male Gennadas elegans, a penaeoid shrimp. Curves were derived from size–
frequency distributions based on collections made at all seasons in Rockall Trough.
Growth is seasonal, but look at the graph closely.
(After Mauchline 1988.)
(^) Childress et al. (1980) provide information about growth and life-cycle timing of
mid-water fishes. You can study growth, if you can determine age. Age in fish is
recorded in the otoliths as depositional layering (growth rings). There are sometimes
daily growth rings, and almost always annual growth rings. Rings are made by
variations in the crystal structure or in the impurities included as the bone grows.
Otoliths are used by the fish as weights suspended on the tips of sensory hairs to
determine the direction of gravity and of centripetal effects. Comparison (Fig. 12.15)
of length vs. age shows that mesopelagic fish that migrate to the surface at night to
feed grow in much the same, decelerating pattern as fish like sardines continuously
resident in surface layers. Fish that stay down both day and night add length in nearly
equal increments at all ages. At least one bathypelagic fish, Poromitra crassiceps,
appears to grow in length progressively faster with age, which is not reported for
animals from any other habitat. Deep-sea fish must get better and better at finding and
capturing food – and thus at growing – as they get bigger.
Fig. 12.15 Length vs. age growth curves for epipelagic (×, +), mesopelagic migrators
(which come well up in the water column at night, open symbols) and bathypelagic
fish (filled symbols). Numbers of otoliths assigned to each age point are in
parentheses.
(After Childress et al. 1980.)