How did the diabolical creatures known as the anglerfish pull off an evolutionary feat that essentially allowed them to dominate the sunless depths of the ocean?
It took special sex—extremely special sex.
Yale University researchers have discovered that 50 million years ago, the rapid diversification of anglerfish began when an ancestral lineage developed a bizarre strategy to ensure successful reproduction in the dark wilderness.
In order to mate, small males clamped their sharp teeth on the abdomens of much larger females. Some males let loose after mating, while others bond 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 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 a press release.
Today, there are more than 300 species of anglerfish, making them the most diverse vertebrate family in the ocean’s no-light zone. This area begins about 1,000 feet below—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 incomprehensibly vast expanse of the environment. By some estimates, the dark zone makes up more than 97 percent of the planetary space inhabited by living creatures, mainly because the ocean plunges 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, so its volume is relatively small.
The anglerfish’s bizarre mating routine is thought to neutralize an otherwise slim chance of finding a mate in the world’s largest ecosystem. It is the only known vertebrate to use sexual parasitism, which has given it an evolutionary advantage.
Over time, the male can physically fuse with the female and connect to her skin and bloodstream. He eventually 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 have changed over time to prevent hosts from rejecting parasitic males.
Hornbills 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 anglerfish species to determine that the spike in diversification occurred during a major global temperature swing between 50 million and 35 million years ago. The world’s oceans were thrown into disarray, and the ancestors of the bottom-walking gills 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 returning to the ocean. It was amazing.”
The team discovered that the fish simultaneously developed their unusual reproductive abilities. Couldn’t tell which came first – temporary or permanent attachment.
Some female anglerfish can grow quite large, reaching over 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 allow them to follow faint trails of female pheromones through the dark to find their mates.
“You’re sniffing out your mate, literally,” he said.