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Sea floor

by  Ashley Rowden

Very few people have explored the sea floor – a mysterious world with canyons, trenches, seamounts and hydrothermal vents, and animals ranging from microscopic worms to corals, sponges, crabs and shellfish.

The sea floor environment

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.

Life on the continental shelf

Factors influencing animal distribution

The distribution and abundance of seabed animals is strongly influenced by the nature and topography of the sea floor. Bryozoans, corals and sponges attach themselves to rock and gravel seabeds, whereas mud or sand sea floors are the realm of burrowing worms and shellfish. In addition, the nature of the overlying water (its temperature, salinity, oxygen content, current strength) and the availability of food influence where and how many animals live on the sea floor.

Ocean depth and animal distribution

There is usually a decrease in the abundance and biomass (weight of living matter) of benthic animals with increasing water depth, from some tens of grams per square metre on the continental shelf to only a few grams on the slope. The number of species of any one group of animals, however, may show a unique relationship with depth.

Sediment-dwelling communities

There are regional differences in the distribution of sediment-dwelling animals on the continental shelf. Seventeen different communities have been described around New Zealand, and their distribution is partly related to the type of sediment. Trough shells (Mactridae) are dominant in the shallow sand and gravel off the southern and south-eastern areas of the South Island, and off the west coast of the North Island.

Communities dominated by types of cockles (Pratulum pulchellum and Pleuromeris zelandica) are found in mud at the outer edge of the continental shelf and are dominant off the east coast of the North Island and northern South Island. More restricted is a bristleworm–wedge shell community, which is confined to organic muds off the South Island west coast at depths of 60–70 metres.

Bryozoan hot spots

Bryozoans are animals that form coral-like clumps on the sea floor. Although relatively widespread on the continental shelf, there are exceptionally rich and diverse bryozoan communities at both ends of New Zealand. Three hundred species, or nearly one-third of all bryozoans known in New Zealand, live on the sea floor at the tip of the North Island. Foveaux Strait, at the southern end of the South Island, is another site of high bryozoan diversity.

The sites contain sea floor sediments dominated by broken skeletons and shells of bryozoans, corals and shellfish, and provide a suitable habitat for bryozoans. They are also areas of high productivity and have strong currents sweeping through them. These factors are probably important in ensuring that there is ample food for the bryozoans and that the colonies are kept free of suffocating sediments.

Life on the deep sea floor

The Chatham Rise

The sea floor of the Chatham Rise, to the east of New Zealand, is one of the few areas where deep seabed life has been studied in detail. Animal communities on the muddy northern flanks of the rise are different from those on the muddy southern flanks. These communities are also distinct from the animals on the rise’s sandy crest.

Biomass is highest on the crest and upper southern flanks. The seabed crest is dominated by crustaceans, especially mobile scavengers or carnivores such as the squat lobster Munida gracilis and sabre prawn Campylonotus rathbunae. Sea cucumbers and brittle stars inhabit the muddy flanks.

The composition of the different communities across the Chatham Rise sea floor seems to be related to the productivity of the Subtropical Front. This is an area above the rise where the cold nutrient-rich waters from the subantarctic meet and mix with warm nutrient-poor waters flowing from the subtropics. The movement of these waters brings nutrients into the upper waters, which supports increased plankton growth. This, in turn, feeds the animals in the lower waters of the rise.

Seamount life

Seamounts are prominent and widely distributed in New Zealand’s marine environment, and are often the focus of commercial fisheries. Seamounts typically have a distinctive cone shape and rise hundreds or thousands of metres above the sea floor. They are usually made of hard rocks, often volcanic in origin.

Although relatively few species are found on seamounts made of pumice or volcanic glass, many seamounts sustain very productive ecosystems. Seamounts with great biodiversity can harbour thickets of deep-water coral that provide a habitat for an array of other organisms such as brittle stars, bristleworms and squat lobsters.

Adjacent seamounts, separated by only tens of kilometres, may have entirely different groups of animals living on them. This is not solely due to differences in seamount composition, but is also related to food supply. Food supply, in turn, is influenced by the productivity of the surface waters above seamounts. Currents that disperse eggs and larvae of seabed animals can also determine the composition of seamount communities.

Animals of the deepest reaches

It is particularly difficult to study the sea floor’s deepest reaches (beyond 6,000 metres). Only a handful of samples have been trawled from the Kermadec Trench, which is the deepest seabed in the New Zealand region. There is life down there: worms, sea stars, sea snails, bivalve molluscs, sea cucumbers, shrimps and barnacles have all been discovered.

Vents and seeps

One of the major biological surprises of the late 20th century was the discovery of a group of seabed organisms that live independently of photosynthesis-based food chains. Instead, they rely directly or indirectly on chemicals flowing from the sea floor.

Animals around vents

These creatures were first discovered in 1977 in the eastern Pacific where they were living in hot water around a hydrothermal vent (an underwater volcanic hot spring) several thousand metres deep. Since then, a number of hydrothermal vents have been discovered around New Zealand. Among the more conspicuous animals found at these sites are:

  • at least two species of large mussel
  • a large, predatory sea star, found nowhere else to date
  • a number of species of blind shrimp
  • two species of large tubeworm
  • a pale eel-like fish
  • crabs.

Life around methane seeps

Similar distinctive communities of seabed animals occur around underwater seeps of hydrocarbons (primarily methane). These cold seeps are found at the undersea margins of continents. At least 3 sites at depths of 900–1,800 metres off the East Coast of New Zealand are known to harbour living representatives of these communities. The dominant animals are shellfish (clams, a type of mussel, and limpets).

Chemosynthetic feeders

Animals that live around hydrothermal vents and cold seeps can exist there because of chemosynthetic bacteria. These synthesise food (sugars) using energy-rich chemicals such as hydrogen sulfide and methane. They are the primary producers within the vent and seep ecosystems. Some animals feed on the bacteria directly, while others have bacteria living on or in their bodies in a symbiotic relationship – the host animals get food and the bacteria gain shelter.

External links and sources

More suggestions and sources

How to cite this page: Ashley Rowden, 'Sea floor', Te Ara - the Encyclopedia of New Zealand, (accessed 24 July 2021)

Story by Ashley Rowden, published 12 Jun 2006