Pelican Eels – The mouth can fit more than the belly can
Meet the Pelican Eel. Image source: MBARI
Life in the deep sea is hard. Near complete darkness, low temperatures and little food means animals must specialise in order to survive. Finding a meal can be tough and many animals have adapted specially in order to ensure that whatever they may find they can eat.
Enter the Pelican Eel or Gulper Eel; a relatively well known example of deep sea life. Like much life at depth, not a huge amount is known about it but scientists are working hard to rectify this. These fish are not ‘true’ eels, as they belong to a different order (Saccopharyngiformes rather than Anguilliformes). They produce leptocephalus larvae though, much like the true eels. Saccopharyngiformes have been found in almost all of the world’s oceans, with the exception of the Southern and Bering seas, it lives in the deep pelagic, up to around 3000m depth.
The most readily recognisable feature of these fish is their extraordinary mouths. These jaws can make up a quarter of the animal’s body length and have a hugely elastic membrane covering them. Many fish in the deep sea have large mouths, when you don’t know where your next meal is coming from it is best to not risk it and hope to swallow anything you may come across. The fish itself has been measured up to 80cm long, making the jaw around 20cm! The skull is only a tiny fraction of this. These fish can eat a large diversity of prey and in their stomachs crustaceans, fish, arrow worms, jellyfish, ribbon worms and others have been found within their stomach.
Eurypharynx pelicanoides. Image source: Wikipedia Commons
Most fish feed by creating a space with negative pressure that sucks water, and prey along with it, into their mouth. It is not known exactly how these gulper eels feed but this method can be ruled out as the muscles in the mouth are almost certainly too weak to create this area of low pressure. An alternative has been proposed based upon the feeding strategy of the fish’s namesake, the pelican. This involves the fish darting towards its prey, followed by a swift upturn of the head and closure of its mouth. As they are swallowed live the prey would struggle in vain against the highly elasticated mouth walls. Saccopharyngiformes possess small teeth that are helpful for prey capture. It is thought that they ‘hover’ in the water column, staying in one place and using the tail as a lure.
Gulper Eel, note the highly visible lateral line ampullae. Image source: Wikipedia Commons
Another unusual feature of the Pelican eel is the red photophore on the tip of its tail. This is used as a lure for unsuspecting predators, they are drawn to the light thinking it is a food source and are snapped up by the waiting jaws of the gulper. The fish also possesses an unusual lateral line. The ampullae project from the body giving a much increased sensitivity used to detect potential prey. Along with no scales, pelvic fins or swim bladder these fish are unusual indeed.
For deep sea life, finding a suitable mate can be very tricky indeed and so special strategies have evolved in order to achieve it. Often extreme sexual dimorphism can be observed. Once the gulper eel male reaches sexual maturity (at what age this is we do not yet know), his teeth begin to shrink and his olfactory organs expand massively. These are known as the nasal rosette, the organ used to smell. In the female these are barely visible but in the male it forms a knob just in front of the eyes. It is thought that once mature the male uses pheromone trails to find a mate. Researchers believe that, like the true eels, Gulper eels reproduce just once in their life cycle. They are known as a semelparous species.
Because the eels live at extreme depths there is still large gaps in our knowledge of their life history. Much of what we do know is based upon damaged specimens caught as bycatch from deep sea fisheries.