On the Benthic Invertebrate Megafauna at the Mid-Atlantic Ridge, in the Vicinity of the Charlie-Gibbs Fracture Zone

Abstract

Little is known about the fauna that inhabits non-chemosynthetic environments associated with mid-ocean ridges. This thesis investigates a ridge and fracture zone system to assess its influence as a barrier to faunal dispersal, and as a unique bathyal habitat. It also describes the ecology of megabenthic communities inhabiting a ridge. Sites were chosen on the Mid-Atlantic Ridge in the vicinity of the Charlie-Gibbs Fracture Zone, at a target depth of 2,500 m. Four superstations were chosen north and south of the Fracture Zone, on either side of the ridge. Different productivity levels and hydrographic features were characteristic for the northern and southern sites. In order to characterise the benthic megafauna 50 ha were trawled and 32,000 m2 of seafloor were sampled with HD video footage, targeting both flat and 10 ? sloped habitats. Holothurians were the most abundant megafauna. In order to assess their evolutionary relationship 43 holothurian specimens were genetically studied by modelling five of their genes (16S, 18S, 28S, COI, H3) in a phylogenetic analysis. All four sites exhibited noticeably different faunal characteristics. The biomass was highest at the SE, and lowest at the NW site. Body sizes differed between sites for most taxa, that were sufficient in numbers to be compared between sites, most likely as a result of different adaptations to food supply. Differences in species richness were observed between the sampling methods, with the highest richness at the SE site in trawl samples, and highest at the NW and SW sites in the video survey. Species densities were highest at the northern sites with both methods. Differences in diversity were also observed, with trawl samples providing a higher taxonomic resolution than the video survey and showing highest diversity at the SE site and lowest at the NE site. Community composition was significantly different between sites. Variations in the composition of megabenthic assemblages were observed between flat and 10 ? sloped habitats, although the effect of slope appears to be site dependent. The genetic analyses revealed a close relationship between individuals from different families. The extent to which the Ridge acts as a faunal barrier was unclear as the southern sites lacked an obvious difference in community composition. Faunal differences to the north and south of the Fracture Zone, however, suggest that this feature is a barrier to dispersal. The contrasting megafaunal assemblages of the sites probably reflect a combination of environmental drivers including sediment type, phytodetrital quality, hydrography, and habitat complexity.

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