Distribution of Pogonophora in Canyons of the Bay of Biscay: Factors Controlling Abundance and Depth Range

Abstract

The Pogonophora are tube-worms of predominantly deep sea distribution. They lack a functioning alimentary canal in the adult stage and are then dependent for nutrition on internal symbiotic chemosynthetic bacteria that occupy tissue derived from the larval endoderm (Southward, 1982, 1987, 1988; Southward & Southward, 1987; Southwardet al.1981). Of the two main subgroups, the small perviate Pogonophora are widely distributed in reducing sediments in the oceans, while the large vestimentiferan or obturate Pogonophora are restricted to hydrothermal vents or cold seeps. The small Pogonophora are often most abundant on steep slopes and in the Bay of BiscaySiboglinum atlanticum(Southward & Southward, 1958) can be a dominant element of the infauna of the sediments on the sides of canyons. This animal, like other small Pogonophora, lies buried in the sediment, in contrast to the vestimentifera which are attached to hard substrates. Until now there has been no fully quantitative information on the distribution ofS. atlanticumand associated pogonophores of the Bay of Biscay. Dredges or trawls have been used for most previous sampling of pogonophores along the continental slope (Southward, 1979, 1985). Even the Plymouth-pattern deep-sea anchor dredge (Southward & Southward, 1963), which is designed to dig into the sediment immediately it is towed on the bottom, may drag for some distance through soft sediments before digging in, and then samples the upper few centimetres over a wider area than its mouth opening. During the last season of operation of R.R.S. ‘Frederick Russell’, before this vessel was disposed of by N.E.R.C., opportunity was taken to make a combined quantitative biological and chemical survey, using box-corers. The samples from the corers were large enough to allow the sample to be used for assessment of the population density of the pogonophores and analysis of sediment chemistry. Some inferences can now be drawn about the factors controlling pogonophore abundance.