Coastal topography drives genetic structure in marine mussels

Abstract

Understanding population connectivity is fundamental to ecology, and, for sedentary organisms, connectivity is achieved through larval dispersal. We tested whether coastal topography influences genetic structure in Perna perna mussels by comparing populations inside bays and on the open coast. Higher hydrodynamic stress on the open coast produces higher mortality and thus genetic turnover. Populations on the open coast had fewer private haplotypes and less genetic endemism than those inside bays. Gene flow analysis showed that bays act as source populations, with greater migration rates out of bays than into them. Differences in genetic structure on scales of 10s of kilometres show that coastal configuration strongly affects selection, larval dispersal and haplotype diversity. KEY WORDS: Dispersal · Coastal topography · ConnectivityResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 368: 189-195, 2008 Bay morphology influences oceanography, not only by retaining water and, thus, changing circulation patterns and consequently affecting larval dispersal (Roughan et al. 2005), but also by creating different selective environments from those of the open coast. In bays, populations are often subjected to high thermal stress, different food concentrations and lower salinities compared to open coast sites (Ricketts & Calvin 1968, Castilla et al. 2002, Largier 2004. However, wave action, which can be the major cause of mussel dislodgement in the intertidal habitat (Paine & Levine 1981), is higher on the open coast than in bays (Ricketts & Calvin 1968).In the present study, we test the hypothesis that the influence of coastal topography on larval dispersal and selective regime produces a discernible and predictable effect on the genetic structure of mussel populations. Our null hypothesis is that open coast sites have the same genetic diversity and number of private haplotypes as sites in bays. Mussels disperse by means of planktotrophic larvae that can stay in the water column for a period of weeks to months before settling to the substratum and being recruited into adult populations (Hicks & Tunnell 1995). In addition, wave force was measured at the same sites on 4 different occasions. Mortality rates of Perna perna were measured at 2 bay and 2 open coast sites after a storm to determine the role of hydrodynamic stress in genetic turnover. MATERIALS AND METHODSSampling, DNA extraction, amplification, and sequencing. Populations of Perna perna (shell length 4 to 5 cm) were sampled at 3 sites inside bays (Plettenberg Bay, Jeffreys Bay, Algoa Bay) and 3 open coast sites (Cape St. Francis, Cape Recife, Kenton-on-Sea; n = 15 each population; Fig. 1) from low intertidal rocky shores of South Africa. These bays are typical of a series of half-heart or log-spiral bays found on this coast. They include a long stretch of sandy beach, with patches of sand-influenced granite rock (Bownes & McQuaid 2006). All populations belong to the same genetic lineage (Zardi et al. 2007)...

Dispersal barriers and stochastic reproductive success do not explain small-scale genetic structure in a broadcast spawning marine mussel

“…DNA was extracted from foot tissue in the 3 molluscs and from cirri in C. dentatus using the CTAB extraction protocol (Doyle & Doyle 1990). The mitochondrial COI gene was amplified as described previously (Nicastro et al 2008); it is matrilineally inherited in all 4 species (Weber et al 2009, Teske et al 2012). Levels of haplotype and nucleotide diversity were determined in DnaSP v5.10.01 (Librado & Rozas 2009).…”

“…Genetic structure between coastal and bay populations was investigated by calculating Φ ST (Michalakis & Excoffier 1996) and G' ST (Nei 1987) in GenoDive v2.0b20 (Meirmans & van Tienderen 2004). For the former, a matrix of uncorrected pairwise differences between haplotypes was generated, while the latter is based on haplotype frequencies between populations.…”

“…While there was no structure between sites on the open coast, sites within bays were genetically distinct not only from those at the coast but also from each other. The observed pattern was attributed to asymmetrical levels of gene flow between bays and the open coast, with bays acting as sources of propagules (Nicastro et al 2008). As the structuring effects of oceanographic barriers can affect species with different levels of dispersal capability in a similar way (Teske et al 2007), genetic structure resulting from higher levels of genetic diversity in bays would be expected to be present in at least some sympatric species.…”

“…In a study comparing genetic structure in female rocky shore mussels Perna perna between several South African coastal sites and bays, Nicastro et al (2008) identified genetic structure using sequence data from the mitochondrial (mtDNA) cytochrome oxidase c subunit I (COI) gene. Unlike many other species in which genetic structure was found at a scale of tens to several hundred kilometres, the genetic structure in P. perna was by no means characterised by 'chaotic genetic patchiness' (sensu Johnson & Black 1984) that likely reflects stochasticity in recruitment, but could be linked to coastal topography.…”

“…It was long believed that the often extended dispersal phase of such species, coupled with very large population sizes and few dispersal barriers in the sea, would result in high levels of connectivity over large geographic scales (Caley et al 1996, Eckman 1996, Roberts 1997. In recent years, numerous studies have challenged this idea, as genetic structure has been found even at small geographic scales (Jørgensen et al 2005, Selkoe et al 2006, Banks et al 2007, Nicastro et al 2008, Hogan et al 2010. In marine invertebrates with sessile adults, genetic structure is usually considered to be the direct result of factors that affect larval dispersal (Johnson & Black 1984, Hedgecock 1994, Selkoe et al 2006.…”