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a b s t r a c tSpatial and temporal biodiversity patterns of free-living marine nematodes were studied in Cienfuegos Bay, a tropical semi-enclosed basin in the Caribbean Sea. Taxonomic (to species level) and functional (biological trait) approaches were applied for describing the assemblage structure and relating it to abiotic environment based on a sampling scheme in six subtidal stations and three months. Biological trait approach added relevant information to species pattern regarding relationships between diversity patterns and the abiotic environment. The most common morphotypes were deposit feeding nematodes, with colonising abilities of 2-3 (in a scale from 1 to 5), tail conical cylindrical or filiforme and body slender; and their abundance were correlated with depth, organic matter and silt/clay fraction. In spite of a high turnover of species, functional diversity of assemblages did not change notably in space and time. A result probably due to sampling of the habitat pool of species and to low heterogeneity of the studied muddy bottoms. Chemical pollution (organic enrichment and heavy metals) and hydrodynamic regime possibly drove the biodiversity patterns. Spatial distribution of assemblages support the existence of two well differentiated basins inside the bay, the northern basin more polluted than the southern one. The low hydrodynamic regime would determine a poor dispersion of nematodes resulting in high spatial variance in the assemblage structure; and also the associated hypoxic conditions and pollutants in sediments can explain the dominance of tolerant nematode species such as Daptonema oxycerca, Sabatieria pulchra, Terschellingia gourbaultae, and Terschellingia longicaudata. A comparison of spatialtemporal patterns of biodiversity between Cienfuegos Bay and other semi-enclosed bays in temperate regions suggests several similarities: nematode assemblages are strongly influenced by anthropogenic disturbance, temporal trends are weak or overridden by spatial ones, and few cosmopolitan genera/ species tolerant to pollution and hypoxic conditions are dominant.

A taxonomic study of the subfamily Stilbonematinae (Nematoda: Desmodoridae) based on collected specimens from a coral reef in the Caribbean Sea revealed two new species, Laxus parvum sp. nov. and Leptonemella brevipharynx sp. nov. L. parvum is characterized by the small body size (2738 μm), large cephalated spicules (63 μm), wide gubernaculum, and coccoid-shaped ectosymbiotic bacteria. The diagnosis of the genus Laxus is emended and a dichotomous identification key is given for the seven valid species. L. brevipharynx is characterized by the shape of the amphidial fovea, 'open' spiral with 1.25 turns, a short pharynx, hook-shaped gubernaculum, and male tail relatively short. Three known sympatric species, Eubostrichus hopperi, Robbea porosum, and Stilbonema brevicolle, were re-described and illustrated based on morphometric features and morphology from light microscope and scanning electronic microscope observations. For each species, relationships are discussed as well as the diagnostic value of morphological features. Phylogenetic relationships amongst desmodorid species were explored based on small subunit rDNA and cytochrome oxidase c subunit 1 partial loci. The three subfamilies within Desmodoridae (Desmodorinae, Spiriniinae, and Stilbonematinae) are polyphyletic; four of the genera of Stilbonematinae proved to be paraphyletic. Convergent evolution would reconcile the presence of glandular sensory organs and ectosymbiosis with the paraphyly of stilbonematins. The cryptic diversity of R. porosum could be explained by morphological stasis owing to obligate ectosymbiosis with bacteria. The current classification of nine genera is still the most tractable system for Stilbonematinae in spite of the evidence of its paraphyletic nature based on molecular data.

The diversity and phylogenetic relationships of the Desmodoridae, a widespread tropical family of free-living marine nematodes, is hitherto poorly known both from molecular and taxonomic points of view. We performed a molecular phylogenetic analysis of marine nematodes to: i) disentangle relationships among tropical desmodorid species; and ii) compare the performance of the nuclear SSU rDNA and mitochondrial COI nucleotide sequences in 42 and 45 nominal species, respectively, to identify species. We generated 27 new sequences of SSU rDNA belonging to five genera not previously sequenced, and 34 new sequences of COI belonging to six genera and four families not previously sequenced. The SSU rDNA tree confirmed the Enoplida to be a monophyletic sister group to the Chromadorida. The family Comesomatidae is a sister group of the Xyalidae within the Monhysterida. Both DNA markers confirmed the congruence between the morphology-and molecular-based phylogenetic inferences for most of the families. Desmodoridae was a monophyletic group, but the relationships within the family could not be recovered; the subfamilies Desmodorinae and Spiriniinae were not monophyletic meanwhile the monophyly of Stilbonematinae was not fully supported due to a few specimens of questionable identity. COI performed better than SSU rDNA to disentangle relationships among closely related species and suggested the presence of cryptic diversity within Desmodoridae. COI is effective to explore cryptic diversity and barcode species within Nematoda, with a possible threshold of genetic distance of 5% between conspecific and interspecific sequences, but DNA barcoding is limited by the poor knowledge of the diversity and taxonomy of the group and the lack of a good reference database of vouchered COI sequences.

Marine nematodes from subtidal tropical sediments of Cienfuegos Bay were subjected to organic enrichment in a microcosm experiment for 32 days. Nematode abundance and diversity decreased, and the taxonomic and trophic structure was altered. The results suggested that the nematodes were not food limited in the microcosms or in their natural environment. Chemical stressors such as ammonia and hydrogen sulfide derived from reduced conditions in sediments may be important factors affecting the assemblages. Hypoxic conditions occurred in all experimental units, as well as in the field, suggesting a nematode assemblage adapted to naturally enriched sediments. However, tolerant species showed a grade of sensitivity to reduced conditions. In agreement with the model by Pearson and Rosenberg (1978), we predict that further organic enrichment in sediments from Cienfuegos Bay may cause a phase shift into a strongly depleted benthic fauna and reduced conditions in water and sediments.