Predicting the Dispersal Potential of an Invasive Polychaete Pest along a Complex Coastal Biome

Abstract

Boccardia proboscidea is a recently introduced polychaete in South Africa where it is a notorious pest of commercially reared abalone. Populations were originally restricted to abalone farms but a recent exodus into the wild at some localities has raised conservation concerns due to the species' invasive status in other parts of the world. Here, we assessed the dispersal potential of B. proboscidea by using a population genetic and oceanographic modeling approach. Since the worm is in its incipient stages of a potential invasion, we used the closely related Polydora hoplura as a proxy due its similar reproductive strategy and its status as a pest of commercially reared oysters in the country. Populations of P. hoplura were sampled from seven different localities and a section of the mtDNA gene, Cyt b and the intron ATPSa was amplified. A high resolution model of the coastal waters around southern Africa was constructed using the Regional Ocean Modeling System. Larvae were represented by passive drifters that were deployed at specific points along the coast and dispersal was quantified after a 12-month integration period. Our results showed discordance between the genetic and modeling data. There was low genetic structure (Φ = 0.04 for both markers) and no geographic patterning of mtDNA and nDNA haplotypes. However, the dispersal model found limited connectivity around Cape Point-a major phylogeographic barrier on the southern African coast. This discordance was attributed to anthropogenic movement of larvae and adult worms due to vectors such as aquaculture and shipping. As such, we hypothesized that cryptic dispersal could be overestimating genetic connectivity. Though wild populations of B. proboscidea could become isolated due to the Cape Point barrier, anthropogenic movement may play the critical role in facilitating the dispersal and spread of this species on the southern African coast.

The role of cryptic dispersal in shaping connectivity patterns of marine populations in a changing world

“…However, their larval transport model showed limited connectivity that coincided with known phylogeographic breaks, indicating that multiple introductory events over a longer time scale, possibly via shipping vectors, could have eroded the phylogeographic signal, giving the illusion of a panmictic population (Dawson et al, 2005). In a similar but more recent study, David et al (2016) found that the aquaculture trade in South Africa was facilitating genetic connectivity in the invasive polychaete Polydora hoplura, which is notorious for burrowing and residing in oyster and abalone shells. The authors found a lack of any clear geographic patterning of haplotypes and low F ST despite the fact that populations were distributed across multiple phylogeographic breaks.…”

“…The authors also found high genetic diversity which was attributed to multiple introductions with international shipping being the culprit vector. Another recent study by David et al (2016) used the cytochrome b gene and a single nuclear locus to detect high genetic connectivity among populations (no geographic patterning of haplotypes) of a shell-boring polychaete distributed across three phylogeographic breaks in South Africa. The movement of oysters among aquaculture farms distributed along the country's ~2000 km coastline was identified as the main driver of this high connectivity (David et al 2016;Williams et al 2016).…”

“…For example, the introduction of the Pacific oyster Crassostrea gigas to Europe resulted in the arrival of more than a dozen NIS, with about five or six eventually becoming established (Wolff & Reise, 2002). In a more While marine invasions are an important consequence of anthropogenic movement of NIS, a more conspicuous phenomenon is the erosion of phylogeographic signal due to continuous and consistent movement of migrants (Wares et al, 2002;Dawson et al, 2005;David et al, 2016;Wrange et al 2016).…”

“…This phenomenon is coined as 'cryptic dispersal' since the anthropogenic effect cannot be definitively detected by genetic patterns alone (David et al, 2016). Cryptic dispersal is primarily driven by propagule pressure and also by the coastal environment, specifically the strength of phylogeographic breaks.…”

Replicated anthropogenic hybridisations reveal parallel patterns of admixture in marine mussels

“…Fourth, a vector risk assessment will be necessary to predict the possible human induced secondary displacements (e.g., Herborg, O'Hara, & Therriault, 2009). Finally, at a local scale, larval dispersal through oceanographic constraints will play a major role in the potential spread of dock mussels and dispersal models for NIS in ports will be needed (see David, Matthee, Loveday, & Simon, 2016, for an example at a large scale). While some studies of water flows, tide or wave physical constraints in ports of the English Channel exist (Guillou & Chapalain, 2011Jouanneau, Sentchev, & Dumas, 2013), none include a biological module.…”