multinucleate cells covered with particles. Some are tangles of mineral-covered tubes
spread across the sediment surface only millimeters thick to the size of dinner plates;
others are smaller balls of tube tangle. Small metazoans are often associated with
these massive “protists”, in the tube mesh or underneath it (Buhl-Mortensen 2010).
(^) Nematoda are both numerous and remarkably diverse. For example, Lampadariou
and Tselepides (2006) found 104 genera (most with numerous species) in just one
closely spaced set of stations in the northern Aegean Sea. These small worms are
found in virtually all Earth habitats: both free-living and as parasites. The meiobenthic
forms move by wriggling against the sediment and feed with an oral complex that
varies by diet. Predators may have puncturing stylets, esophageal suction bulbs or
circlets of miniscule teeth. Many with simpler mouths ingest whole sediment,
digesting from it bacteria, diatoms, detritus, and also absorbing dissolved organic
matter. Experiments suggest capability for moving toward preferred foods guided by
olfaction (Moen et al. 1999). Meiobenthic copepods mostly belong to the order
Harpacticoida and occupy the lower limit of copepod adult sizes, about 0.5 mm. They
move by digging with their feet and by body flexion. Platyhelminthes (“flat worms”,
often termed Turbellarians) are significantly abundant and a diverse part of the
meiofauna, particularly the order Acoela, with a tiny mouth that opens through the
epidermis to a solid column of digestive tissue rather than to an open gut. Acoels are
worm-like in smaller sizes, while larger forms are flatter; all of them move through
sediment largely by ciliary action.
(^) The complexity of the meiofaunal taxonomic scheme duplicates virtually all of
phylum-level zoology from the cnidarians to the chordates, and sediment (or at least
very tightly confined spaces) is the only habitat of at least the five “minor” phyla
listed above. For extended information, we again recommend Giere (2009).
Gradients with Depth and Surface Productivity
Depth in the Sediment
(^) On the continental slope and deeper, the meiofauna and macrofauna within the
sediment are found very close to the sediment surface. Macrofauna stir the sediment
to depths of at most 10 to 15 cm, with reducing conditions excluding them from
below that level. Exceptions are larger forms that excavate and then irrigate a burrow,
so that the oxidative layer can line it for perhaps a centimeter to either side. The bulk
of meiofauna are found even closer to the sediment surface (Snider et al. 1984),
within about 4 cm, but members of the more abundant meiofaunal groups are also
found in vastly reduced numbers in deeper layers with significant sulfide
concentration. In all groups, there is a strong shift in the genera and species