Genealogical approaches to the temporal origins of the Central American gap: Speciation and divergence in Pacific <i>Chthamalus</i> (Sessilia: Chthamalidae)

Abstract

Abstract:A large section of the tropical Eastern Pacific coastline is nearly devoid of reef or consolidated habitat, and is known as the Central American Gap as it is associated with a biogeographic transition in fish and invertebrate species. We analyze phylogeographic data for intertidal barnacles (Chthamalus) to identify relevant temporal patterns that describe the origins of this biogeographic transition (the Mexican-Panamic Transition Zone). These contrasts of populations on either side of the transition zone include two pairs of closely related species (C. panamensis and C. hedgecocki; C. southwardorum and a Southern form of C. southwardorum), as well as gene flow data within one species (C. panamensis) that currently is found on both sides of the boundary between provinces. Using sequence data from a prior phylogenetic study, we used traditional (net nucleotide divergence) measures as well as coalescent analyses that incorporate the isolation-migration model to identify the likely time of separation between Northern and Southern taxa in two species pairs. A total of 67 individuals were sequenced at two mitochondrial (cytochrome c oxidase i, 16S) and one nuclear (elongation factor 1-alpha) gene regions. Our analyses indicate that the regional isolation of these intertidal barnacles occurred approximately 315-400kya, with subsequent expansion of C. panamensis from the Southern region into the North much more recently. There are insufficient survey data to conclusively document the absence of species from this group within the Central American Gap region near the Gulf of Tehuantepec. However, appropriate habitat is quite sparse in this region and other environmental factors, including upwelling and water temperature, are likely to be associated with isolation of many species in the Mexican and Panamic provinces sensu stricto. Some taxa may maintain gene flow across this region, but very few genetic studies have been completed on such taxa. Until further work is done, distinguishing between prior hypotheses of a faunal gap, or a faunal transition zone, is somewhat speculative. Additional taxonomic revision will be necessary in Chthamalus but is beyond the scope of this paper. Rev. Biol. Trop. 61 (1): 75-88. Epub 2013 March 01.

The oceanic concordance of phylogeography and biogeography: a case study in Notochthamalus

“…Hellberg () showed that the range overlap among recently diverged lineages of the gastropod Tegula followed isolation between wave‐exposed and wave‐sheltered environments, a pattern that is also found in some tropical eastern Pacific barnacles (Meyers et al. ). Abundant evidence shows that “ecological” speciation or divergence is possible in marine systems (Sanford et al.…”

“…For example, complex patterns may develop when distinct coastal environments promote the divergence of populations. Hellberg (1998) showed that the range overlap among recently diverged lineages of the gastropod Tegula followed isolation between wave-exposed and wave-sheltered environments, a pattern that is also found in some tropical eastern Pacific barnacles (Meyers et al 2013). Abundant evidence shows that "ecological" speciation or divergence is possible in marine systems (Sanford et al 2003;Schmidt et al 2008), so the interaction between dispersal and fitness is an important component for describing the origins of marine biodiversity.…”

On the importance of habitat continuity for delimiting biogeographic regions and shaping richness gradients

“…This suggests that many TEP species are capable of dispersing across the CAG and may have relatively high dispersal potential (Robertson & Cramer 2009). But in order to more fully assess the degree to which the CAG and other tropical habitat gaps may limit dispersal, population genetic studies, coupled with field surveys of abundance, distribution, larval dispersal potential (plus its interaction with temperature; O'Connor et al 2007) and recruitment success for tropical species are needed (Meyers et al 2013;Marchant et al 2015). Nevertheless, the total range extents of tropical/subtropical species tend to be larger than extratropical species (measured as total km of coastline between species' range limits) partly because many TEP species appear capable of dispersing across large habitat gaps such as the CAG.…”

Why not do phylogeography on every chthamalid barnacle? The case of Jehlius cirratus

“…A remaining question is why particular environmental boundaries appear to influence the connectivity among locations for some species but not others. As with the comparison between N. scabrosus, which exhibits strong population structure along the Chilean coast (Zakas et al 2009;Laughlin et al 2012), and J. cirratus, which does not, we can also consider the structure of Balanus glandula (Sotka et al 2004) (not a chthamalid barnacle, however) against the lack of structure in the codistributed Chthamalus dalli (Wares and Castañeda 2005), the structure in C. montagui across a broad geographic range (Dando and Southward 1981) and lack of structure across a similar range in C. stellatus (O'Riordan et al 2010), along with other examples (Zardus and Hadfield 2005;Tsang et al 2008;Wares et al 2009;Meyers et al 2013). Are the differences in structure based on larval behavior or developmental time (Burton and Feldman 1982)?…”

“…Here, we add sequences from subsequent samples within this range, as well as samples from the edges of the distribution (Häussermann and Försterra 2009), now spanning close to 3000km of the species' range, to explore more fully the potential for spatial diversity in intertidal taxa from this coast (see Figure 1). As chthamalid barnacles have a propensity to harbor cryptic genetic diversity (Dando and Southward 1981;Zardus and Hadfield 2005;Tsang et al 2008;Wares et al 2009;Meyers et al 2013), we specifically look for any phylogeographic structure that may add to our understanding of coastal biodiversity in Chile.…”