The Eels Song from The Mighty Boosh includes an emotive verse that goes: “Eels up inside ya, finding an entrance where they can.” The ditty appears to apply to one species above all others: the snubnosed eel (or pugnose eel, Simenchelys parasiticus), which has been found in all sorts of places, including the hearts of sharks.
A particularly harrowing account of the heart-burrowing antics of snubnosed eels was published in the journal Environmental Biology of Fishes in 1997. It describes the dissection of a 395-kilogram (871 pounds) shortfin mako shark that was investigated in 1992. Like some sort of grisly Kinder surprise, the researchers were amazed to find the deceased but otherwise healthy carcasses of two snubnosed eels. After a little bit of dissection Inception, an analysis of the eels’ stomach contents revealed they had survived long enough inside the mako’s heart to ingest some blood.
A similar observation was made in a 2008 study that was also published in the journal Environmental Biology of Fishes. In this instance, a snubnosed eel was found to have made its way into the heart of a small-tooth sand tiger shark. They say butterflies in your stomach is a sign of nerves, anyone know what eels in your heart represent?
Exactly how the eels made it into the sharks' hearts wasn’t obvious, though some eels have been found to burrow through the body of their attacker to attempt escape after entering the stomach still alive. This is seen among snake eels, also (appropriately) known as burrowing eels, whose mummified remains were found inside the body cavities of 11 different species of predatory fish. These animals are characterized by a hard tip to their tail that they used to rapidly reverse and carve through the seabed. This same tail allows them to burst out of the stomachs of fish that eat them, only to end up getting stuck elsewhere in the body cavity.
As part of #EelOrNoEel, a trending challenge among Fish Twitter, Alabama Museum of Natural History director and #TeamFish enthusiast John P Friel wrote of the "true eels", “Juveniles [snubnosed eels] feed on copepods and amphipods while adults appear to be scavengers, using their powerful jaws to tear away chunks of flesh from carcasses. They can also be 'facultative parasites' that opportunistically enter sick and dying fishes.”
Snubnosed eels are thought to feed in a manner comparable to cookie cutter sharks, as they bite off circular plugs of flesh and twist their body at high speed to break it off from the dead animal, as can be seen in the above Tweet.