Factors effecting reproduction and larval development of Ostrea angasi (Sowerby 1871) : advancing hatchery production

The flat or native oyster, Ostrea angasi (Sowerby, 1871), was once a conspicuous inhabitant of many coastal bays and estuaries in south eastern Australia and formed the basis of a thriving wild fishery. Overfishing led to population collapses in NSW, Victoria, Tasmanian and South Australia, which in turn saw local demand for oysters being met through the production of Sydney rock oysters and the introduced Pacific oyster. As markets have developed and factors such as disease have impacted the production of other oyster species, there has been a renewed interest in production of O. angasi within the southern states. O. angasi are sequential protandric hermaphrodites and larviparous, with brooding adults retaining larvae on the gill filaments in the brachial chamber for approximately half the larval life cycle. Wild catch of O. angasi spat is low and unreliable and the cultivation of this species is reliant upon hatchery production. To date production techniques for O. angasi have been loosely based on those used for Saccostrea glomerata and Crassostrea gigas, and have varied in success, with many hatcheries still reporting poor outcomes. In part this has arisen from a lack of systematic assessments of the fundamental variables that determine hatchery success. Specifically, improvements in O. angasi production output and reliability require an improved understanding of the reproductive biology of the species and knowledge of conditions required to optimise late larval development and metamorphosis. In the first instance hatchery production is underpinned by the reproductive biology and ecology of this species. Knowledge of the temporal frequency of reproduction within New South Wales (NSW) estuaries is essential for larval acquisition and the development of a hatchery program. To address this, oysters were sampled over a 12 month period, across four estuaries, and examined histologically to reveal marked differences in gametogenic patterns to that previously reported in the southern states of Australia. The frequency of brooding oysters among farmed populations of O. angasi, in NSW waters, extended beyond that recorded in the Southern states of Australia. In the most northern and southern estuaries examined, brooding oysters were found between eight to nine months of the year with little synchronicity observed in reproductive timing within and between estuaries. Temperature, previously thought to halt gametogenic activity, was not restrictive to O. angasi reproduction in NSW farmed populations. For oviparous oyster species, assessments of larval rearing techniques frequently address specific ontogenetic stages, which begin with fertilisation success and culminate in competency and metamorphosis. For O. angasi, advanced stage larvae are acquired directly from brood-stock and greater emphasis is on the settlement and metamorphosis. This study sought to better understand the influence of the larval rearing environment on the morphological changes and physiological mechanisms associated with metamorphosis in O. angasi. Accordingly, techniques to reliably induce metamorphosis in competent larvae were assessed, and based upon these outcomes the impact of diet, temperature and salinity on later stage larvae were assessed, all with a view to improve O. angasi hatchery outcomes. Several neuroactive catecholamines (epinephrine, epinephrine bitartrate, L-3,4- dihydroxyphenalalanine and gamma amino butyric acid) were trialed to induce metamorphosis of O. angasi larvae. The optimal dose and duration of exposure of epinephrine and epinephrine bitartrate was determined by induced metamorphosis of O. angasi larvae. The capability to induce rapid metamorphosis in competent O. angasi, aside from single seed production for industry, offers an invaluable tool to assess the effects of rearing conditions on larval development and acquisition of competency. Using epinephrine induced metamorphosis, the influence of algal diet, rearing salinity and temperature on survival, growth and development of hatchery reared O. angasi, larvae was investigated. A comparison of dietary requirements for early stage larvae (140 - 230˜í¬¿m shell length) and late stage larvae (230 - 340˜í¬¿m shell length) was made. A series of uni, binary and ternary algal diets were assessed for their effect on the development of competency by induced larval metamorphosis. Optimal salinity (30 to 35) and temperature (26 to 29oC) were determined for larval rearing, larvae reared outside these salinity and temperature range exhibited, reduced growth, survival and/or delayed development. The effects of short term (1h) reduction in larval rearing temperature from 26oC significantly increased larval metamorphosis without affecting larval survival. However, short term increase in larval rearing temperature from 26oC adversely effected larval survival and metamorphosis, high lighting the need to assess rearing conditions during this critical stage of development. To ensure repeatability in outcomes, tests showed larvae sourced from different estuaries did not vary significantly in their metamorphic response to short term temperature manipulation and epinephrine induced metamorphosis. Collectively the work in this thesis has significantly increased our understanding of O. angasi reproductive patterns and larval biology. The seasonal availability of larvae and the effects of the main environmental factors purported to influence reproduction of O. angasi have been examined. The environmental factors that can be easily and economically adjusted for greater larval rearing success were determined. Additionally, this work adds to the broader understanding of the acquisition of competency and larval metamorphosis in bivalve molluscs. Commercially, O. angasi hatchery outcomes have been highly variable; however, beyond having developed protocols for the use of settlement inducers, significant improvements in hatchery success have accrued. At the Port Stephens Fisheries Institute, a facility that has produced commercial quantities of O. angasi spat for over 15 years, the cumulative improvements in larval rearing technology described in this thesis have increased the percentage of larvae that successfully metamorphose from an historical average of approximately 57% for the four most successful rearing attempts to an overall average of 68% for nine larval rearing attempts.