Chapter 12
Adaptive complexes of meso- and bathypelagic
organisms
Mid-water habitat, or the mesopelagic zone, ranges from the bottom of the euphotic
zone (or somewhat deeper, say 200 m) to 1000 or 1200 m. Carol Robinson et al.
(2010) have reviewed the ecology and biogeochemistry of this zone, pointing out that
much remains to be learned, including full description of both microbial and
metazoan diversity. For example, relative abundances of distinct genome types
(primarily DNA coding ribosomal RNA) of bacteria and archaea shift progressively
downward in the mesopelagic (aristegui et al. 2009). About 90% of organic matter
exported from the euphotic zone as sinking particles is metabolized above 1200 m,
but estimates of organic-matter utilization by picoplankton (bacteria, archaea, protists)
and zooplankton both exceed the influx from above as measured with neutrally
buoyant traps (Steinberg et al. 2008). Partly, that is because quantifying pulsed and
spatially erratic input events is difficult. Also, application of uncertain growth-
efficiency measures is involved. Steinberg et al. propose that much of the supply gap
may be filled by vertically migrating zooplankton and nekton feeding at the surface,
then both respiring and dying at depth. Precise budgets will continue to elude us. We
focus in this chapter on some of the adaptations that permit animal survival below 200
m and that make this zone a showcase of evolutionary problem-solving.
(^) Light in the mesopelagic is insufficient for positive net photosynthesis but remains
enough for vision to be important. Since the mesopelagic depends for its trophic
support upon organic matter transported from the layers above, the inhabitants must
eat detritus as it falls through, participate in a DOC-based food-chain, migrate up to
feed in the euphotic zone or eat each other. The last of these possibilities means that
modes of predation suitable to the near-dark have evolved, and there are elaborate
specializations for “hiding” from predators. The habitat can be described as
“trophically dilute”: there is very little food, as shown by classic Russian zooplankton
biomass estimates from subpolar and subtropical areas in the Pacific (Fig. 12.1). Thus,
mid-water animals must have adaptations for surviving on slim rations and for
enduring long periods of starvation. Furthermore, the ocean’s oxygen-minimum zones
center in the mesopelagic (Plate 1.1), with strongly hypoxic levels in the eastern
tropical Pacific and northern Indian Ocean. Animals must either avoid these levels or
be specially adapted to acquire or live without oxygen: larger, more subdivided gills,