How did the fearsome creatures known as anglerfish pull off an evolutionary feat that allowed them to occupy the sunless depths of the ocean?
I needed some unusual sex. It was extremely unusual sex.
Scientists at Yale University have discovered that the diversification of anglerfish began about 50 million years ago, when an ancestral lineage developed a bizarre strategy to ensure successful reproduction in the dark wilderness.
To mate, the small male clamps his sharp teeth against the belly of the much larger female. Some males are released after mating, while others permanently fuse with the female. The attached male became a permanent organ for sperm production.
“We know that a set of traits, including those necessary for sexual parasitism, enable angler fish to invade the deep sea,” said Chase D. Brownstein, lead author of the study and a graduate student in the Department of Ecology and Evolutionary Biology at Yale University. “I found it.” From a press release.
Today there are over 300 species of anglerfish, making them the most diverse vertebrates in the lightless ocean region. This region begins about 1,000 feet below the glacial zone and extends several miles down, where it receives enough sunlight to support photosynthesis and most marine plants. The team's research was published last week in the journal Current Biology.
Finding a mate in the deep sea can be very difficult because the environment is incomprehensibly vast. According to some estimates, the Dark Zone covers more than 97% of the planet's inhabited space, largely because the oceans sink to a maximum depth of nearly seven miles. In contrast, terrestrial habitats make up less than 1% of Earth's biosphere. This is because the range of life is so narrow that its volume is very small.
The anglerfish's bizarre mating routine appears to be a response to the slim chances of finding a mate in the world's largest ecosystem. It is the only known vertebrate to use sexual parasitism and to have given it an evolutionary advantage.
Over time, the male can physically fuse with the female, connecting to her skin and bloodstream. Eventually he lost all of his internal organs except his eyes and testicles. A female can carry multiple males on her body.
The Yale research team documented how the immune system that attacks external threats changes over time to ensure that female hosts are less likely to reject parasitic males.
The name Anglerfish comes from the way the females use their glowing, rod-like appendages to lure prey into their needle-like teeth. They are the fish that catch fish. Their mouths are so large and their bodies so flexible that they can swallow prey twice as large as they are.
The Yale research team used fossils and genetic data from more than 100 living anglerfish species to determine that an explosion of diversity occurred during a global heat surge between 50 and 35 million years ago. The world's oceans were in turmoil, and the ancestors of the bottom-dwelling angler fish began to explore the wider ocean world.
“This happened in the blink of an evolutionary eye about 50 million years ago,” Mr. Brownstein said in an interview. “It was like a whale returning to the sea. It was great.”
At the same time, the team discovered that the fish had developed unusual reproductive abilities. I couldn't decide which came first: temporary or permanent attachment.
Some female anglerfish grow quite large, reaching more than 3 feet in length, but most are smaller. Free-swimming males are typically several inches long.
How do they find females in the eternal darkness of the deep sea?
Mr. Brownstein said males have enlarged nasal organs that allow them to follow the faint trail of female pheromones in the dark to find a partner.
“You literally smell your spouse,” he said.