filtering particles from water above the sediment. At greater depths, eulamellibranchs
are progressively replaced (Fig. 13.9) by protobranchs, clams with smaller, simpler
gills not involved in feeding. Protobranchs are deposit feeders, sorting sediment with
their labial palps and ingesting portions found suitable. Both Gastropoda (snails) and
Scaphopoda (tooth shells) are found in and on sediment at all depths. Some snails
deposit feed, others are predators, particularly those that bore into bivalves.
Scaphopods can in places be a significant fraction of the deep-sea fauna. Their soft
tissue is protected by a gently flaring calcareous tube buried wide end down in the
sediment. The head and a muscular, digging foot extend from this lower end. Action
of the foot moves the shell and body through the sediment. The head bears a tuft of
captacula, thread-like tentacles with sticky, sensory pads at the tips. The foot pushes a
cavity into the sediment, and the captacula search along and into the cavity surface for
suitable food, which is then transported along the tentacle to the mouth by ciliary
bands (Gainey 1972). Most of the diet is foraminifera (Bilyard 1974). Aplacophora, a
primitive group of mollusks related to chitons and snails, are found in small numbers
at abyssal depths.
(^) Several phyla of worms reach significant abundance in deep-sea sediments.
Sipunculids (phylum Sipuncula or Spunculida) look like peanut shells and are fondly
called “peanut worms”. The body is an elongate sac bearing a tuft of tentacles around
the mouth at the narrow anterior end. In some the tentacles are for gas exchange, in
others they are active in feeding. Most are non-selective deposit feeders.
Micro- and Meiofauna
(^) Compared to the water column above, marine sediments are rich in bacteria (<2 μm)
microfauna and in a complex assemblage of protozoa and small metazoa termed
meiofauna. Bacteria adhere to smooth surfaces of clay, silt, and sand grains, and
burgeon in cracks and striations on the mineral bits. They are abundant in interstitial
water. In surficial, oxygenated layers of the sediment bacteria metabolize organic
matter, with particular importance for breakdown of more refractory constituents.
They are responsible for a majority of sedimentary oxygen consumption. Their
numbers continue to be high down the sedimentary column into anoxic layers where
they continue to oxidize organic matter by stripping oxygen from nitrate and,
quantitatively more important, from sulfate. The sulfide produced gives sediments and
estuarine mud their characteristic, rotten-egg odor. At still deeper levels, bacterial
abundance remains high, although their “biogeochemical” activity may not. The
proportions that are active metabolically remain an issue; some may be effectively
dead but show up in epifluorescent counts of preparations with nuclear stains. It was
established long ago that many, sediment-dwelling deep-sea bacteria are obligate
cryophiles and barophiles, requiring the cold (<3°C) and hydrostatic pressure of the