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2014The state of the art of Xyalidae (Nematoda, Monhysterida) with reference to the Brazilian records Marine Biodiversity, v. 44, p. 367-390, 2014

The majority of studies have advocated that diversity of marine nematodes increases with increasing sediment grain size, although the opposite trend has also been suggested. The controversy is partially caused by not taking into account the effect of density on patterns of diversity and by analyzing datasets from different environments. The present study investigated nematode assemblages from sediments varying from very fine sand (mean grain size of 0.12 mm) to very coarse sand (1 mm) in shallow sublittoral marine environments. Contrary to previous studies, species richness was constant along the granulometric spectrum, despite significant changes in composition. The dominant genera were separated into five groups according to their optimum distribution and there was little overlap between these groups. Concepts from the niche theory explain to some extent the observed patterns. For instance, some of the coexisting genera were from different feeding types.

The first report of Apistobranchus Levinsen, 1883 (Family Apistobranchidae) in Antarctica was presented by Hartman (1967). Two species were later described: Apistobranchus glacierae Hartman, 1978 and Apistobranchus gudrunae Hartmann- Schröder & Rosenfeldt, 1988, which differed from A. glacierae mainly by having compound setae. Subsequently, ecological studies in Antarctica have identified both of these species. On the status of Antarctic Apistobranchus, we concluded that there is up to now, only one valid species, A. glacierae. The character ‘compound-setae’ referred for A. gudrunae is in fact simple limbate ones eventually splintered as described for A. glacierae. Other characters, also previously considered as diagnostics for A. gudrunae, did not differ in both species as shown by the observation of several specimens of different sizes and type material of A. glacierae and A. gudrunae. All the reports on the densities of Antarctic apistobranchids, including ours, show that they have higher values in finer sediments of 20 and 40 m depth. The need of additional work, including the rearing of specimens in the laboratory and plankton analysis, is emphasized.

Chromadoridae is a widespread family of mostly free-living marine nematodes. This systematic review provides for each genus: a historical background, an updated diagnosis and a list of species. Our review recognizes 37 valid genera, 395 valid species, 57 descriptions without enough morphological information for accurate identification (species inquirenda) and 10 species incerta sedis. We also recognize 21 species as nomena nuda. Additionally, polytomous keys were constructed for the subfamilies and for the genera of the three major subfamilies (Chromadorinae, Euchromadorinae and Hypodontolaiminae) using the most important diagnostic characters. A phylogenetic analysis based on rDNA sequences of species available in the GenBank was also conducted. Phylogenetic trees based on the 18S and 28S rDNA confirmed the classification into three subfamilies (Spilipherinae, Hypodontolaiminae and Chromadorinae), despite the absence of defined synapomorphies. Phylogenetic relationships at lower taxonomic level are problematic given the large number of sequences not identified to species level.

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