Acantholaimusis a species-rich genus of deep-sea nematodes, often with dozens of species found at the same locality but each represented by single or few individuals. Species discrimination by morphological characters in this genus is therefore often difficult due to transitional forms that may be referred to several species because of lack of data on intraspecific variability. The aim of this study was to evaluate the intraspecific variability of morphological characters that are most often used inAcantholaimustaxonomy, in order to distinguish those which are most informative for species differentiation. A reverse taxonomic approach was applied for initial species discrimination. Two loci, one each from small and large subunits of rRNA, were sequenced for 59Acantholaimusspecimens from two deep-sea locations. Twenty-seven Molecular Operational Taxonomic Units (MOTU) were identified, of which 12 were represented by more than one individual. These were then analysed for intraspecific variability in morphological characters. Some of the examined characters showed high intraspecific variability; specifically: length of cephalic setae; distance from anterior end to amphid; shape of anterior setae; position and arrangement of cervical setae. In the absence of genetic data, these characters should be used with caution for differential diagnoses or species discrimination. Other characters were more conservative within the same MOTU: body proportions; length of outer labial setae; amphidial diam.; appearance of lateral field; general arrangement of cervical setae; and shape of tail. These characters may be successfully used for species discrimination in the absence of molecular data.
Assessing the reliability of fatty acid-specific stable isotope analysis for trophic studies