The mystery of the missing fish heads

Scientists studying the diets of shortfin mako sharks in the Atlantic found a bit of a surprise when they opened up a bunch of shark stomachs — nearly all of the fish the shortfin makos had eaten were missing heads and tails.

The research team, led by Sebastián Biton Porsmoguer of Aix-Marseille University in France, was investigating the diets of shortfin mako sharks in the waters between Portugal and the Azores. Though shortfin mako sharks have declined in numbers around the world and are considered critically endangered in the Mediterranean, they are fished legally in these Atlantic waters and sold as food in Spain and other European countries. Scientists want to know what the sharks eat to better understand the sharks’ place in the food web and, perhaps, the animals’ future.

The researchers obtained stomachs from 113 sharks, mostly juveniles, that had been caught by longline fishermen based in Vigo, Spain (no sharks were killed specifically for the study). The sharks had been caught at sea and refrigerated until the fishing boats returned to land, anywhere from four days to several weeks later. Porsmoguer and colleagues then got ahold of the stomachs and analyzed their contents.

Forty-two sharks had empty stomachs, but the others had lots to look at. The main prey for sharks in this area was teleost  fish (82.4 percent of total prey). Of those fish, 87 percent was Atlantic saury , a type of fish that lives near the sea surface. But nearly all of the Atlantic saury were missing heads and tails. Only one fish had a head, without its eyes, and a tail, both in an advanced state of digestion, the researchers report  February 12 in PLOS ONE .

Even weirder, the scientists found lots of crystalline lenses of fish eyes in the shark stomachs.

What’s going on? The researchers have three hypotheses:

1. “Sharks do not eat the head but only the posterior of the body, with the tail.” In this scenario, the shark is chasing a school of fish and manages to catch its prey, but only the part behind the head. The tail is missing because it decomposes more quickly during digestion. The eye lenses could belong to other prey species the shark had previously eaten.


2. “Sharks eat the head of their prey.” The head and tail, unprotected by the muscle found in a fish’s body, decompose rapidly in the acidic gastric juices of the stomach. The eye lenses, which take longer to digest, remain behind.


3. “The absence of a head and tail could be an artifact due to a post-mortem particular digestion process occurring between capture and landing.” But the researchers ruled this out because they found a mackerel with an undigested head and tail in one shark’s stomach.

“There is no obvious explanation for this unexpected and unrecorded pattern of digestion,” the researchers write. It’s a mystery.

Science News’ life sciences writer Susan Milius and I came up with our own hypothesis: There are zombies among the Atlantic saury in this region of the ocean, and they’re cracking into the heads of their still-living comrades to eat their brains. The shortfin mako sharks are just mopping up after the massacre.

OK, probably not. But if I had to guess, I’d bet that hypothesis number two is the most likely. After all, simpler is usually better when it comes to these sorts of explanations.