Seasonal changes in the assembly mechanisms structuring tropical fish communities.

Despite growing interest in trait-based approaches to community assembly, little attention has been given to seasonal variation in trait distribution patterns. Mobile animals can rapidly mediate influences of environmental factors and species interactions through dispersal, suggesting that the relative importance of different assembly mechanisms can vary over short time scales. This study analyzes seasonal changes in functional trait distributions of tropical fishes in the Xingu River, a major tributary of the Amazon with large predictable temporal variation in hydrologic conditions and species density. Comparison of observed functional diversity revealed that species within wet-season assemblages were more functionally similar than those in dry-season assemblages. Further, species within wet-season assemblages were more similar than random expectations based on null model predictions. Higher functional richness within dry season communities is consistent with increased niche complementarity during the period when fish densities are highest and biotic interactions should be stronger; however, null model tests suggest that stochastic factors or a combination of assembly mechanisms influence dry-season assemblages. These results demonstrate that the relative influence of community assembly mechanisms can vary seasonally in response to changing abiotic conditions, and suggest that studies attempting to infer a single dominant mechanism from functional patterns may overlook important aspects of the assembly process. During the prolonged flood pulse of the wet season, expanded habitat and lower densities of aquatic organisms likely reduce the influence of competition and predation. This temporal shift in the influence of different assembly mechanisms, rather than any single mechanism, may play a large role in maintaining the structure and diversity of tropical rivers and perhaps other dynamic and biodiverse systems.

Molecular phylogeny of thorny catfishes (Siluriformes: Doradidae).

Doradidae is a monophyletic catfish family endemic to continental South America, and composed of 93 valid species here placed in 31 genera. Existing phylogenetic hypotheses for Doradidae are derived from comprehensive analyses of morphological data, and a single molecular-based study on a limited subset of taxa. To provide a robust molecular phylogeny commensurate with those based on morphology, we gathered original and published sequence data for 86 species-level taxa (at least 70 valid species plus 16 new or questionably nominal species) and all genera of Doradidae, as well as 10 species (nine genera) of Auchenipteridae and three species and genera of Aspredinidae as outgroups. 3011 base pairs were aligned for two mitochondrial genes (cytochrome c oxidase subunit 1, and 16S ribosomal RNA) and one nuclear gene (recombination activating gene 1), and analyzed for a total of 143 specimens (130 doradids, 10 auchenipterids and three aspredinids). Tree topologies generated by Maximum Parsimony, Maximum Likelihood, and Bayesian analyses were largely congruent, and are compared to existing phylogenies based on morphology and molecules. Although many of the relationships supported by our molecular analyses corroborated those based on morphology, others are newly hypothesized or remain in conflict. The monotypic Wertheimeria, Franciscodoras and Kalyptodoras, for example, form a newly proposed clade, and the subfamily Astrodoradinae is placed at the base of the doradid tree. The monotypic Doraops and Centrochir, endemic to Caribbean drainages north and west of the Andes, are sister to Pterodoras and Platydoras, respectively, two genera that are widely distributed in Atlantic drainages. Additional biogeographic implications are discussed for hypothesized relationships among doradids. Molecular evidence strongly supports synonymization of monotypic Merodoras with Amblydoras, and transfer of Amblydoras bolivarensis to genus Scorpiodoras. Furthermore, we consider Opsodoras ternetzi to be more properly placed in the genus Nemadoras. The genus Opsodoras may warrant synonymization with Hemidoras, and the monophyly of genus Ossancora is not supported; however, we refrain from taxonomic decisions regarding those taxa until a broader spectrum of doradids can be submitted to further morphological and molecular phylogenetic analyses.

Platydoras brachylecis, a new species of thorny catfish (Siluriformes: Doradidae) from northeastern Brazil

Platydoras brachylecis, new species, is described from coastal drainages of northeastern Brazil (Pindaré to Parnaíba rivers), and diagnosed from congeners by the unique combination of: pale yellow to white stripe beginning above orbits, continuing midlaterally on body and onto middle rays of caudal fin; skin in axil of each midlateral thorn without concentration of pigment forming small dark spot, midlateral scutes shallow (depth of 10th scute 8.8-11.9 percent of SL), and midlateral scutes on caudal peduncle distinctly separated by strip of skin from middorsal and midventral caudal-peduncle plates. Three additional species of Platydoras are recognized as valid: P. armatulus (lower Orinoco, Amazon and Paraguay-Paraná drainages), P. costatus (coastal drainages of Suriname and French Guiana), and P. hancockii (upper Orinoco, Negro, Essequibo and Demerara drainages). The nominal species P. dentatus and P. helicophilus are tentatively treated as junior synonyms of P. costatus. A key to species of Platydoras is provided.

Platydoras brachylecis, espécie nova, é descrita para as drenagens costeiras do nordeste do Brasil (rios Pindaré a Parnaíba) e diagnosticada de suas congêneres pela exclusiva combinação dos seguintes caracteres: faixa amarelo-pálida a branco iniciando acima das órbitas, continuando médio-lateralmente sobre o corpo e atingindo os raios medianos da nadadeira caudal; pele das axilas dos espinhos médio-laterais sem concentração de pequenas pintas negras; escudos médio-laterais baixos (altura do décimo escudo 8.8-11.9 por cento do comprimento padrão) e escudos médio-laterais do pedúnculo caudal distintamente separados das placas médio-dorsais e médio-ventrais da mesma região por uma faixa de pele. Mais três espécies de Platydoras são reconhecidas como válidas: P. armatulus (distribuída pelas drenagens do baixo Orinoco, Amazônia e Paraguai-Paraná), P. costatus (drenagens costeiras do Suriname e Guiana Francesa) e P. hancockii (drenagens do Negro, Essequibo, Demerara e alto Orinoco). As espécies nominais P. dentatus e P. helicophilus são provisoriamente consideradas sinônimos juniores de P. costatus. Uma chave de identificação para as espécies do gênero Platydoras é apresentada.