Using morphological and molecular tools to identify megalopae larvae collected in the field: the case of sympatric Cancer crabs

Abstract

Studies of recruitment dynamics in meroplanktonic organisms are dependent on the correct identification of each ontogenic stage of each species. This is particularly difficult when studying the larval stages, which are not easy to identify due to their lack of resemblance to conspecific adults and their high degree of similarity with congenerics at the same stage of development. This is the case with the crustacean megalopae of the genus Cancer along the coast of the south-eastern Pacific. This fact represents a serious limitation on ecological studies of populations of these species which constitute a heavily exploited local resource. In this study we describe in detail field collected megalopae larvae of three sympatric crab species of the genus Cancer (C. edwardsii, C. setosus and C. coronatus). As a result of this analysis we were able to identify easily visible diagnostic characters which allow the species to be distinguished from one another. The megalopae were easily distinguished by the form of the cheliped and the presence of spines on these. Cancer edwardsii has an elongated globulose cheliped, whereas C. coronatus has a subquadrate one. Both species possess a prominent ischial spine, which is absent in C. setosus. We corroborated the utility of these diagnostic characters by comparing the COI gene sequences of mitochondrial DNA of larvae identified by morphology with sequences taken from samples of the adults of all species of Cancer found in the region. We discuss the morphological variations between larvae found across the region (i.e. at sites separated by more than 800 km) and between megalopae obtained from the field versus those cultivated in the laboratory. We conclude that the simultaneous use of morphological and molecular tools for identification of decapod larvae appears useful for the study of cryptic species.

Description, seasonal morphological variation, and molecular identification of Paraxanthus barbiger megalopae obtained from the natural environment

“…Orensanz and Gallucci (1988) and DeBrosse et al (1990) found diVerences in size, appendage setation pattern, and timing of settlement in megalopae of C. magister from oVshore and inshore populations. Pardo et al (2009) recorded greater sizes in megalopae of C. setosus collected during the autumn as compared with those from spring collections. A similar pattern of seasonal size variation has been detected in other groups of decapods, such as anomuran megalopae (Gebauer et al 2007).…”

“…Other traits such as rostrum shape and the hook-like spine on the isquium of chelipeds appear invariant, making them ideal as an "easy to locate" key character of identiWcation (e.g. Pardo et al 2009). …”

“…The identiWcation of species is the foundation for more in-depth ecological studies. The challenge is then to obtain information on the diVerent life-cycle stages of a species in its natural environment by integrating the traditional methods of laboratory cultivation with the collection of specimens in the wild and using tools such as DNA sequencing which have been demonstrated to be fast and eYcient for crustaceans (Makinster et al 1999;Pardo et al 2009). Better morphological information on larvae and brachyuran development, which are susceptible of variation due to environmental factors, will augment the reach and precision of studies on larval dispersion, recruitment, and population ecology.…”

“…Makinster et al 1999;Pardo et al 2009). Larvae were obtained from a year-round sampling program; therefore, a morphological comparison of larvae from diVerent seasons was possible.…”

Morphology of the megalopa of the mud crab, Rhithropanopeus harrisii (Gould, 1841) (Decapoda, Brachyura, Panopeidae), identified by DNA barcode

“…The use of molecular markers has demonstrated to be a powerful tool in providing accurate identifications for plankton specimens (Pan et al 2008;Pardo et al 2009;Ampuero et al 2010;Marco-Herrero et al 2013a). The identification of megalopae has traditionally been based on morphological characteristics, but sometimes, it is impossible to get an accurate identification with this approach.…”