depths the pressure is 300 to 600 atmospheres. At any one site it is nearly invariant
from a biological perspective. So far as is known, these conditions have been
consistent since at least the end of the Wisconsin glaciation (∼8000 YBP).
(^) We can predict, before we begin reviewing the observations, that variations among
benthic habitats will depend upon:
(^1) substrate type − rock or sediment, and within sediment there will be
habitat variation with particle size, which we characterize as gravel > sand >
silt > clay (and much sediment is characterized as diatom or foram “ooze”);
2 depth;
3 food supply.
(^) Rock supports assemblages of attached fauna. Until recently, with the advent of
submersibles and remotely operated vehicles (ROVs), these have been very difficult
to study. Sediments support both infauna, animals that nestle in or move through the
substrate, and epifauna that slide or run about on the surface.
(^) Since most of the seafloor is sediment-covered, and sediments are readily sampled
by grabs, corers, or dredges, a majority of studies of “the benthos” have been of
infauna. In order to see the animals brought up by a grab, say, the sediment must be
removed from around them. A sample is placed on a screen and the sediment is gently
washed through. Animals not going through with the sediment are those larger than
the mesh, so mesh-size divisions define “ecological” categories:
(^) NAME GENERAL SIZESCREEN
Megafauna 1 mm Any practical size
Macrofauna>1 mm Held by 0.5 mm
Meiofauna 0.1–1 mm Passing 0.5 mm, but held by 0.062 mm
Microfauna <0.1 mm
(^) These groups are usually constituted of different animal taxa, and since it is usual
for one worker to master only one or a few large groups of organisms (say amphipods,
annelids, or clams in the macrofauna, or nematodes, harpacticoid copepods or
foraminifera), they are usually studied by different people. The life stories among
these size categories are even considered to be separate problem areas in ecology.
Megafauna (sometimes defined as visible in photos or on ROV video) and
macrofauna are easier to study (less microscopy), so they are rather better known.
Meiofauna are important ecologically, and they are under active study. The microbiota
of bacteria and smaller protozoa are much more numerous than in the water column
and account for a majority of overall benthic metabolism.