Getting Fed Down Deep
(^) Mesopelagic fish that eat zooplankton can have quite regular meals (Moku et al.
2000). For example, Stenobrachius nannochir, an 11 cm myctophid fish living
between 500 and 700 m in the subarctic Pacific, feeds almost exclusively on copepods
and, at least in August, eats primarily the large Neocalanus spp. that descend in huge
numbers for diapause at these depths in mid-summer. Over half the S. nannochir
collected at all hours of the day and night have some food in the stomach, although it
amounts to a small fraction (0.1% on average) of body mass. Other myctophids that
migrate at night to surface layers eat more. For example, stomach content of
Stenobrachius leucopsaurus is typically 1.5% of body weight: numerous copepods
and a greater mass of euphausiids.
(^) However, for many permanent residents, particularly predatory fish in the deep
mesopelagic, meals can be few, and waiting times between them long. Tolerating this
dietary regime requires special adaptations. Most mid-water fish have very weak
bodies. All bones, except the jaw, are weakly ossified, which saves on weight and thus
on swimming. Musculature is reduced and has a higher proportion of water than in
surface fish. It is not unusual for the body to be reduced to a huge jaw with a wisp of
tail appended. Squid are characteristically flaccid, often with body cavities filled with
tissue fluid lightened by replacement of sodium with ammonium. In Chiroteuthis, for
example, there are large coelomic spaces in the ventral arms filled with buoyant fluid.
(^) Dramatic, though infrequent, feeding events are the rule for a number of predatory
forms. Some mid-water fish have adaptations like terrestrial snakes: a jaw that
unhinges to admit relatively enormous prey, elastic throats, and big folded bags for
stomachs. Several species can swallow other fishes substantially larger than
themselves. James Childress (pers. comm.) found that Chiasmodon niger, the “Black
Swallower”, adds a new growth layer on its otoliths (ear bones) each time it gets a
meal. The evidence is that the outer layer is different when there is a meal in the gut
of a captured specimen than when it is empty. The sequence of the two types of layers
implies that this species eats about 14 meals in growing from the earliest juvenile
stage to reproductive adulthood.
(^) Capture tactics are elaborate, and become more so with depth. As stated above,
many fish hang in wait or cruise slowly, their upward-directed eyes finding slight
mismatches between downwelling light and counter-illuminating prey – meals to grab
with upward-directed mouths. There are anglerfish with bioluminescent lures that
dangle before their mouths. Deeper-living mid-water fishes have extraordinarily long,
saber-like teeth (Fig. 12.12). The jaws distend and stretch to get prey between the tips
of these fangs.