Marlin built to take advantage of periodically plentiful prey
Marlin are carnivores, and when hungry, they are fearless and undoubtedly the top predator teleost of the oceans. Their stomach contents show that they are opportunistic feeders – feeding on almost anything and everything. Almost every available species has been identified from gut content analyses.
Accordingly, their gastrointestinal tract shows features common to most carnivores, including large-capacity stomachs and short intestines. The abdominal organs of marlin make up about 4-10 percent of the total body mass. Because carnivores feed discontinuously, the stomach needs to be large to take advantage of periodic food availability.
The intestines are short because meat or fish is more easily digested than vegetation. Marlin ingest food items whole, without chewing, and therefore must have a digestive system capable of breaking down bones, scales and skin as well as the proteins, fats and carbohydrates in their diet. How long this takes is unknown, but two facts suggest it is rapid.
The first is that marlin are very active fish and need a large food supply. The second is that in order to take advantage of periodic food availability, they need to take in and process as much food as possible when it is available. Large-capacity stomachs and rapid digestion will help them take advantage of periodically plentiful prey.
Marlin are occasionally seen by anglers with their stomachs everted. The stomach is inside out and usually hanging from the corner of the mouth. When a marlin is brought rapidly to the surface from a depth of more than a few meters, the gas bladder will expand significantly and may cause eversion on the stomach (stomach throwing).
Stomach throwing is commonly seen in marlin and other teleosts. The absence of mesenteric attachments and blood vessels other than those at the esophagus allow stomach eversion with no obvious damage. The mechanism by which they might swallow the stomach again is unknown, but anatomically it appears feasible and may involve the longitudinal bands of smooth muscle on the outer stomach walls.
Stomach throwing may be a normal activity to help marlin get rid of indigestible items such as hooks, wood, squid beaks and bony debris that they tend to accumulate in order to inhibit injury to billfish stomachs.
However, they do have a tendency to accumulate ulcers at the end of their stomach, possibly due to injuries from spines of prey and other indigestible matter.