Emphasizing the diversity of North Sea hydromedusae by combined morphological and molecular methods

Abstract

Hydromedusae are widespread and diverse representatives of the gelatinous zooplankton, but are often neglected because of their inconspicuousness and difficulties with identification. Here, we used an integrated approach combining both morphological and molecular genetic analyses of North Sea hydromedusae. Morphological identification was successfully carried out on living material, and preservation in 4% formaldehyde allowed re-examination of most morphological features. Ethanol and DESS were adequate fixatives for DNA analyses but led to distortion of morphological characters. In most cases, morphological species identifications were confirmed by molecular data (COI partial sequences) and the latter approach led to valid discrimination where morphological characters were insufficient. In comparison with 22 morphologically identified entities, COI analysis revealed 25 clades with a pronounced difference of ≥5.4% between intra- and inter-specific variability. Specimens morphologically identified as Obelia spp. were attributed to O. geniculata, O. dichotoma and O. longissima, while Clytia spp. were allocated to C. hemisphaerica and C. languida by the comparison to hydroid and medusa sequences retrieved from GenBank. Our results highlight the molecular approach as a powerful tool, extending the possibilities for valid species discriminations where morphological identification is difficult, for example, in species with a similar or identical morphology, in early life stages with insufficient identifying features and in linking different generations (hydroid and medusa). However, genetic analysis cannot replace morphologically based taxonomy in studies on species' population dynamics, physiology and ecology. Thus, most information is achieved by combining both methods in integrative studies using both morphological and molecular taxonomy. © The Author 2013.