crustaceans (squares) and cephalopods (filled circles), mostly squid, plotted against
the shallowest depth at which the animal is commonly caught, usually the upper range
of any vertical migration. Most measurements were for animals captured off Southern
California or near Hawaii.
(^) (After Seibel et al. 1997.)
More recent results with chaetognaths (Thuesen & Childress 1993), jellyfish
(Thuesen & Childress 1994), and copepods (Thuesen et al. 1998) seem to be
qualitatively different from results for larger crustaceans, fish, and squid. Childress
(1995) has reviewed the significance of these comparisons. In contrast to the larger
animals, there is no fall-off of metabolic rate or metabolic-enzyme activities with
depth in chaetognaths (arrow worms), jellyfish, or copepods (Fig. 12.19). Apparently,
the mesopelagic habitat as seen by chaetognaths or jellyfish does not differ so much
from the surface as it does for fish, squid, and shrimp. Thuesen and Childress (1993;
also Childress 1995) attribute the difference to mode of prey finding in the two
groups. The fish–squid–crustacean group mostly finds food visually. Exponential
reduction in irradiance with depth shortens the distances from which information is
received, so that possible reaction distance is also shortened. The short jumps of the
deep habitat require much less metabolism than do the long jumps and pursuit
swimming needed in the well-lighted surface. In chaetognaths and copepods, prey are
mostly detected from vibrations, and the prey of jellyfish simply run into the
tentacles. Thus, these animals require the same reaction capability and, hence,
metabolism at all depths. According to Childress (1995), this explains the end of the
metabolic decline for the sighted animals at about 800 m; once available light is
reduced to occasional photons, vision becomes relatively less useful and reaction
distance ceases to change with depth. There is contrary evidence: Ikeda et al. (2007;
see Chapter 7) compiled copepod respiration results, finding a strong reduction in
temperature-adjusted respiration rates of mesopelagic copepods. This generated an