Abstract
Excirolana braziliensis is a coastal intertidal isopod with a broad distribution spanning the Atlantic and Pacific tropical and temperate coasts of the American continent. Two separate regional studies (one in Panama and one in Chile) revealed the presence of highly genetically divergent lineages, implying that this taxon constitutes a cryptic species complex. The relationships among the lineages found in these two different regions and in the rest of the distribution, however, remain unknown. To better understand the phylogeographic patterns of E. braziliensis, we conducted phylogenetic analyses of specimens from much of its entire range. We obtained DNA sequences for fragments of four mitochondrial genes (16S rDNA, 12S rDNA, COI, and Cytb) and also used publicly available sequences. We conducted maximum likelihood and Bayesian phylogenetic reconstruction methods. Phylogeographic patterns revealed the following: (1) new highly divergent lineages of E. braziliensis; (2) three instances of Atlantic–Pacific divergences, some of which appear to predate the closure of the Isthmus of Panama; (3) the distributional limit of highly divergent lineages found in Brazil coincides with the boundary between two major marine coastal provinces; (4) evidence of recent long‐distance dispersal in the Caribbean; and (5) populations in the Gulf of California have closer affinities with lineages further south in the Pacific, which contrasts with the closer affinity with the Caribbean reported for other intertidal organisms. The high levels of cryptic diversity detected also bring about challenges for the conservation of this isopod and its fragile environment, the sandy shores. Our findings underscore the importance of comprehensive geographic sampling for phylogeographic and taxonomical studies of broadly distributed putative species harboring extensive cryptic diversity.
Highlights
Documentation of cryptic species has increased exponentially in the last couple of decades and is associated with the widespread application of DNA sequencing (Bickford et al 2007)
Four mitochondrial gene fragments were amplified as follows: a ~439-bp fragment of the 16S rDNA gene and a ~480-bp fragment of the 12S rDNA gene using the primers reported in Podsiadlowski and Bartolomaeus (2005); a 430-bp fragment of the cytochrome b gene (Cytb) using the primers reported in Merritt et al (1998); and a 710-bp fragment of the COI gene using the primers reported in Folmer et al (1994)
Two taxonomic revisions based on traditional morphology, which included a comprehensive representation of its entire range, concluded that E. braziliensis constitutes a single species (Glynn et al 1975; Brusca et al 1995)
Summary
Documentation of cryptic species (i.e., those that are difficult or impossible to distinguish on the basis of morphology) has increased exponentially in the last couple of decades and is associated with the widespread application of DNA sequencing (Bickford et al 2007). Recent phylogeographic studies of intertidal isopods indicate that several species presumed to occupy broad distributions, instead correspond to diverse species complexes comprised of multiple highly divergent cryptic lineages, which often have very restricted geographic ranges (Sponer and Lessios 2009; Hurtado et al 2010, 2013, 2014; Varela and Haye 2012; Santamaria et al 2013, 2014, 2016) Their phylogeographic patterns are of biogeographic relevance because they retain signatures consistent with past geological and climatic events, environmental factors, as well as instances of overwater dispersal. Given the broad distribution of currently valid species of several coastal isopods, the extensive cryptic diversity they tend to harbor, and the restricted ranges of cryptic lineages, accurate inferences regarding their phylogeography, evolution, and diversity require comprehensive sampling throughout their distributional ranges
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