Observations on the composition and larval developmental modes of polydorid pests of farmed oysters (Crassostrea gigas) and abalone (Haliotis midae) in South Africa

This study provides preliminary insights into how the culture method of molluscs and the larval developmental mode of pest polydorids may determine which species become pests. A total of 10 species were identified on oysters and abalone in offshore and oysters in onshore culture on the west coast of South Africa. One-factor PERMANOVA showed that abalone and oysters cultured offshore housed similar communities of worms, but that they differed significantly from the communities housed by oysters cultured onshore. Oysters from onshore were infested mainly by Polydora hoplura, but also by Polydora cf. websteri and Boccardia pseudonatrix. All these species produce adelphophagic larvae or a mixture of these and planktotrophic larvae. Adelphophagic larvae usually only emerge soon before they settle, which would probably enhance their ability to recruit locally and establish local populations. By contrast, worms from molluscs grown offshore were infested by high abundances of P. hoplura and lower abundances of up to five more species per individual host. All hatched broods observed with these female P. hoplura contained planktotrophic larvae which have a long planktonic phase before settling. This would presumably enhance their ability to reach hosts in the offshore.