Browsing by Author "David, Andrew Anthony"

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    An integrated larval development and population genetics approach for predicting the establishment and dispersal potential of a recently introduced polychaete (Annelida: Spionidae) in southern Africa
    (Stellenbosch : Stellenbosch University, 2015-03) David, Andrew Anthony; Simon, Carol A.; Matthee, Conrad A.; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Boccardia proboscidea is a recently introduced polychaete in South Africa (SA) where it is a notorious pest of commercially reared abalone. The species was restricted to abalone farms distributed in three biogeographic regions up until 2011, when the first wild population was detected in the southern part of the country. If Boccardia proboscidea becomes invasive, it could pose a threat to the intertidal marine ecosystem of SA. The overarching aim of this thesis was therefore to predict the establishment and dispersal potential of B. proboscidea. The first objective was to assess the feasibility of using a closely related species to ground truth in the predictions. In Chapter 2, reproductive experiments were integrated with molecular studies to show that the nonindigenous oyster pest Polydora hoplura, like B. proboscidea can produce both planktotrophic and adelphophagic larvae (poecilogonous development). Due to a similar reproductive strategy along with its status as an aquaculture pest, P. hoplura was chosen as the “predictor” species. In Chapter 3 I investigated the effect of temperature on larval development of P. hoplura and B. proboscidea using temperature regimes reflective of the SA coast to determine establishment potential. Results showed that temperature significantly affected survivorship and developmental rate of planktotrophic and adelphophagic larvae for both species. For P. hoplura, survivorship of both larval types was highest at the intermediate to high temperature treatments (21 and 24°C) and was generally lower at the lower temperatures (12 and 17°C). Boccardia proboscidea exhibited a difference in survival optima where low temperatures favoured high planktotroph survival but low adelphophagic larval survival. Conversely, increased temperatures favoured high adelphophagic larval survival but low planktotroph survival and this was most likely driven by increased rates of sibling cannibalism. There was also a positive relationship between temperature and developmental rate for both larval types of both species. Polydora hoplura’s response to experimental temperatures is congruent with its present distribution. Based on this I predicted that B. proboscidea should become established along a large section of the SA coast and differences in survival optima may also facilitate its establishment in colder waters where P. hoplura appears to be absent. In Chapter 4, I investigated the phylogeography of P. hoplura using mtDNA (Cyt b) and nDNA (ATPSα) gene fragments. Results showed genetic connectivity among all sampling sites distributed across two biogeographic regions. I hypothesized that the low genetic structure observed was likely due to anthropogenic dispersal mechanisms rather than natural dispersal. Finally in Chapter 5, I discussed the potential for natural dispersal of B. proboscidea. Based on temperature-specific planktonic larval duration and current velocities along the SA coast, B. proboscidea could potentially cover hundreds of kilometres in a single generation from each of its three point sources. However once the discrepancy between potential and effective dispersal was accounted for based on the literature, planktotrophic larvae would be expected to cover considerably shorter distances. When compared to the historical movement of other introduced marine invertebrates in the region, these adjusted distances appear to better reflect the reality of larval dispersal along the SA coast. Boccardia proboscidea benefits from a versatile reproductive strategy which may aid the worm in its attempt to invade the SA coast but anthropogenic dispersal could be a critical factor facilitating its widespread dispersal.