One of the least explored environments in New Zealand is the sea floor. Beyond the intertidal coast and limits of scuba diving (about 30 metres depth), few New Zealanders, other than marine scientists, have had any type of access to the sea floor.
New Zealand’s Exclusive Economic Zone is extensive, encompassing an area of sea floor that is 16 times larger than the dry land that forms its islands. The sea floor around New Zealand consists of:
- a relatively shallow continental shelf
- a slope that is dissected by underwater canyons
- a deep sea floor containing an array of prominent features such as trenches, ridges and seamounts.
Bottom dwellers
Organisms that live on or in ocean-floor sediments are known as benthic dwellers (from the Greek word benthos meaning ‘bottom’). They range in size from microscopic bacteria and diatoms, burrowing nematode worms and copepods a few millimetres long, through to larger seaweeds and invertebrates such as anemones, bryozoans, corals, sponges, crabs, shrimps, sea stars, sea urchins, worms, and bivalve and gastropod molluscs.
Food sources
In shallow waters where the light penetrates, benthic seaweeds and diatoms produce their own food through photosynthesis. In deeper waters where there is no sunlight for photosynthesis, much of the food eaten by benthic organisms arrives from above, in the form of clumps of dead plankton, faeces and corpses of marine animals, and the sunken remains of seaweeds and plants.
Typical feeding relationships
Bottom-dwelling animals are often characterised by the way they obtain food, for instance:
- The kina or sea urchin (Evechinus chloroticus) is a herbivore and eats seaweeds.
- Natatolana rossi, a sea slater, is a scavenger and feeds on other animals’ leftovers.
- The polychaete bamboo worm (Asychis trifilosus) feeds on deposits, digesting fragments of organic matter from the sediments it consumes.
- The zigzag cockle (Tawera spissa) is a filter-feeding shellfish, which extracts tiny particles of food from water passing over its gills.
- The predatory comb star (Astropecten polyacanthus) is a carnivore and feeds on sediment-burrowing invertebrates.
The animals of the sea floor are not only linked to one another through a benthic food chain, but also, are dependent on the life and death of organisms in the waters above them.