Mitochondrial and nuclear genetic analyses of the tropical black-lip rock oyster (Saccostrea echinata) reveals population subdivision and informs sustainable aquaculture development

The black-lip rock oyster (Saccostrea echinata) has considerable potential for aquaculture throughout the tropics. Previous attempts to farm S. echinata failed due to an insufficient supply of wild spat; however, the prospect of hatchery-based aquaculture has stimulated renewed interest, and small-scale farming is underway across northern Australia and in New Caledonia. The absence of knowledge surrounding the population genetic structure of this species has raised concerns about the genetic impacts of this emerging aquaculture industry. This study is the first to examine population genetics of S. echinata and employs both mitochondrial cytochrome c oxidase subunit I gene (COI) and single nucleotide polymorphism (SNP) markers

A non-destructive tissue sampling technique for holothurians to facilitate extraction of DNA for genetic analysis

Increasing demand and overfishing of high-value species has promoted interest in both conservation and aquaculture initiatives supporting stock restoration programs for tropical sea cucumbers. Accordingly, there is a need for baseline information on the genetic structures and relatedness of sea cucumber populations to support sustainable implementation of mariculture and conservation programs, which often involve coastal communities in developing countries. Identification of a non-destructive tissue sampling technique for sea cucumbers that allows extraction of high-quality genomic DNA in a sustainable and culturally appropriate way is thus required. Six sampling techniques were assessed for their suitability to collect tissue for DNA extraction from sandfish (Holothuria scabra): core needle biopsy, punch biopsy, shave biopsy, buccal swab, anal swab, and evisceration. The quantity, quality, and purity of extracted DNA were compared to assess the relative merit of each sampling method. The swab biopsy method produced the best quality DNA agarose band image, and evisceration resulted in the highest yields of DNA, at an average of 525.9 (±98.0) μg g⁻¹. However, when considering all criteria assessed, the swab biopsy methods (both buccal and anal) proved superior. Swabbing not only produced the best quality DNA agarose band image, it was also the only technique that produced DNA that amplified 100% of the time across both extraction protocols. We hope that the non-destructive sampling techniques evaluated in this study provide a foundation for the genetic analysis of sea cucumber stocks to support their conservation and management

Evaluating spawning induction methods for the tropical black-lip rock oyster, Saccostrea echinata

When developing a species-specific hatchery protocol it is important to investigate the triggers for spawning, which is the foundation of the production cycle. This study evaluated multiple spawning induction techniques to optimise the spawning success of black-lip rock oyster, Saccostrea echinata, broodstock. Initially, the most effective method for non-destructively opening broodstock was determined, to allow intramuscular injection of chemical stimuli. Following this, the efficacy of seven spawning induction treatments, including two physical shock techniques, three chemical induction techniques, a combination of physical and chemical induction and strip spawning were assessed. Based on the outcomes of these trials, the combined and independent effects of reduced salinity and addition of sperm, as well as the potential of neuropeptides to trigger spawning, were evaluated. Results demonstrated that a concentration of 30 g/L of MgCl2 is an effective muscle relaxant for commercial and research application for S. echinata. It was determined that salinity reduction, rather than temperature increase, is a key factor for spawning induction. Strip spawning is a viable option for S. echinata; however, sperm motility was significantly affected, and fecundity and fertilisation rates were lower (although not significant, P > 0.05) when compared to other induction methods tested. A combination of physical and chemical induction, involving reduced salinity and addition of sperm, was the most successful treatment; 80 % of broodstock spawned within 19 min of salinity drop and addition of sperm. This method delivers significant production improvements for S. echinata, particularly regarding the time taken to induce spawning, and is therefore recommended for application in the hatchery

Elucidation of fine-scale genetic structure of sandfish (Holothuria scabra) populations in Papua New Guinea and northern Australia

The development of a sandfish (Holothuria scabra) mariculture industry within Papua New Guinea (PNG) is of great socio-economic importance. However, the lack of knowledge surrounding the current population genetic structure throughout the region has raised concern about the genetic impacts of hatchery-augmented sea ranching on already diminished wild populations. The present study evaluated the current population genetic structure of sandfish within PNG, and more broadly across northern Australia, to inform sustainable mariculture practices and provide baseline genetic data within these regions. Microsatellite-based population genetic analyses were used to determine the genetic diversity within subpopulations. This analysis found that although microsatellite loci varied widely in the number of alleles (3-28), the overall allelic diversity was similar among all populations sampled. The level of genetic substructuring among all populations sampled was low, although significant (F-ST = 0.037, P = 0.000). Most of these differences were driven by distinctness of the Australian populations from those in PNG, whereby results indicated that PNG populations exhibited a panmictic stock structure. No distinct patterns of genetic isolation by distance were detected among the populations examined. Information obtained from the present study will improve the management of restocking programs and support a sustainable future for the PNG sandfish mariculture industry

Cadmium uptake and zinc-cadmium antagonism in Australian tropical rock oysters: potential solutions for oyster aquaculture enterprises

Variable and occasionally high concentrations of cadmium in wild oysters at a remote location with the potential to develop aquaculture enterprises motivated research into the distribution and sources of metals in oysters, seawater, sediment, suspended solids and phytoplankton. Saccostrea mytiloides and Saccostrea mordax contained cadmium concentrations exceeding the food standard maximum level (ML) at three of four sites. At one site with high zinc levels in sediment, oyster cadmium levels were below the ML. Metal levels in seawater were not correlated with cadmium levels in oysters but high cadmium/zinc ratios were measured in Trichodesmium erythraeum blooms. We suggest that oysters accumulate cadmium mainly from annual phytoplankton blooms except at sites where zinc availability is sufficiently high to prevent uptake though a mechanism of antagonistic exclusion. Knowledge of the source and uptake mechanisms of cadmium in oysters should lead to new management strategies to reduce cadmium levels in farmed oysters

Growth of <i>V. parahaemolyticus</i> in Tropical Blacklip Rock Oysters

The opportunistic pathogen Vibrio parahaemolyticus poses a significant food safety risk worldwide, and understanding its growth in commercially cultivated oysters, especially at temperatures likely to be encountered post-harvest, provides essential information to provide the safe supply of oysters. The Blacklip Rock Oyster (BRO) is an emerging commercial species in tropical northern Australia and as a warm water species, it is potentially exposed to Vibrio spp. In order to determine the growth characteristics of Vibrio parahaemolyticus in BRO post-harvest, four V. parahaemolyticus strains isolated from oysters were injected into BROs and the level of V. parahaemolyticus was measured at different time points in oysters stored at four temperatures. Estimated growth rates were −0.001, 0.003, 0.032, and 0.047 log10 CFU/h at 4 °C, 13 °C, 18 °C, and 25 °C, respectively. The highest maximum population density of 5.31 log10 CFU/g was achieved at 18 °C after 116 h. There was no growth of V. parahaemolyticus at 4 °C, slow growth at 13 °C, but notably, growth occurred at 18 °C and 25 °C. Vibrio parahaemolyticus growth at 18 °C and 25 °C was not significantly different from each other but were significantly higher than at 13 °C (polynomial GLM model, interaction terms between time and temperature groups p V. parahaemolyticus growth data will inform regulators and assist the Australian oyster industry to develop guidelines for BRO storage and transport to maximise product quality and safety