iso (61%) and not influenced by food concentration. Decreased CR and AE in response to a rapid reduction in water temperature during summer, reflected the 35% lower CR and 47% lower AE observed during the colder austral winter, suggesting temperature contributes to differences in suspension feeding between seasons.

The high feeding capacity of P. penguin raised the issue that in order to undertake brood-stock conditioning, hatcheries would require the facilities to culture large volumes of live microalgae. Pearl farms that use P. penguin are typically located in regional areas where the technical capacity for mass algae culture is not available. An experiment was conducted to assess the viability of using commercially available concentrated microalgae and a unique flow-though aquarium system to condition brood-stock prior to the natural spawning season. Fifteen P. penguin of a similar size were distributed between 5 identical 30 L 'flow-though aquaria'. A mixed diet of concentrated microalgae from the Instant Algae® range was supplied to brood-stock for a period of 40 days with periodic increases (10 day intervals) in food concentration and water temperature up to a maximum of 40 x 10 cells mL⁻¹ and 28°C. Histological examination of gonad tissue was conducted at the conclusion of the study so that the reproductive condition of each oyster could be categorized using the five stages for pearl oyster gonad development, ranging from inactive to ripe. The same process was conducted for 15 similar sized P. penguin that were held in ocean culture for the same time period. The reproductive state of conditioned animals suggested that male P. penguin produced spermatozoa at a rate exceeding that observed in a wild environment over the same time period. The production of mature oocytes in experimental females was less reliable, attributable to the period for conditioning being too short for the production of energetically expensive ooyctes.

In order to develop techniques for hatchery culture of juvenile P. penguin, it was first necessary to understand the processes of embryogenesis and larval development for this species. Following standard methods used in the hatchery culture of other pearl oyster species, P. penguin eggs were spawned, fertilized and incubated until they hatched into shelled veliger larvae. Larvae were then fed a diet of live microalgae until developing a foot and being deemed competetent to settle. Embryos and larvae were sampled periodically during hatchery culture to be examined under scanning electron microscope (SEM). The resulting high resolution images were then used to map the approximate timing of developmental stages. These stages included the first cleavage (1 h post-fertilisation; hpf), morula (2.5 hpf), blastula (4.5 hpf), gastrula (5.5> hpf), trochophore (7> hpf), D-stage (20 - 22 hpf), prodissoconch II (3 - 6 days post-hatching into D-stage; dph), umbone (10 - 12 dph) and pediveliger (22 dph). Comparison with patterns of embryogenesis and larval development in other oviparous oyster species revealed a similar sequence of key events, with differences occurring in the timing of developmental stages, shell structure and shell shape.

Embryo incubation is a period of pearl oyster culture that is typically characterised by excessive mortality. This study addressed the issue of embryo mortality by examining the effects of egg stocking density and the application of antibiotics during incubation. A factorial experimental design combined three egg densities (10, 50 and 100 mL⁻¹) and three antibiotic treatments (Control - no antibiotic; 5 mg mL⁻¹ streptomycin-sulfate; 5 mg mL⁻¹ tetracycline:erythromycin 2.5:2.5 mg mL⁻¹). Antibiotics were added to the culture medium as a single dose and fertilised eggs were incubated for a period of 24 h. Tetracycline:erythromycin (1:1) improved mean survival (23%), but yielded an average of only 9% more veliger larvae than control aquaria due to interference with development. The antibiotic streptomycin-sulfate improved mean survival by 16% when compared to control aquaria, without significantly compromising development. A high egg density of 100 mL⁻¹ did not significantly reduce survival, but resulted in a 5% reduction in normal development to D-stage. It is recommended that eggs be stocked at a density ≤50 mL⁻¹ and mortality be minimised by treating the culture medium with the antibiotic streptomycin-sulfate.

Hatchery culture in regional areas is often impossible because farms cannot afford the facilities required to produce the live microalgae used as a food source for larvae. Concentrated algal paste supplied by Instant Algae® has been successfully trialled as an alternative food source during hatchery culture of P. penguin, but the feeding regime that promotes optimal larval growth and development is yet to be determined. Experiment 1 assessed the combined effects of stocking density and feed ration on the survival and growth of P. penguin larvae during D-stage (1-8 days post-fertilisation). Experiment 2 examined the effects of the same treatments on the survival and growth of larvae during umbo-stage (8 - 17 days post-fertilisation). Both experiments used a factorial design combining 3 egg stocking densities (Experiment 1: 2, 6 and 10 larvae mL⁻¹; Experiment 2: 1, 3 and 5 larvae mL⁻¹) and 3 levels of feed ration (Experiment 1: 5, 10 and 15 x 10³ cells mL⁻¹; Experiment 2: 10, 15 and 20 x 10³ cells mL⁻¹). Survival during D-stage was significantly enhanced (by 105%) in aquaria stocked at <10 larvae mL⁻¹, whereby a density of 6 mL⁻¹ maximised larval production per volume of culture medium. An intermediate feed ration of 10 x 10³ cells mL⁻¹ maximised both survival and growth during D-stage. Increasing the initial stocking density of umbostage larvae from 1 to 3 mL⁻¹ resulted in a significant reduction of both survival (360%) and growth (16%). Growth of umbo-stage larvae stocked at 1 mL⁻¹ increased significantly (7%) when feed ration remained below 20 x 10³ cells mL⁻¹.

Optimising the rate of larval settlement during pearl oyster hatchery cultivation is critical to maximising the production of juvenile spat for commercial use and is reliant on providing suitable stimuli. This study used two experiments to investigate the effects of (1) treating the culture medium with alternate concentrations of three chemical compounds (Serotonin; GABA; KCl) both in the presence/absence of a bio-film and (2) exposure to five substrate types (red nylon mesh with 5 mm and <1 mm pore sizes; black fibreglass mesh with 3mm and 1mm pore sizes; transparent smooth plastic) both in the presence/absence of a chemical cue (KCl), on recruitment of P. penguin pediveliger larvae. After 48 h, settlement was 65% greater in aquaria containing a substrate covered by a naturally formed bio-film than in control aquaria. After 72 h, settlement of larvae in aquaria treated with serotonin (10⁻³M) or KCl (20 mM) was significantly greater than in control aquaria by 75% and 84%, respectively, while exposure to GABA had no effect. Settlement in response to 20 mM KCl was enhanced by the presence of a red nylon mesh substrate with 5 mm pore size.

The findings of this PhD project provide practical knowledge regarding techniques for efficient hatchery culture of P. penguin. The specific aims of this study place emphasis on facilitating hatchery propagation in regional communities within the Indo-Pacific. This research will aid in increasing pearl production from hatchery bred P. penguin and therefore alleviate much of the pressure currently being placed on overexploited wild populations.

Item ID:	29260
Item Type:	Thesis (PhD)
Keywords:	Pteria penguin; pearl oyster; hatchery culture; survival development; settlement; optimum culture techniques
Related URLs:	http://eprints.jcu.edu.au/22224/ http://eprints.jcu.edu.au/20788/ http://eprints.jcu.edu.au/22225/
Additional Information:	Publications arising from this thesis are available from the Related URLs field. The publications are: Chapter 4: Wassnig, Matthew, and Southgate, Paul C. (2012) Embryonic and larval development of Pteria penguin (Röding, 1798) (Bivalvia: Pteriidae). Journal of Molluscan Studies, 78 (1). pp. 134-141. Chapter 5: Wassnig, Matthew, and Southgate, Paul (2011) The effects of egg stocking density and antibiotic treatment on survival and development of winged pearl oyster (Pteria Penguin, Röding 1798) embryos. Journal of Shellfish Research, 30 (1). pp. 103-107. Chapter 7: Wassnig, Matthew, and Southgate, Paul C. (2012) Effects of settlement cues on behaviour and substrate attachment of hatchery reared winged pearl oyster (Pteria penguin) larvae. Aquaculture, 344 . pp. 216-222.
Date Deposited:	12 Sep 2013 00:15
FoR Codes:	05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050209 Natural Resource Management @ 50% 06 BIOLOGICAL SCIENCES > 0608 Zoology > 060808 Invertebrate Biology @ 50%
SEO Codes:	83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8301 Fisheries - Aquaculture > 830104 Aquaculture Oysters @ 50% 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
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