Aim Traits affect the survival and reproduction of individuals in different habitat conditions, ultimately altering their distributions. In the oceans, changes in environmental conditions with bathymetry may influence the occurrence of specific traits. Therefore, characterizing trait variation with depth can illuminate drivers related to the distribution of diversity of forms, functions, and life histories. We aimed to investigate patterns of variation in the diversified life histories and morphologies of hydroids with depth, integrating these patterns with the natural history of the group and ecological principles of the deep sea. Location Atlantic Ocean and adjacent polar seas, from 50 to 5,330 m deep. Time period Present day. Major taxa studied Hydrozoa. Methods Analyses were based on 14 traits collected for a total of 4,668 specimens of hydroids, belonging to 438 species. Records were divided into 12 depth strata for comparisons. We evaluated: how each trait varies with depth; whether variation in some traits is affected by the presence of other traits; how traits covary; and similarities in trait compositions among depth strata. Results Traits of hydroids vary with depth, with more pronounced differences for regions deeper than 1,000 m. Hydroids are generally smaller, infertile, solitary, meroplanktonic, and devoid of protective structures with increasing depth. The relationship, however, is not always linear. Also, some covariation and correlation between traits was evident. For example, depth may affect size differently according to the presence of specific traits such as structures protecting against predation. The lower proportion of fertile specimens recorded in the deep sea suggests that chances for genetic recombination are reduced in deep‐sea populations, ultimately leading to a slower rate of evolution. Main conclusions We identified novel trends in hydroid trait variation with depth by combining observations on morphology, ecology, and life history, clarifying selection pressures on hydroids in the deep sea.

An important component of species diversity which has recently received significant attention is the change in species composition across space, or beta diversity. Temporal turnover of species is an equally important component of species diversity, which can by itself account for diversity patterns, but has received much less attention, especially in marine communities. Indeed, patterns of diversity of marine epifaunal assemblages may be better understood through the study of variability in richness and composition of recruits that colonize and survive long enough to start a population, but before species interactions mask the diversity initially settled. Here we document temporal patterns in richness and composition of recruits of a speciose benthic cnidarian assemblage, contrasting a tropical and a subtropical site to evaluate simple predictions about latitudinal diversity and richness stability. Recruitment variation was quantified during five three-month periods at two harbour areas (Pecém at 3°32′S and Ilhabela at 23°46′S) in Brazil separated by 20°latitude. Results show that: (1) species richness of benthic cnidarian assemblages was higher at the more tropical location, as expected from general patterns in marine species global distribution; (2) contrary to the expectation of a negative association between richness and species turnover, temporal turnover of species was similar at both sites, although more seasonal at the higher latitude site; and (3) species temporal persistence was greater at the subtropical site. Results are discussed in the context of how different environments and different regional richness may influence temporal patterns in species turnover and local richness, and how this may vary across latitudes.

The bathymetric variation in the oceans and associated environmental changes impose limits on the distribution of species, modulating the occurrence of individuals with different forms, functions and life histories according to depth, and is therefore important for the understanding of marine biodiversity patterns. This study aims to infer patterns of hydroid distribution in the Atlantic Ocean and adjacent Arctic and Antarctic seas at more than 50 m deep, seeking to contribute to the understanding of the diversification and structuring associated with the bathymetric variation that favored the occupation of the different environments by the group. We present for the first time inferences on the bathymetric ranges of distribution of the species, on the variation of functional traits of individuals and species with depth, and on the distribution of the species composition along depth and latitude. Together, the results indicate that the distribution of hydroids in the deep Atlantic is related to historical factors and to the environmental gradients associated with latitudinal and bathymetric variations. Reduced sizes and low fertility in deep sea suggest that colonization and evolution of hydroids along depth are mainly influenced by food availability and low population densities. Also, the greater proportion of solitary species and individuals in the deep sea and the greater use of unconsolidated substrates suggest influence of substrate availability. The proportion of species capable of releasing medusae below 50 m deep is generally lower than in shallow coastal waters, but the proportion increases with depth, especially below 1,500 m. The release of medusae would be disadvantageous in an environment with low population densities, by increasing the uncertainty of fertilization given by the dispersion of gametes, and expending more energy for reproduction in a scenario of few food resources. Wide bathymetric distributions suggest vertical dispersal capacity and high tolerance to the environmental changes associated to the bathymetric variation. The results also indicate that colonization of hydroids in the deep sea occurs in a source-sink system in which deep-sea populations would be sustained by shallower water immigration. We show in this study that hydroids are important inhabitants of the deep sea and that the understanding of the diversity of the group in this environment will benefit from investigations in areas still poorly sampled, such as southern tropical latitudes and depths below 1,000 m.

Aos meus pais. AGRADECIMENTOSÀs amigas Amarílis, Lígia e Bia, por terem me recebido como irmã e terem tornado o período que passei em Fortaleza tão agradável.À Ariane Bettim, por ter me ajudado a dar os primeiros passos no mestrado.A todos do LEM (Amanda, Thaís, Lucília, María, Nathalia, Juliana Bardi, Juliana Rossi, Camila, Max, José Eduardo, Luciana, Daniel e Marcos), por toda ajuda, contribuição, amizade, apoio, e por tornarem os dias no laboratório muito alegres.Aos meus amigos do IB, pela amizade e confiança.Ao Felipe, pelo amor, força e incentivo.Aos meus pais e à minha irmã, pelo importante e total apoio em todos os meus caminhos.