Does life history predict past and current connectivity for rocky intertidal invertebrates across a marine biogeographic barrier?

Abstract

The southeast Australian coast potentially includes a complex biogeographic barrier, largely lacking exposed rocky shore that may limit the dispersal of rocky intertidal taxa and contribute to the maintenance of two biogeographic regions. Surprisingly, within the 300-km barrier region, several species considered exposed rocky shore specialists occurred within sheltered sites. We analysed COI sequence variation for 10 rocky intertidal invertebrate species, with a range of life histories, to test the hypotheses that larval type and habitat specificity are strong predictors of gene flow between biogeographic regions. Our data revealed that the southeast corner of Australia includes a strong barrier to gene flow for six of eight species with planktonic larvae, and a coalescence analysis of sequence differentiation (IM model) suggests that a barrier has existed since the Pleistocene. In contrast, two direct developers were not affected by the barrier. Our comparative approach and data from earlier studies (reviewed here) do not support the hypothesis that larval type predicts gene flow across this barrier, instead we found that the ability to utilize sheltered habitat provides a clearer explanation of the phylogeographic break. Indeed, the species that displayed little or no evidence of a phylogeographic break across the barrier each displayed unexpectedly relaxed habitat specificity.

Patterns of demography for rocky-shore, intertidal invertebrates approaching their geographical range limits: tests of the abundant-centre hypothesis in south-eastern Australia

“…Finally, a five-degree sea-surface temperature difference, which also occurs in this region (Knox 1963;Ridgway and Dunn 2003), may contribute to limiting both the early and long-term survival of individuals of our study species on the shore. This may be particularly true for range-limit populations of T. rosea and M. marginalba, because their connectivity to populations from Bermagui at more northern locations of the coast (Ayre et al 2009) may potentially render them less able to adapt genetically to 'marginal conditions' at the range edge (Sagarin et al 2006). Assessing post-settlement mortality of recruits and adults from the middle to the limit of the range may be an appropriate future study to investigate this hypothesis.…”

“…This hypothesis would be consistent with the results of Caffey (1985), who examined recruitment of T. rosea in the middle of its geographical distribution, and found more frequent recruitment events at more northern locations along the NSW coast than in the south. A recent survey of mitochondrial DNA-sequence variation in samples of T. rosea and M. marginalba collected from sites arrayed along the coast from Bermagui in the north to Cape Conran in the south revealed minimal divergence (y ¼ 0), suggesting that there is a high degree of genetic connectivity among populations of these species along this coast (Ayre et al 2009). Given the predominantly southward flow of the EAC, this implies that populations at the range limits for these species are likely to comprise recruits from upstream 'source' populations (sensu Pulliam 1988).…”

“…This change in potential habitat suitability, followed south by the Ninety Mile Beach biogeographical barrier, may reduce the ability of the species to colonise more southern locations. Although all three study species have planktonic larvae that spend up to a few weeks in the water column (Wisely and Blick 1964;Underwood 1974;Caffey 1985), recent genetic investigations into the connectivity of populations of rocky, intertidal marine species across the Ninety Mile Beach biogeographical barrier have revealed that degree of population connectivity across this barrier is not related to the mode of larval dispersal, but rather, to flexibility in habitat utilisation (Ayre et al 2009). The present study supports the mounting evidence (Sagarin and Gaines 2002b;Gilman 2005;Sagarin et al 2006) that the abundant-centre hypothesis is too simplistic a model to predict the abundance distributions of planktonically developing benthic marine invertebrates.…”

“…The range limit of our three species also corresponds to the northern limit of a biogeographical barrier for rocky-shore, intertidal marine species (Hidas et al 2007;Ayre et al 2009), composed of a 300-km stretch of sandy shore (Ninety Mile Beach) and mangrove forest (between Cape Conran and Wilsons Promontory; Fig. 1), separating the nearest natural rocky intertidal habitats.…”

Detecting range shifts among Australian fishes in response to climate change

“…In aquatic environments, range expansion and contraction events can be inferred by matching information on the phylogenetic separation of taxa with information on oceanic circulation and physical barriers that might limit dispersal (e.g. Barber et al 2000;Dawson 2001;Ayre et al 2009;Faulks et al 2010b). Such approaches are now widespread; however, there is also an increasing move towards relying not only on the use of neutral markers to assess historical patterns of gene flow, but towards the detection of genes that are actively being selected for along environmental gradients.…”