Stable associations of more than one species of symbiont within a single host cell or tissue are assumed to be rare in metazoans because competition for space and resources between symbionts can be detrimental to the host. In animals with multiple endosymbionts, such as mussels from deep-sea hydrothermal vents and reef-building corals, the costs of competition between the symbionts are outweighed by the ecological and physiological flexibility gained by the hosts. A further option for the coexistence of multiple symbionts within a host is if these benefit directly from one another, but such symbioses have not been previously described. Here we show that in the gutless marine oligochaete Olavius algarvensis, endosymbiotic sulphate-reducing bacteria produce sulphide that can serve as an energy source for sulphide-oxidizing symbionts of the host. Thus, these symbionts do not compete for resources but rather share a mutalistic relationship with each other in an endosymbiotic sulphur cycle, in addition to their symbiotic relationship with the oligochaete host.
The meiofauna of hydrothermal deep-sea sediments in the North Fiji Basin (NE Pacific) was investigated. Nematodes were the dominant taxon. The structure of the hydrothermal nematode communities from the Fiji Basin is compared with (1) the communities from adjacent oxic deep-sea sediments. ( 2 ) other hydrothermal vent areas and (3) shallow reduced environments such as cold seeps and subsurface anoxic sediments of eutrophic bottoms. Although the genus composition of the hydrothermal area and the biodiversity on a generic or functional-morphological level did not deviate greatly from the control areas, we found differences at the species level and in the size spectra and the species diversity. None of the species found in the hydrothermal sediments occurred in the surrounding areas. The size spectra of the vent communities was shifted towards larger nematodes, and species diversity was much lower in the hydrothermal area. The similarity in nematode genus composition between hydrothermal sites and control areas stands in contrast with the presence of a specialized endemic hydrothermal macrofauna. The absence of a planktonic life stage, in combination with small size, makes it more difficult for nematode species to migrate between isolated hydrothermal patches.
Hydrothermal vents, at depths varying from the littoral zone to water depths of 115 m, have been explored around the islands of Milos and Santorini in the Hellenic Volcanic Arc. The biota were surveyed by scuba divers and with a remote operated vehicle as well as by soft-bottom sampling with grabs and corers. No species specific to hydrothermal areas were found at any of the sites investigated. Animals in the immediate vicinity of the vents belonged to opportunistic species, such as the polychaetes Capitella capitata, Microspio sp. and Spio decoratus. The nassariid gastropod Cyclope neritea was the dominant macrofaunal species found in the bacterial mat areas at Milos which overlay hot brine seeps. At all rocky hydrothermal sites deeper than 35 m water depth the echiuran Boniellia cf. viridis was observed. Around the periphery of the seeps the meiofaunal community was dominated by the nematode Onchlaimus camplyoceroides. Few nematodes were found in the centre of the hydrothermal brine areas. At Santorini the bacteria at the venting sites were dominated by iron bacteria, whereas at Milos large globular sulphur bacteria, Achromatium volutans, covered the hydrothermal brine seeps. Unlike deep-sea vents, little of the biomass production was due to symbiotic associations between animals and chemoautolithotrophic bacteria, although a few stilbomematine nematodes were found at the periphery of the brine seeps at Milos. Macrofaunal biomass reached a maximum of 80 g m -2 around the vents, compared with ~>c 500 g m -2 at hydrothermal vents elsewhere.
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