There are some things humans can cope without. We can lose a kidney or a lung, a limb or two, and of course our appendix, and live perfectly well. But some fish put us to shame. They don't have stomachs.
One such fish is the stout longtom, a frankly misnamed creature judging by this illustration. The group it belongs to carries a more appropriate name: the needlefish.
All needlefish lack stomachs. Their ancestors had them, but at some point in evolutionary history they were lost. If an entire group of fish losing its stomach isn't peculiar enough, consider this: the stout longtom and its brethren are carnivores and so eat a lot of protein – yet for most animals, digesting protein is exactly what stomachs are for.
The stout longtom can reach 1.3 metres in length, and lives near the sea surface. Like all needlefish, it can jump out of the water to escape predators, or simply to leapfrog floating obstacles. Tropical fishermen are sometimes injured by needlefish, or "living javelins", jumping over their boats: in 1977, a 10-year-old Hawaiian boy was killed when an unidentified needlefish penetrated his brain.
The longtom eats smaller fish, plus a few crustaceans. Its teeth are good at gripping prey but hopeless at chewing, so it swallows fish whole, head-first – adding to the mystery of how it then digests them.
Ryan Day of the University of Queensland in Brisbane, Australia, and colleagues wanted to know how the longtom digests its meaty meals without a stomach, so they ran chemical tests to see what enzymes were at work in their gut.
Stomach juices are extremely acidic, thanks to cells in the lining that secrete hydrochloric acid, while intestines are alkaline. These differing conditions suit different sets of enzymes: the stomach breaks down proteins into smaller molecules called peptides, while the intestines break down the peptides still further and also deal with other food groups like carbohydrates and fats.
Day's results show that the longtom simply does without the acid-driven digestion that the stomach normally performs. It uses an enzyme called trypsin that can break down proteins without acid – although the approach is less efficient than using a stomach.
Because it's a carnivore, the longtom gets a lot of protein in its food, so it can afford this slightly less efficient system for absorbing it. Two plant-eating fish that Day studied actually had higher levels of trypsin in their intestines, as their food was low in protein and they had to catch every last molecule.
Day thinks that the longtom and its stomachless relatives might actually have arrived at an energy-saving solution. Making acid in the stomach involves pumping ions, and the acid has to be neutralised before it reaches the intestine. He says that although the stomach's emergence was critical for the rise of vertebrates, the organ is "a fairly expensive organ to run" – perhaps explaining why some animals have ditched theirs.
In one study of pythons, for instance, making acid and enzymes in the stomach burned 55 per cent of the energy the snakes used to digest a meal.
Journal reference: Journal of Comparative Physiology B: Biochemical, Systemic and Environmental Physiology, DOI: 10.1007/s00360-010-0546-y