How did the evil creatures known as minnows achieve the evolutionary feat that allowed them to essentially colonize the sunless depths of the ocean?
It took special sex—extremely special sex.
Scientists at Yale University have discovered that an explosion of anglerfish diversification began about 50 million years ago after the ancestral line developed a strange strategy to ensure successful reproduction in the dark desert.
In order to mate, small males clamped down on the abdomens of much larger females with sharp teeth. Some males are released after mating while others mate permanently with females. Males that remained attached became permanent sperm-producing organs.
“We found that a cascade of traits, including those required for sexual parasitism, allowed ospreys to colonize the deep sea,” said Chase D. Brownstein, a graduate student in Yale’s department of ecology and evolutionary biology, of who was the lead author of the study. in a press release.
Today, there are more than 300 species of fish sharks, making them the most diverse family of vertebrates in the ocean’s light-free zone. The region begins about 1,000 feet down—just below the photic zone, which receives enough sunlight to support photosynthesis and most sea 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 incomprehensible size of the environment. By some estimates, the dark zone makes up more than 97 percent of the planetary space inhabited by living things, 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 band of life is so narrow, making its volume quite small.
The kingfisher’s strange mating routine is seen as a countermeasure to 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 gave it an evolutionary advantage.
Over time, the male can physically fuse with the female, bonding with her skin and bloodstream. Eventually, he loses his eyes and all internal organs except his testicles. A female can carry several males on her body.
The Yale team documented how the immune systems that attack foreign threats changed over time so that female hosts did not reject male parasites.
Anglerfish get their name from the way females use rod-like appendages with flared 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 own size.
The Yale team used fossils and genetic data from more than 100 living species of sharks to determine that the burst of diversification came during a major increase in global heat between 50 and 35 million years ago. The world’s oceans were thrown into turmoil, and the bottom-walking ancestors of the kingfisher began to explore the wider oceanic world.
“This happened in the blink of an evolutionary eye about 50 million years ago,” Mr. Brownstein said in an interview. “It was like whales returning to the ocean. It was amazing.”
The team found that, at the same time, the fish developed their unusual reproductive abilities. He was unable to determine which came first – the temporary attachment or the permanent attachment.
Some female kingfishers can grow quite large, reaching more than three meters in length, but most are smaller. Free-swimming males are usually several inches long.
How do they find women in the perpetual darkness of the deep sea?
Mr. Brownstein said males have enlarged nasal organs that are thought to allow them to follow faint trails of female pheromones through the dark in order to find their mate.
“You smell your friend, literally,” he said.