description of crack propagation. Application to animals other than polychaetes
remains sketchy. Possibly, for example, the curved, flattened back of mud-dwelling
amphipods serves as a cracking wedge, but detailed observations are required to
explain their motility in polymer-stabilized mud. It has been suggested that cracking
demands far less energy than pushing through mud, if mud is modeled as a massive
and extremely viscous semi-fluid. Perhaps, but the energy costs of burrowing remain
to be adequately quantified.
(^) As sediment ahead liquefies or cracks, the newly accessible space ahead of a
burrower can be explored by tactile and olfactory senses for particularly desirable
food particles: organic-rich and bacteria-laden clots of sediment, nematodes, and other
meiofauna are ingested, generally along with plenty of the sediment matrix. Digestion
extracts organic matter, but the mass of benthont fecal matter can be nearly
indistinguishable from the mass ingested. Nutrition is derived from small fractions of
relatively large throughput. On an Oregon (USA) sand beach, with ∼6 mg POM per
gram of sand, gut throughput for the polychaete Euzonus mucronata, which ingests
bulk sediment, is mostly mineral. However, uptake by the worms from traces of ^14 C-
labelled organic detritus was sufficient that an apparently realistic 10% assimilation
efficiency for all POM could supply its needs (Kemp 1986). Most deposit-feeders
consume particles from very near the sediment surface, but there are also specialists
that live on organic components considerably deeper, at least toward the bottom of the
most active bioturbation (see below) zone. The capitellid polychaete Heteromastus
filiformis orients head down and feeds on deposits 15 cm and farther below the
sediment surface (cm bss), recovering modest fractions of usable organic matter from
a largely refractory supply, possibly depending upon bacteria as food. Clough and
Lopez (1993), using isotope labeled bacteria from sediment >15 cm bss, estimated
their retention efficiency at 8%. Oxygen for metabolism is apparently acquired by the
tail of the worm closer to the water column.
(^) The bulk of deposit-feeders, both macrobenthos and megafauna, mostly select and
assimilate organic matter quite recently arrived from euphotic zone production. This
has been demonstrated for the upper slope offshore of Cape Hatteras (DeMaster et al.
2002; samples from 1996) and the upper slope at 64°S in the Bellinghausen Sea
(Purinton et al. 2008; samples from 2000). The comparisons were of the ^14 C contents
of upper ocean particulates (largely plankton) with that of sediment organic matter.
The upper-ocean organic carbon has been enriched by +50 to +170 ppm (relative to
wood from 1890) by bomb-test ^14 C, which was increased in the atmosphere during
the 1950s and 1960s by +700 ppm. The variability is due to varying proportions of
upwelled water with old (low ^14 C) carbon. Offshore from Cape Hatteras, surface algae
(Sargassum) using this enriched source have ^14 C content of ∼+109 ppm. Organic
carbon in the sediment, however, was quite old, ^14 C of −41 to −215 ppm, while that in
benthic animals (six families of polychaetes and a fish) was +40 to +83 (one worm