Deep-Sea Biodiversity: A Morphological Dimension

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Research on the biodiversity of deep-sea soft sediment habitats has centered largely on a?diversity, that is estimating local species diversity and its geographic variation in the ontext of traditional explanations involving disturbance, biological interactions and environmental heterogeneity. Little is known about the natural history and distributional patterns of the species themselves - information that is vital to understanding the roles species play in community organization, and the relationship between local diversity and the regional-scale processes that ultimately regulate it. Here we propose two avenues of research to relate biogeographic and life-history characteristics of species to deep-sea species diversity. The first is to introduce morphological variation in shell architecture of gastropods as a new dimension of biodiversity. The second is to explore the relationship of body size to depth within and among species of prosobranch snails. Both add a new level of understanding about how deep-sea communities are structured that could not be attained by analyses of simple a-diversity. The proposed research directly addresses the NSF Biological Oceanography theme of Biological Diversity in Marine Systems, particularly the areas of distribution, abundance, and life history of marine organisms from deep-sea ecosystems; adaptations of marine organisms; and their population ecology.1. Morphological Variation. We present a preliminary analysis of bathymetric ranges and variation in shell architecture in lower bathyal and abyssal assemblages of gastropods from the western North Atlantic. The abyssal fauna is revealed to be merely a subset of the bathyal fauna consisting of species that extend their ranges deeper and out onto the abyss. In gastropods, shell architecture reflects feeding type, mode of development, predator defense mechanisms, locomotion and calcium carbonate conservation. We construct both empirical and theoretical morphospaces of shell form by using an eigenshape analysis and Raups model of shell geometry. Both approaches yield the same result. In terms of the range of basic shell types, the abyssal fauna appears to be a random subset of the richer bathyal fauna. However, the abyssal fauna is dominated numerically by a few core deposit-feeding species of bathyal origin with shells that are preadapted for carbonate conservation below the CaCO3 compensation depth. The combination of morphological variation, bathymetric range, feeding type and dispersal ability suggests a new hypothesis for abyssal biodiversity that is radically different from the ecological determinism of current theories. We propose that bathyal and abyssal environments comprise a source-sink system in which the abyssal fauna largely represents a mass effect. The abyssal fauna appears to be an attenuation of the bathyal fauna with a small minority of abundant species, and a majority of extremely rare species that may represent nonreproducing populations maintained by dispersal. We will test this hypothesis by extending the analysis to four other deep-sea basins, and by determining the reproductive state of bathyal and abyssal populations of the same species. Since the great abyssal plains represent such a huge ecosystem, this research has very significant implications for the genesis and maintenance of diversity, and for estimating the level of global deep-sea biodiversity.2. Body Size. Body size is an issue of fundamental significance in biology, and one that has an important bearing on abundance, distribution, macroecology and biodiversity.Results from prior NSF support (OCE-9301687) showed that, in contrast to the prevailing view, individual gastropod species increase in size with depth. Size-depth clines decrease in slope with depth and bathymetric ranges expand. We present a conceptual model that integrates this very consistent trend within species with the observation that average size among species appears to decrease with depth. We propose measuring the average size component of the model. This research has the potential to reconcile a very basic paradox in deep-sea ecology, and will provide, by far, the largest and most well-controlled database on size in the deep-sea benthos.