Anglerfish survived the ocean's midnight zone by becoming sexual parasites

Anglerfishes colonised the ocean some 55 million years ago by becoming sexual parasites, a new study has revealed. The unique fishes, which are found in the ocean’s midnight zone, adapted to the deep sea environment during a period of extreme global warming, the researchers said.

Anglerfishes are one of the most diverse vertebrate groups in the deep sea. They have unique features including bioluminescent lures. The light from these organs attracts prey towards them, which they use as their weapon to prey on other species.

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Many anglerfish species roam on the seafloor, a zone ranging from the near shore to depths of thousands of feet. They walk along the bottom using modified fins that look like legs. But others live in the deep open water of the bathypelagic, or midnight zone, which is 3,000 to 13,000 feet (900 to 4,000 meters) below the surface.

How anglerfishes survived the ocean’s midnight zone?

The recent study by BioRxiv suggests anglerfish of the group Ceratioidea colonised this midnight zone during the Paleocene-Eocene Thermal Maximum, which occurred 55 million years ago and lasted for around 200,000 years.

The period, researchers believe, may have been initiated by volcanic events that released methane into the atmosphere. The temperatures were so extreme that polar seas reached temperatures of up to 23 degrees Celsius, while sea surface temperatures may have gotten as warm as 36 degrees Celsius.

This led to a wipe-out of numerous deep-sea organisms that likely opened up new ecological niches, the study suggests. It was this window that allowed ceratoid anglerfish to adapt to new surroundings and exist even under changing climates.

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"What we found is that they went into the deep ocean, much like whales going back into the ocean from walking ancestors," lead author Chase Brownstein, a first-year graduate student at Yale, told Live Science.

"Anglerfish just did it in reverse. They were walking on the ocean floor and they went back up into the water column."

Living in the midnight zone means having no real home as there are no reefs, caves, seaweed or other substrate to lean on to. Hence, this environment is not conducive to mating, but researchers suggest anglerfish adopt new breeding strategies to come out of this problem.

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First, they seem to locate each other by scent. And then males fuse to their potential partners permanently, by merging with the bodies of females. In some cases, only one male fuses with the female. In others, even multiple males were found attached to the female.

This unique reproductive strategy is the result of immune system deficiencies, which proved advantageous during this period of radical ecosystem change, which allowed anglerfish to survive in the depths of the ocean.

(With inputs from agencies)