Traits and depth: What do hydroids tell us about morphology and life‐history strategies in the deep sea?

Abstract

AbstractAimTraits 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.LocationAtlantic Ocean and adjacent polar seas, from 50 to 5,330 m deep.Time periodPresent day.Major taxa studiedHydrozoa.MethodsAnalyses 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.ResultsTraits 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 conclusionsWe 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.