Calyptraeotheres garthi (Fenucci, 1975)

Ocampo, Emiliano H., Robles, Rafael, Terossi, Mariana, Nuñez, Jesús D., Cledón, Maximiliano & Mantelatto, Fernando L., 2013, Phylogeny, phylogeography, and systematics of the American pea crab genus Calyptraeotheres Campos, 1990, inferred from molecular markers, Zoological Journal of the Linnean Society 169 (1), pp. 27-42 : 38-39

CALYPTRAEOTHERES GARTHI

Calyptraeotheres garthi has a high degree of genetic homogeneity across its distribution range, as indicated by the AMOVA and the haplotype network. Genetic structure is observed when ongoing or historical processes have limited gene flow ( Avise, 2009). However, marine species commonly exhibit low levels of global population differentiation, even when physical barriers limit gene flow ( Ward, Woodwark & Skibinski, 1994; Waples, 1998; Fernández Iriarte et al., 2011). Although the San Matias Gulf has been considered to be a closed system with larvae retention ( Guerrero & Piola, 1997), this does not seem to lead to genetic structure in C. garthi . Similarly to almost all other pinnotherids, C. garthi has a long larval cycle and requires about 30 days to settle ( Ocampo et al., 2011), giving the species sufficient time for marine dispersal and to maintain connectivity amongst populations across its geographical range.

Calyptraeotheres garthi displayed high haplotype (28 haplotypes in 30 individuals) and low nucleotide diversity (overall 0.009), which may be attributable to rapid population growth and accumulation of mutations after a period of low effective population size. In support of this idea, the mismatch distribution showed a unimodal distribution that, together with the results of the neutrality tests (negative and significant), can be interpreted as indicators of sudden expansion. Population expansion of C. garthi started ~300 Kya according to the estimate from the mismatch distribution, and ~220 Kya using BSP analysis.

Climate changes in the Quaternary shaped the diversity of marine and terrestrial species ( Avise, 2000, 2009). One of the climate events that played a crucial role in determining the abundance and distribution of natural populations was the last glacial maximum (LGM) in the late Pleistocene (~20 Kya; Hewitt, 2004; Provan & Bennett, 2008). Although strongly disputed by some authors ( Rabassa, 2008; Lessa, D’Elía & Pardiñas, 2010), during the LGM many species are generally thought to have remained in refuges and then expanded when the glaciers retreated. Recent population expansions resulting from post-LGM recolonization have been detected mainly for terrestrial ( Hewitt, 2000, 2004) and for some marine species (see examples given by Marko et al., 2010). Nevertheless, our results show that C. garthi underwent a population expansion earlier than the LGM (~220 Kya based on BSP and ~300 Kya calculated by T). Thus, if this expansion was a result of climate change, this event would have occurred before the LGM. Consistent with our results, several marine species underwent population growth before the LGM in the south-western Atlantic ( Fernández Iriarte et al., 2011; Ceballos et al., 2012) and elsewhere ( Marko, 2004; Wilson, 2006; Wang, Li & Li, 2008; Marko et al., 2010).

Glaciations in the mid-Pleistocene appear to have left traces in the evolutionary history of several marine species ( Wilson, 2006; Marko et al., 2010). Indeed, after the Great Patagonian Glaciation during the Miocene (~1 Mya; Rabassa, 2008) and before the LGM, three glacial periods strongly affected the region. Two of these occurred at or around 145 Kya ( Kaplan et al., 2005) and 260 Kya ( Hein et al., 2009). In these periods, the decrease in sea level and water temperature and changes in marine currents may have forced species to take refuge at lower latitudes, as has been suggested for the sub-Antarctic fish Eleginops maclovinus (Cuvier, 1830) ( Ceballos et al., 2012) and the south-western Atlantic fish Cynoscion guatucupa (Cuvier, 1830) ( Fernández Iriarte et al., 2011) . Calyptraeotheres garthi may have undergone a northward retraction during glacial phases, similar to the process that has been suggested either for marine species such as those mentioned above or for other continental organisms in Patagonia ( Ruzzante et al., 2008; Lessa et al., 2010). Calyptraeotheres garthi populations expanded at the end of the glacial period, by ~260 Kya, either recolonizing or colonizing higher latitudes for the first time.

Given the species’ obligate symbiont lifestyle, a C. garthi population requires the prior existence of its hosts for growth and survival. It is plausible that the evolution of this crab is intimately related to the history of its host. The presence of limpet hosts [e.g. Crepidula protea (d’Orbigny, 1841) , Crepidula onyx (Sowerby, 1824) ] on the Argentinean coast dates from the Miocene (~20 Mya; Aguirre, 1993; Aguirre & Farinati, 1999), which would have enabled the crab population to establish. However, whether these host species either expanded or maintained stable population sizes over the last 300 000 years is unknown. It would be interesting to assess, in future studies, the historical demography of the limpet hosts of C. garthi .