Abstract
The nematode genus Acrobeles is composed of two morphological groups distinguished by the presence ('double' cuticle) or absence ('single' cuticle) of the refringent inner layer of the cuticle. In the present study, four species of this genus, two with 'single' cuticle (Acrobeles ciliatus and Acrobeles cylindricus) and two with 'double' (Acrobeles aenigmaticus and Acrobeles complexus) are studied from coastal dunes in Spain. This study provides detailed morphological and morphometrical analyses for the four species, while molecular analysis, based on 18S and 28S ribosomal DNA, is provided for A. complexus. The four species are studied with scanning electron microscopy, which is obtained for the first time for A. cylindricus. These analyses revealed morphological and molecular differentiations between both groups, appearing as two related monophyletic entities. The subgenera Acrobeles and Seleborca, formerly considered as separate genera, are proposed to accommodate both groups.
Highlights
The nematode genus Acrobeles von Linstow, 1877 (Rhabditida, Cephalobidae) was proposed by Otto F.B. von Linstow (1877) to include one species having oral expansions
As a result of the present study, the following important morphological differences have been found: Acrobeles ciliatus vs. A. cylindricus Both species with ‘single’ cuticle are very similar, having similar body size (484–553 μm vs. 402–528 μm), but they can be distinguished by the width of lip region, similar at the adjacent part of the body in A. ciliatus and visibly narrower in A. cylindricus, excretory pore located anteriorly (at metacorpus level vs. at procorpus)
A very important character that differentiates both species is the position of the vulva (ventrally centred in A. complexus vs. left sublateral in A. aenigmaticus)
Summary
The nematode genus Acrobeles von Linstow, 1877 (Rhabditida, Cephalobidae) was proposed by Otto F.B. von Linstow (1877) to include one species having oral expansions. Thorne (1937) revised again these species and divided the genera Acrobeles according to the morphology of the labial probolae and proposed five new genera, maintaining in Acrobeles only those species with long bifurcate labial probolae. Thorne (1925) revised this genus and included 40 species, all of them with oral expansion (labial probolae) with variable morphology. This genus is mainly characterized by having a lip region with six asymmetrical triangular lips and an oral opening surrounded by three bifurcated labial probolae, all of which – lips and probolae – are bordered by triangular processes.
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