When you think of parasites, you probably think of worms and insects… but in the unforgiving realm of the ocean anything seems possible.
Not even sharks are safe there, and some rare instances prove how tough life can be under the waves. In the hearts and organs of sharks, scientists have occasionally recorded a rare, yet possible, parasite: the snub nose (Simenchelys parasitica).
Recorded in one case in 1997a large shortfin mako shark (Isurus oxyrinchus) was cut open to reveal two eels nestled in the shark’s heart and crammed full of shark blood. Ten years later, in 2007snubbed eels were found in the heart, body cavity, and muscles of a small-toothed sand tiger (Odontaspis ferox).
What makes these cases interesting is that the eels – the only species in their genus – need not be parasites. They can live quite happily in the ocean, just scrounging around, cleaning up the dead remains of marine animals on the sea floor.
But no. Snub-nosed eels also like to dig their way into the flesh of larger fish, just for the sake of the halibutwhere they wallow and feast to their heart’s content.
The eel was not known to parasitize sharks until a male shortfin mako shark was recovered from the western North Atlantic seabed in June 1992 and brought ashore in Montauk, New York.
The shark was a large one, weighing 395 kilograms; it was wrong addicted on a longline, meaning the hook had caught it somewhere other than its mouth, and it was already dead when it was brought aboard.
Discoloration suggested the shark had been lying on the muddy sea floor for some time. It was placed in a cold room until the next day so that it could be studied carefully to determine how it had died – from disease or parasites, for example.
The next day, when the shark was cut open by biologists Janine Caira of the University of Connecticut and Nancy Kohler of the Northeast Fisheries Science Center, it contained a surprise, like some kind of nasty Kinder egg.
There, nestled in the lumen of the shark’s heart, were two chubby and glistening blunt-nosed juvenile female eels, measuring 21 and 24 centimeters (8.3 and 9.4 in) in length, respectively.
They were dead, of course; they had been fished from the ocean and stored refrigerated. But they seemed to have been healthy before that. In addition, there was evidence that the eels had been merrily in the heart of the shark for some time.
“The stomachs of both eels were filled with blood, suggesting they were at least in the shark long enough to feed,” wrote a team led by Caira in a paper published in 1997. “The stomach contents of both eels consisted entirely of clotted blood. No intestinal contents (food or parasites) were found in either eel.”
There was other evidence of their stay. There was damage to the shark’s heart that was not present in the hearts of six other, eel-free shortfin mako sharks the researchers examined. But interestingly enough, the scientists couldn’t find any signs of how the eel got into its heart from outside the shark.
The team believes what may have happened is that the eels found the shark injured or dead by the longline hook and took the opportunity to feast.
“Having been compromised by being hooked, the shortfin mackerel was dangling from the catch line on or just above the ocean floor around the time of its death,” they wrote in their newspaper.
“Either just before or after its death, the two eels found the shark and dug into it in the same general area somewhere around the gills or throat. The eels entered the circulatory system (either through an afferent artery or the ventral aorta) and traveled to the heart. At some point during this process, the eels consumed blood.”
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We know even less about the sand tiger. Few details about the case were shared in a 2007 paper on the shark’s population range and density: A female shark, 3.7 meters (12.1 feet) long, was found in the ocean, dead, near Fuerteventura on the Canary Islands. Several snub-nosed eels were found “in the heart, body cavity, and adjacent dorsal musculature” of the shark.
“This shark was presumably mature, but the ovaries were completely missing, either eaten by the eel or naturally degenerating,” wrote a team led by shark biologist Ian Fergussonwho is also a meteorologist with the BBC in the UK.
“It is possible that the eels contributed to the cause of death, as no other obvious external or internal causative factors were found.”
Two cases are not a trend, and the eel seems to thrive on non-living food sources. On a whale carcass sighted off the coast of Japan, for example, snub-nosed snails made up more than half of the total number of animal sightings that came to feed. The video above shows how they do it. But it might be worth taking a closer look at this deep-sea bottom feeder.
At the moment, the current strategy is what is known as facultative parasitism; the eels do not need to feed on live animals. But the ability to do this could be analogous to how other organisms developed obligate parasitism or the absolute necessity of crawling onto another living organism in order to survive.
Like Caira and her colleagues remark“This ability to live even briefly in another organism has important potential evolutionary significance, as it opens up future opportunities for phylogenetic and ecological changes that could ultimately lead to obligate parasite lines and true hosts.”
The newspaper, published in 1997, appeared in Fish environmental biology.