Alexei Ruiz-Abierno

Effects of organic enrichment on nematode assemblages in a microcosm experiment.

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.

Systematics and DNA barcoding of free-living marine nematodes with emphasis on tropical desmodorids using nuclear SSU rDNA and mitochondrial COI sequences

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.

Does Morphology of Host Marine Macroalgae Drive the Ecological Structure of Epiphytic Meiofauna

Does Morphology of Host Marine Macroalgae Drive the Ecological Structure of Epiphytic Meiofauna? We explored the relationships between the epiphytic meiofauna and the morphology of five host macroalgae species. Four morphological attributes of the macroalgae (fractal complexity, area, perimeter, and amount of trapped sediment) were measured and correlated with the density and diversity of meiofauna and nematodes. The meiofauna was similar to epiphytic communities described in other studies with harpacticoid copepods and nematodes being the two more abundant taxa. The structural complexity of the macroalgae did not correlate with the meiofauna neither with the nematode assemblages.

A morphometric analysis of the genus Terschellingia (Nematoda: Linhomoeidae) with redefinition of the genus and key to the species

The cosmopolitan and often ecologically dominant genus Terschellingia (Nematoda, Linhomoeidae), with 37 nominal species, is taxonomically a problematic taxon. Its species show high morphological plasticity, possess few diagnostic morphological characters and identification keys are lacking. A revision of the genus was carried out based on morphological and morphometric data from the literature and from light and electron microscopic observations of specimens collected in Cienfuegos Bay, Caribbean Sea, Cuba. The diagnosis of the genus Terschellingia is emended. Of the current 37 nominal species, 15 are considered as valid species based on morphological characters related to size and position of amphidial fovea, presence/position of cephalic and cervical setae, presence/ size/ shape of oesophageal bulb, shape of spicular apparatus and shape of tail. Tabular and pictorial keys were provided based on these characters. Three sympatric species: T. communis, T. gourbaultae, and T. longicaudata were redescribed based on recently collected Cuban specimens. Each of them showed relatively large differences in body size in comparison with the respective type specimens, suggesting possible variation due to local environmental differences. The highest intraspecific variation pertains for the most widely spread cosmopolitan species T. longicaudata, suggesting that morphological plasticity enhanced adaptation to different environmental conditions. The notable taxonomic inflation within the genus (13 species inquirendae, 9 junior synonyms), probably also present in other highly specious genera of marine nematodes, can lead to an overestimation of the alpha-diversity for some taxa.

Coral reef habitats strongly influence the diversity of macro- and meiobenthos in the Caribbean

Macro- and meiobenthos contribute substantially to the diversity of marine habitats and should account for a significant fraction of the diversity in coral reefs. The aims of the present study are to characterize macro- and meiobenthic communities in a Caribbean coral reef and to analyze the effects of habitat type on the α-, β-, and γ-diversity of free-living nematodes. Two reef sites with four habitat types each were selected: seagrass bed, bare sand, coral rubble, and algal turf. Habitats within sites were adjacent to each other and characterized by their physical architecture, hydrodynamic regime, and foundation species. The diversity of marine communities was high, with eight phyla represented in the macrobenthos and 18 phyla in the meiobenthos. The structure of macrobenthos was strongly associated with the habitat type. This relationship was weaker for meiobenthos, which is likely related to ecological drift, hydrodynamic regime, and macrobenthic influence. The nematode species richness was high at both studied scales: α-diversity ranged from 31 to 83 species per habitat and γ-diversity for the whole reef was 156 ± 4 species. The nematode assemblages consisted of few dominant and many rare species, which is typical of hyperdiverse faunas. The β-diversity was large in the reef with few shared species and the presence of distinctive nematode assemblages adapted to the physical architecture and food availability of each habitat. The results imply that the physical structure and heterogeneity of the coral reef habitats are important for maintaining the high diversity of small invertebrates, especially regarding the richness and turnover of nematode species.

Body size distribution of free-living marine nematodes from a Caribbean coral reef

Body size is a fundamental property of organisms but its distribution is almost unknown for marine nematodes. We measured the length and width of 7446 nematodes of 186 species to describe their morphological landscape and the relationship between abundance and body size. The body shape has a bimodal distribution with two morphotypes, suggesting adaptations to lifestyle. In fine sediments (seagrass bed), slender nematodes dominated, whereas sandy sediments had mostly stout nematodes but also slender forms. Seaweed turf from hard bottom substrates may favour mostly slender nematodes, whereas dead coral harbours both morphotypes, probably as a result of high heterogeneity of the substrate. The size spectra of abundance vs mass class shows a negative exponential relationship, suggesting that the energetic equivalence hypothesis holds for nematodes. The shape and position of the size spectra depended on the type of habitat. Body size is an important organismal trait that offers valuable information for disentangling ecological patterns in Nematoda.