Unconventional sex Let the sea hook conquer the deep ocean

How did the eerie creatures known as hookfish pull off the evolutionary feat that allowed them to take over the sunless ocean depths?

Unusual sex was needed – extremely unusual sex.

Yale University scientists have discovered that the spurt of diversification of hookfish began some 50 million years ago when an ancestral line developed a bizarre strategy to ensure successful reproduction in the dark wilderness.

In order to mate, the tiny males would grab onto the abdomens of much larger females with their sharp teeth. Some males would be released after mating, while others would permanently fuse with the females. Males that remained attached became permanent sperm-producing organs.

“We found that a cascade of traits, including those required for sexual parasitism, allowed anglerfish to invade the deep sea,” said Chase D. Brownstein, a graduate student in Yale’s Department of Ecology and Evolutionary Biology who was lead author of the study. in the press release.

Today, there are more than 300 species of fish, making them the most diverse family of vertebrates in the ocean’s no-light zone. The area starts about 300 meters down—just below the photic zone, which receives enough sunlight to support photosynthesis and most marine plants—and descends for miles. The team’s study was published last week in the journal Current Biology.

Finding a mate in the deep sea can be extremely difficult due to the unfathomably vast size of the environment. By some estimates, the dark zone is more than 97 percent of the planet’s habitable space, largely because the ocean sinks to a maximum depth of nearly seven miles. In contrast, terrestrial habitats make up less than 1 percent of the planet’s biosphere because the belt of life is so narrow, making its volume quite small.

The anglerfish’s bizarre mating routine is thought to reduce the otherwise slim chances of finding a mate in the world’s largest ecosystem. It is the only vertebrate known to use sexual parasitism, which has given it an evolutionary advantage.

Over time, the male can physically fuse with the female, bonding with her skin and bloodstream. In the end, he is left without eyes and all internal organs except testicles. A female can carry several males on her body.

The Yale team documented how immune systems that attack foreign threats have changed over time to keep female hosts from rejecting parasitic males.

Hookfishes get their name from the way females use stick-like appendages with glowing tips to lure prey into their needle-like teeth. They are fish that fish. Their mouths are so large and their bodies so flexible that they can swallow prey up to twice their size.

The Yale team used fossils and genetic data from more than 100 living fish species to determine that the burst of diversification occurred during a major global thermal jump between 50 and 35 million years ago. The world’s oceans were thrown into turmoil, and the ancestors of the bottom-walking fish began to explore the wider ocean world.

“It happened in the blink of an evolutionary eye about 50 million years ago,” Mr. Brownstein said in an interview. “It was like whales going back to the ocean. It was amazing.”

The team discovered that the fish developed their unusual reproductive abilities at the same time. It was not possible to determine which came first – temporary or permanent attachment.

Some female fish can grow quite large, reaching more than three feet in length, but most are smaller. Free-swimming males are usually several inches long.

How do they find females in the eternal darkness of the deep sea?

Mr Brownstein said males have enlarged nasal organs which are thought to enable them to follow the faint trail of female pheromones through the darkness to find their mate.

“You smell your partner, literally,” he said.