depths to function. At least some forms that can be cultured do revive and grow
(slowly) when recompressed in a laboratory after a trip to the surface in a grab or
corer.
(^) Feeding on the bacteria, on microparticulate detritus, possibly on DOM and on each
other are the meiofauna. The array of such animals is studied by suspending a
sediment sample in a larger volume of seawater and picking through it under a
microscope with a pipette. Identification also requires microscopy. A thorough but
accessible treatment of what these animals are like, of their biology and ecology, is
given by Giere (2009). We only touch on those aspects here. Fully 20 phyla of
animals are included. The Gnathostomulida, Kinorhyncha, Loricifera, Gastrotricha,
and Tardigrada are found only as meiofauna, some of them also in fresh water,
tardigrades even in moist habitats like moss, as well as in marine sediments. The rest
are miniscule representatives of phyla usually thought of as much larger animals, for
example, gastropods and holothurians. Apart from larvae of macrofauna and with
exceptions like Loricifera, small size in metazoans is usually attained by reducing cell
numbers, not by reduction of cell size. Rather complex, functional anatomy can be
constructed from fewer than 1000 cells. Loricifera, however, pack more than 10,000
cells into ovoid bodies only 0.5 mm long. Typically, organs are reduced in number
(one gonad instead of two, for example) and relative size. Numbers of eggs in females
ready to spawn are few; Giere shows that ranges of one to four are typical. The low
fecundity is partly compensated by (i) keeping larval stages in the sediment rather
than as more risky planktonic stages, and (ii) developing very fast to reduce the time
until the full adult defensive modes (spines, motility, secretions ... ) are in place. As
among zooplankton, many groups are or include hermaphrodites – no individual is
lost to either reproductive function – but cross-fertilization remains the rule.
Copulatory sperm transfer is most common, but other means of mating include
deposition of sperm packets to be found by conspecific mates ready for female
function. Most mate finding and sperm transfer likely involves chemical signaling,
but little is known as yet of the specifics.
(^) Many, but not all, meiofauna living in sand (which is mostly restricted to subtidal
and slope areas) are very thin and elongate, particularly compared to their relatives in
other habitats, adapted to twist through and perhaps anchor in the contorted interstitial
spaces of the sediment, and some have a long, skinny tail, likely for the same purpose
(Fig. 13.10). Some have adhesive pads or viscous surface secretions for attachment to
sediment grains, raising their effective mass and, thus, their ability to stay put as the
sand sifts. Some have epidermal spicules or internal stiffeners to enhance abrasion
resistance or to enable them to penetrate sediments. In softer silts and clay, the bodies
of meiofauna are mostly not exaggeratedly long, but various modes of stiffening in
the body wall remain prominent (Fig. 13.11). Mechanisms for moving through
sediments tend to emphasize ciliary locomotion in sand; and modes like press, pull