Culture potential and population structure of the indigenous oysters Striostrea margaritacea and Saccostrea cucullata in South Africa

[摘要] ENGLISH ABSTRACT: Oyster culture has a long standing history with mankind and movement of indigenous oysters, in South Africa, dates back to the 1600s. Crassostrea gigas (the Pacific oyster) accounts for 80% of global oyster culture and is the sole oyster species cultured in South Africa. Marine environments are vulnerable to invasion, not only by the introduced species but also their associated epifaunal organisms and parasites, as well as the potential for intraspecific disease transmission between introduced and indigenous species. The culture of indigenous oysters, within their range, could reduce these risks as well as relieve commercial harvesting pressure on the current wild stocks. Of South Africa's five indigenous oysters the two most palatable ones were chosen for this study, Striostrea margaritacea (Cape rock oyster) and Saccostrea cucullata (Natal rock oyster). To develop a new culture species requires knowledge of population densities, distribution, biology, growth rates, and settlement success. To minimize the potential impact of such mariculture, genetic structure and diversity should also be examined. This study focused on genetic structure and diversity, as well as spat settlement and the impact of a Harmful Algal Bloom on spat settlement.Samples of S. margaritacea were taken from five equidistant locations throughout the species' range in South Africa, from the Breede River to Westbrook in KwaZulu-Natal; the same was done for S. cucullata from three equidistant locations from Mtakatye in the Eastern Cape up to Umdloti in KwaZulu-Natal. Genetic analyses assessed the CO1 and 16S mtDNA gene regions and both showed that S. margaritacea populations have relatively high genetic diversity and high levels of gene flow along South Africa's coastline (Fst>0.02, p>0.05 in both cases). The populations that were thought to be S. cucullata have a high level of fixation between populations for CO1 and 16S (Fst>0.86 in both cases, p<0.01); the fact that this Fst value is so high suggested that these populations are not related.What was thought to be Saccostrea cucullata in South Africa is now show to be two distinct species (S. cucullata and, what appears to be, S. mordax) that appear to be separated at some point between Port Edward (south KwaZulu-Natal) and Mtakatye. However, there appears to be an overlap in range as one sample from Mtakatye groups with those from the northern populations. Although the northern population samples do not form a single clade with S. mordax in the phylogenetic tree, there is a shell morphological characteristic (radial groves) that seems to be shared between the two that is not shared by other Saccostrea species. This is the first record of more than a single Saccostrea species on South Africa's coastline. If Saccostrea were to be used for mariculture the species identity would have to be confirmed before breeding in hatcheries could occur. Both S. cucullata and S. mordax are exceptionally similar morphologically and the most accurate method of identification would be using CO1 barcoding. A small tissue sample could be extracted by anesthetising the oyster with magnesium chloride and extracting a small section of mantle tissue. Further study should be conducted to confirm the identity of the two Saccostrea species; this study should include a taxonomic study and genetics using nuclear and mitochondrial markers. The true range of the Saccostrea species should also be determined as they may have overlapping geographic ranges.Spat settlement trails were performed in Port Alfred in the Eastern Cape over a six month period (November 2013-April 2014). Two temporally separate trials were undertaken throughout the oyster spawning season from 6 November 2013 to 11 February 2014 and 21 February to 23 April 2014 respectively. Few true oysters settled throughout both trials (total of 74 true oysters) and only 19 of the targeted S. margaritacea settled. The majority of the oyster settlement occurred during the second trial period. As oysters, along with other marine bivalves, settled on all culch types and the fact that these culch types are used globally for oyster settlement, it can be assumed that the key to the lack of settlement lay elsewhere. During the settlement trials an extensive Harmful Algal Bloom (HAB), comprising of predominantly the toxic dinoflagellate Lingulodinium polyedrum, occurred; this bloom ranged from Mossel Bay to East London, its epicentre was at Port Elizabeth (approximately 125 km west of the settlement trail site in Port Alfred), and it lasted from November 2013 to February 2014. Harmful Algal Blooms have an impact on several environmental conditions (pH, temperature, dissolved O2 and CO2 levels, and ammonium levels), which may have affected the settlement of oyster spat.To assess the impact of the HAB (if any) on spat settlement at the study site in summer 2013-2014, three locations within Marine Protected Areas (MPAs) were chosen to perform a size class analysis. One of these, the southern-most, was within the range of the algal bloom (Gulu), another fell on the border of the HABs extent (Gonubie), and the third, northern-most location fell outside of the bloom range (Kei Mouth). I expected a gradient, with Gulu having the lowest number of recruits and Kei Mouth (outside of the HAB range) the highest. However, pre-existing population structure (most likely due to anthropogenic factors) seemed to influence the number of recruits more than the HAB. The location with the most recruits was Gonubie, within a densely human-populated area, whereas the other two sites (north and south of Gonubie) had far fewer recruits and have a smaller human population. The low settlement experienced during the settlement trials show the unreliable nature of wild oyster settlement as it is subject to unpredictable natural phenomena. A more reliable method for oyster cultivation would be to use hatcheries where most, if not all, conditions could be regulated and monitored.This study is the first step in the investigation into indigenous oyster culture, and growth trials and other studies still need to be performed. The finding that there are two Saccostrea species in South Africa, indicating a need for further resolution of oyster taxonomy in this country, is important for both biodiversity conservation and mariculture.Key words: Oyster culture, CO1 and 16S, spat settlement, HAB, size class analysis.

[发布机构] Stellenbosch University