Snub-nose eels can burrow into the hearts of sharks and survive by digesting the host's blood, Nature reported on June 26.
Snub-nose eels typically live at depths of 500 - 1,800 m. Photo: Weird Animals
In the hearts and viscera of sharks, scientists occasionally encounter a rare parasite called the snub-nose eel ( Simenchelys parasitica ). In one 1997 case, two eels nested in the heart of a large shortfin mako shark ( Isurus oxyrinchus ), digesting the shark's blood. Ten years later, in 2007, snub-nose eels were found in the heart, body cavity, and muscles of smalltooth sand sharks ( Odontaspis ferox ), according to Science Alert . In fact, snub-nose eels don't need to be parasitic. They can live comfortably underwater, feeding on dead animals on the seafloor. But snub-nose eels prefer to burrow into the flesh of larger fish.
Researchers did not know that eels inhabited sharks until a male shortfin mako shark was collected from the bottom of the North Atlantic in June 1992 and brought ashore in Montauk, New York. The shark was a large one, weighing 850 pounds (395 kg), tangled in fishing line and dead when brought aboard. Its pale color suggested that it had been on the muddy seabed for some time. The mako shark was placed in a cold room for researchers to carefully examine to determine its cause of death.
The next day, when biologists Janine Caira of the University of Connecticut and Nancy Kohler of the Northeast Fisheries Science Center cut open the shark’s belly, they found two subadult female snubnose eels, measuring 21 and 24 centimeters long, nesting in its heart. They were both dead from being taken out of the ocean and put in cold storage, but they had seemed healthy before. There was also evidence that the eels had been hiding in the shark’s heart for some time. According to the team led by Caira, the stomachs of both eels were full of blood, indicating that they had been inside the shark long enough to feed. The shark’s heart also had damage that was not present in the six other unparasitized shortfin mako sharks.
However, scientists have been unable to find evidence of how the eels enter the shark’s heart from the outside. They speculate that the sunken fish find the injured or dead shark and take advantage of the situation to feed. Before or after the animal dies, the two eels enter the gills or throat. They then enter the circulatory system through the efferent artery or aorta and travel to the heart. During the process, they digest blood to survive.
In 2007, researchers found the body of a 3.7-meter-long female sand tiger shark floating in the sea near Fuerteventura in the Canary Islands. It had several snub-nose eels in its heart and musculature in its back. The shark was an adult but had completely lost its ovaries, which may have been eaten by the eels or had naturally degenerated, according to the team led by biologist Ian Fergusson. It is possible that the eels contributed to the shark’s death, as no external or internal injuries were found. Both cases reflect the snub-nose eel’s survival strategy as a facultative parasite.
An Khang (According to Science Alert )
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