Biologists refer to species dwelling on the sea floor at any depth as benthic organisms or ‘the benthos’. They are also known as bottom dwellers. Fish are common in benthic communities in the mesopelagic and bathypelagic zones, but there are none on the deeper sea floors of the abyssopelagic and hadopelagic zones. Energy is in scarce supply here, deriving mainly from falling food that has made it past all the waiting mouths above.
Bacteria are probably the base for most benthic food chains, as they are able to release nutrients that other life forms cannot make use of directly. For example the armour-like exoskeleton of zooplankton such as krill contains carbohydrates locked up in the tough chitin or cellulose that only bacteria can break down. These nutrients can then be used by slightly larger organisms referred to as the meiofauna, which live in the fine sediment that covers most of the deep-sea floor. They in turn concentrate nutrients and make them available to larger organisms. The occasional bonanza, such as a dead whale, may provide food for decades.
Cold seeps occur in places along the edge of continental shelves between 300 and 600 metres deep. Methane (natural gas), hydrogen sulphide and other compounds seep out of the sea floor in a stream of bubbles at about the same temperature as the surrounding water. The original source of some or all of the methane is archaea (similar to bacteria) living in sediments as deep as 750 metres under the sea floor. Other archaea and bacteria use these compounds for energy by a process called chemosynthesis. There is a specialised ecosystem of creatures such as clams and tubeworms based on these chemosynthesising organisms.
Hot deep-sea vents
Also known as hydrothermal vents, these are similar to geysers, producing superheated, mineral-laden water. They are found along zones of tectonic or volcanic activity such as mid-ocean ridges where hot magma is near the sea floor. The minerals are used by chemosynthesising organisms, forming the basis of a unique community that does not depend on sunlight. The giant tubeworm Riftia pachyptila, found around vents, can grow 1.5 metres long, yet has no mouth or anus and only a vestigial gut. Both Riftia and the large clam Calyptogena harbour dense colonies of bacteria. The relationship is mutually beneficial: the animals get a food source, and the bacteria get a place to live.
Nutrients at deep sea vents are much more abundant than at cold seeps, and the surrounding sea floor is crowded with life forms.
Emerging from the deep
In New Zealand’s southern waters around Fiordland, light is reduced by humic-stained fresh water on the surface, enabling species that usually live in deeper water to be observed within a safe scuba-diving range. The sponge Symplectella rowi may be found at depths of 30–50 metres on cliff walls in Doubtful Sound. In other parts of Fiordland a small mollusc (Melanella luminosa) is frequently found on the scarlet sea cucumber (Ocnus brevidentis), itself a species normally living at much greater depths.
Echinoderms are the most obvious deep-sea species. They include starfish (sea stars) and brittle stars, sea urchins and sea cucumbers. Sea cucumbers may move by swimming or squirting water. Others walk on the sea floor using tube feet. In 1986 a new group of echinoderms, the Concentricycloidea, was discovered on wood in deep water off the New Zealand coast (although some taxonomists no longer consider them a new class).
Crustaceans of various kinds, polychaete worms and bivalve molluscs are also reasonably common. Some deep-sea invertebrates – most notably some isopods and pycnogonids (sea spiders) – are gigantic.
Among the benthic fish species is the strange tripod fish (belonging to the family Triacanthidae), which has long feelers on its pectoral and caudal fins. The ventral and caudal fins act like a tripod, holding the fish on the sea floor, while the pectorals are held sideways and detect any water movements caused by prey. These fish are virtually blind. Other bottom-dwelling species are fished commercially and end up on the table – grenadiers (or rat-tails), orange roughy, oreos and cusk eels (sold as ling). Less savoury characters include hagfish, brotulas, toadfish and snailfish.