features, as well as pelagic primary production, have been discussed in earlier
chapters and need no review here.
Approaches to What Animals Eat
Gut-Content Analysis
(^) What specifically does a particular class or species of animal eat? The obvious way to
find out is to collect some of them when they have been eating, open their stomachs
and see what is there – an approach termed gut-content analysis. Second best is to
poke through feces for identifiable residues from prey. These are obvious approaches,
but they involve modest difficulties. First, meals taken in the course of capture, rather
than before, must be identified somehow. “Net feeding” is very common in the
capture of zooplankton and fish with nets, and is likely the best explanation for
observations like copepods holding chaetognaths in their mouthparts (Davis 1977).
For many predators, such meals can be taken out of consideration by their forward
position in the gut, lack of digestion, or simply improbability. Second, regurgitation or
defecation of partly digested food under the stress of capture is demonstrated for
many animals, reducing feeding rates estimated from frequency of prey in the guts of
individuals. And third, a substantial part of gut contents is likely to be unrecognizable
due to chewing and enzymatic breakdown, leading to food categories like “green
matter” or “gray stuff”. To date, specific identification with DNA sequence probes
and PCR of prey that have been converted to gray stuff has been hampered by the
very rapid digestion of DNA.
(^) Despite its shortcomings, gut-content analysis provides useful information. Sullivan
(1980) examined the meals in collections of chaetognaths from the Gulf of Alaska.
These tubular animals impale their prey with grasping hooks, manipulate them
through the jawless mouth, then rapidly transfer them along the gut to just ahead of
the anus (at the trunk–tail septum), where they are digested. An undigested residue is
eliminated after several hours as a compact fecal mass. Thus, meals eaten well before
capture can be examined by cutting across the body anterior to the digestion site, then
pressing out the meal. Virtually all meals of Sagitta (Parasagitta) elegans and
Eukrohnia hamata were copepods. So, the next questions were: which copepods and
do the predators eat selectively from the smörgåsbord of copepods available? To
answer those, it is necessary to identify copepod remains after nearly complete
digestion, which turns out often to be possible. One anatomical part, the tooth-bearing
jaw, is resistant to digestion, and copepod jaws are distinctive among genera and often
among species (Sullivan et al. 1975). Their size gives the developmental stage.
(^) Sullivan found for juvenile Sagitta (4–14 mm long, living mostly in the top 25 m)