In the pitch-black depths of the sea, a truly unexpected sight has been spotted: a swarm of over 100 eels.
In a new study, researchers from the University of Hawaiʻi at Mānoa argue that this observation is the largest aggregation of fishes ever recorded in the abyssal deep sea. Their video footage shows a swarm of 115 cutthroat eels, which is almost double the previous record for the largest aggregation of fishes in the deep sea, according to the researchers.
Given the extremely scarce supplies of food found at these depths, it’s unusual to see such a large number of the same species in a single “herd,” so it's thought this new observation could change a lot of what marine biologists know about the mysterious abyssal deep sea.
"Our observations truly surprised us. We had never seen reports of such high numbers of fishes in the sparsely-populated, food-limited deep-sea,” lead author Astrid Leitner, who conducted this work as a graduate researcher at the UH Mānoa School of Ocean and Earth Science and Technology (SOEST), said in a statement.
Reporting the discovery in the journal Deep Sea Research Part I: Oceanographic Research Paper this week, the team caught this unexpected sight in the depths of the Pacific Ocean between Hawai'i and Mexico, a region around an unmapped seamount, a deep underwater mountain that often attracts marine life, at a depth of over 1,000 meters (3,280 feet). The eels were attracted by a small bait package containing about 1 kilogram (2 pounds) of mackerel, which they feasted on before escaping the scene.
Interestingly, the eels were identified as Ilyophis arx, also known as cutthroat eels, a species that scientists know remarkably little about due to their extreme choice of habitat. In fact, it’s thought fewer than 10 specimens of this species have been collected worldwide.
Despite living deep down on the seabed in the middle of the Pacific, these creatures are not immune to the pressures of human activity. The region they were found in, called the Clarion-Clipperton Zone (CCZ), is being explored by the deepsea mining industry for minerals and metals such as copper, cobalt, zinc, and manganese. Since very little is known about the biodiversity of this area, it’s unclear what effects this might have on the seafloors’ inhabitants, but marine biologists are understandably concerned. As previous research has highlighted, mining can be heavily disruptive to sea-bed ecosystems and it's thought the damage is slow to heal.
"If this phenomenon is not just isolated to these two seamounts in the CCZ, the implications on deep-sea ecology could be widespread," said Leitner. "Our findings highlight how much there is still left to discover in the deep sea, and how much we all might lose if we do not manage mining appropriately."