The hadal community, an introduction

Abstract

1. (1) During the last ten years the Danish Galathea and the Russian Vitjaz Expeditions have made biological investigations in thirteen deep-sea trenches at depths exceeding 6000 m. Due to the complicated configuration of the trenches trawling is often very difficult and can only be undertaken after careful echo sounding. 2. (2) Environmental features including extent, sediments, pressure, oxygen, salinity, temperature, and food supply were examined. 3. (3) The quantitative composition of the bottom fauna seems to vary considerably from one trench to another, depending on the abundance of food supply in the photic zone above. Quantitative bottom samples indicate a biomass as high as 10–40 g/m 2 in certain trenches. 4. (4) In eight of the trenches a total of 250–310 species have been found. The decrease in the number of species with increased depths within these eight trenches is smaller than might be expected (probably at the most 50 per cent). Dominant groups are actinians, polychaetes, isopods, amphipods, gastropods, lamellibranchs, and holothurians. Considering the total number of species in each group, the most important are : Pogonophora, echiuroidean worms, holothurians, and isopods. Decapod crustaceans, brachiopods, and turbellarians have no hadal representatives, and other groups (including fishes and bryozoans) are very insignificant in the hadal zone. 5. (5) So far 127 species (and subspecies) from depths exceeding 6000 m have been identified. Of these 74 (58 per cent) are endemic to these depths. The upper limit of the hadal zone might better be set at 6800–7000 m resulting in 62 endemic species (43 per cent). As thus defined, fourteen genera and two families (of actinians and pelagic amphipods) seem to be restricted to the hadal zone. 6. (6) Nine exclusively hadal species are found in more than one trench (two of them in three trenches), but most of these are close to one another geographically. 7. (7) The communities in the hadal zone are listed in Appendix 3. Several species (especially of holothurians and Pogonophora) must occur in great numbers. 8. (8) Both the benthic and pelagic hadal animals are without pigmentation and are blind. In probably all crustacean groups living at great depths a remarkable gigantism has been found, probably due to the effect of the hydrostatic pressure on the metabolism. 9. (9) The hadal fauna is certainly derived from the abyssal zone. Endemic families, genera and perhaps also some species may be considered as relics of a preglacial abyssal and hadal fauna, but the main invasion of eurybathic species came after the onset of the first glaciation. 10. (10) Very little is so far known of the hadopelagic fauna. The few hauls with closing nets seem, however, to indicate a high percentage of endemism.