Author: Warwick James Noble
Noble, Warwick James, 2014 Aspects of life history and ecology of Dicathais orbita Gmelin, 1781 related to potential aquaculture for bioactive compound recovery, Flinders University, School of Biological Sciences
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World aquaculture production, including the production of pharmaceuticals and nutraceuticals, is increasing to supplement fisheries harvest from wild stocks. Muricids (Neogastropoda) are widely fished around the globe and produce a range of interesting bioactive compounds (Chapter 1). Several species of muricids have been successfully cultured to supply seafood markets. The southern Australian muricid, Dicathais orbita, is recreationally harvested for food and more recently has been shown to produce potent bioactive compounds of interest for development as pharmaceutical leads. Successful aquaculture of D. orbita would provide a sustainable supply for ongoing development of pharmaceutical leads, as well as for seafood markets. Information on aspects of the life history of D. orbita will underpin successful aquaculture production. Neogastropods are dioecious, but morphologically identical when in their shell. In order to non-destructively assess effective population size of wild stocks of D. orbita and manipulate lab held stock, it was necessary to develop a means to identify the sex of the species. A suite of anaesthetics were trialled for their efficacy in relaxing D. orbita out of the shell to identify sex organs and for stimulating bioactive compound production through a stress response (Chapter 2). Magnesium chloride proved most effective in relaxing D. orbita specimens enough to identify sex. Benzocaine and the carrier solvent ethanol were less effective for identifying sex, but stimulated expulsion of the bioactive precursors. The presence of bioactive brominated indoles in the expelled mucus of D. orbita was confirmed by liquid chromatography/mass spectrometry and provides a novel, sustainable means for obtaining these compounds without killing the snails. The reproductive cycle of D. orbita was studied using a gonadosomatic index in wild and captive populations, along with spawning and post hatching larval development (Chapter 3). D. orbita follows an annual reproductive cycle, peaking in early summer (December) in South Australia. Female D. orbita spawn ~ 40 egg capsules in a session and each capsule contains an average of 5542 eggs with an average diameter of 105.2 µm. Post-hatching larval development proceeded through 5 stages over 41 days with average shell length increasing from 253 to 974.3 µm and shell width increasing from 203.8 to 980.5 µm. Information on the reproductive cycle and larval development patterns of D. orbita will allow for enhancement of reproductive condition and larval production. Larval rearing experiments, to determine the effects of temperature and diet on the growth and survival of D. orbita larvae under laboratory conditions, used five different unicellular algal diets for larvae maintained at 16µC and 22µC (Chapter 4). Larvae reared at 22µC on a mixed diet, or diatoms alone, performed significantly better than those reared on green microalgal diets alone. Trials with settlement cues were undertaken on newly hatched to 38 day old D. orbita larvae to determine when larvae become competent. An array of natural cues (carrion, Xenostrobus pulex, adult mucus and Ulvella lens), as well as concentrations of KCl were tested. 20 mM KCl induced the greatest settlement, however no larvae metamorphosed under the conditions provided. In conclusion, D. orbita can be grown under laboratory conditions and are highly fecund. Bioactive compounds can be extracted non-destructively from D. orbita, providing an extra resource that can be collected to value-add to seafood aquaculture. Larvae are planktotrophic with higher development rates at the higher water temperatures within their local range and grow to greater size on a diet comprised of both green microalgae and diatoms. D. orbita is iteroparous with an annual reproductive cycle. High fecundity indicates that larval survival and recruitment is the limiting phase of the life cycle for D. orbita. Further studies will optimise culture conditions and cues for settlement and metamorphosis in order to close the life cycle of D. orbita for aquaculture production.
Keywords: Dicathais orbita,muricid,whelk,aquaculture,seafood,bioactive compound
Subject: Biology thesis
Thesis type: Doctor of Philosophy
School: School of Biological Sciences
Supervisor: James Harris