Body Size Versus Depth

Regional and Taxonomical Variation in Deep-Sea Meio- and Macrofaunal Organisms

Jesse M.A. van der Grient1; Alex D. Rogers    Department of Zoology, University of Oxford, Oxford, United Kingdom
1 Corresponding author: email address: jesse.vandergrient@zoo.ox.ac.uk

1 Introduction

Body size (weight per individual) is one of the most important properties of an organism as it is related to, and can be used to predict, many other co-varying characteristics, such as physiology, life history, and ecology (Peters, 1983). The relationship between body size and abundance can link individual- and population-level species traits with the dynamics and structure of ecological communities (Woodward et al., 2005). Furthermore, body size is a determinant of resource use as it is related to metabolism, and thus can aid in predicting resource partitioning (Brown et al., 2004). Body size relationships have been extensively studied in terrestrial environments, but less so in the oceans and in particular the deep sea (depths greater than 200 m). The deep sea is the largest ecosystem on Earth and is increasingly recognized as important in global biogeochemical cycling (Dunne et al., 2007; Giering et al., 2014). Benthic ecosystems, especially those of ocean margins, are important in carbon burial (Dunne et al., 2007) and can have remarkably high levels of biodiversity, especially considering they are generally recognized as food-limited environments (Grassle and Maciolek, 1992). Body size and temperature have been identified as having a significant influence on the metabolic rate of deep-sea organisms, as well as on aspects of life history such as longevity and population turnover (McClain and Barry, 2010) supporting the Metabolic Theory of Ecology (Brown et al., 2004).

Since the 1950s, quantitative sampling and ecological investigation in the deep sea have documented general patterns in community structure and functional ecology (Gage and Tyler, 1999). One of the most widely recognized of these patterns is the decline in community biomass and abundance, and individual body size, with increasing depth and decreasing surface-derived particulate organic matter, in other words, food (Rex et al., 2006; Wei et al., 2010a). Understanding how such patterns of community attributes and organismal traits vary both spatially and temporally is important to understanding how climate change effects, such as ocean warming, and other human impacts, like deep-sea trawling, may influence ecosystem function within these communities, ultimately with implications for predicting changes in the biological cycling of important nutrients within the Earth system.

Early work in deep-sea ecology resulted in the recognition of two trends of body size with increasing depth: dwarfism and gigantism (Berkenbush et al., 2011; Danovaro et al., 2002; Galeron et al., 2000; Jones, 1969; Pequegnat et al., 1990; Pfannkuche, 1985; Rex and Etter, 1998; Rowe et al., 1991; Schwinghamer, 1985; Shirayama and Horikoshi, 1989; Soetaert and Heip, 1989; Thiel, 1979; Thurston, 1979). Of these two, dwarfism is thought to be more common (Madsen, 1961; Pfannkuche, 1985; Rex et al., 2006; Soetaert and Heip, 1989; Thiel, 1975). Rowe and Menzel (1971) are often acknowledged as the first to have quantitatively demonstrated the trend of decreasing body size with increasing depth. Their results, however, showed no significant decrease in body size with increasing depth; rather, they accepted the hypothesis that the slope of decreasing abundance with increasing depth was different from the slope of decreasing biomass with increasing depth based on a non-significant p-value (P = 0.13). The trend of no change in body size with increasing depth has been supported by some studies for macrofaunal and meiofaunal organisms (Polloni et al., 1979; Shirayama, 1983; Vanhove et al., 2004), whilst other studies have found an increase in size (Alongi, 1992; Rex et al., 1999). Alternatively, parabolic patterns of body size with depth have been observed as well (McClain et al., 2005). This variety of trends may indicate that there is no general rule; rather, we should look to explain the observed differences.

The conflicting results may reflect differences in oceanographic regions. Many studies present their data and trends as a universality, rather than as a local or regional phenomenon in ecology. However, when regions differ in food availability, for example if food is vertically transported or laterally advected or is subject to resuspension in different parts of the continental slope (Dell'Anno et al., 2013), then changes in body size may not correlate with depth. There are also potential regional variations in the efficiency of transfer of surface primary production to the deep sea (Buesseler et al., 2007).

If there is a taxonomic signal in body size, then changes in the taxonomic composition of communities as a result of changes in depth or other physical parameters (e.g. sediment grain size; Rohal et al., 2014), could also influence estimates of average community body size. Community composition is expected to differ in space and time, and this has been observed (Billett et al., 2010; Blake and Grassle, 1994; Cosson-Sarradin et al., 1998; Danovaro et al., 2010; Flach and de Bruin, 1999; Ruhl et al., 2008). Varying community composition can have contrasting influences on the nutrient recycling and carbon burial of a particular region, for example through differences in bioturbation potential (Braeckman et al., 2011; Dauwe et al., 1998).

Two size classes often studied in the benthic marine system are the meiofauna and the macrofauna. A rough distinction between the two is that macrofauna can be seen when they are lying in your hand, but are not visible in pictures of the environment; meiofauna require microscopes to see them. A more scientific distinction is the range of sieve sizes that can be used to separate them: 0.5–1 mm for macrofauna and 0.063–0.5 mm for meiofauna. These sizes are based on shallow-water ecosystems. It has been found that in the deep sea, this does not adequately capture the benthic fauna, and thus smaller sizes are used. For example, from 0.25 to 1 mm sieve meshes are used for macrofauna, while meiofauna can be determined from 0.025 to 0.5 mm meshes. An important observation in this is that the sieve sizes can overlap.

Many marine invertebrates have indeterminate growth and might be present in different size classes through different stages of their lives. Studies differ in their approach to this phenomenon: some include all organisms captured by a specific sieve size, while others look at the predetermined fauna senso stricto meaning that they can potentially, for example, exclude nematodes from macrofauna and polychaetes from meiofauna.

When looking at the two size classes, a bimodal distribution in the frequency-size relationship is observed. There is much debate about whether this distribution shows biological adaptation to disruptive selection for two different lifestyles, resulting in the meio- and...