How did the eerie creature known as the anglerfish evolve to the point where it can rule the virtually sunless depths of the ocean?
It was strange sex, extremely strange sex.
Yale University scientists have discovered that an explosion in anglerfish diversification began about 50 million years ago, and that their ancestors developed a strange strategy to ensure reproduction in the dark wilderness.
To mate, the tiny male bites into the abdomen of the much larger female with his sharp teeth. Some males release the female after mating, while others fuse permanently with her, forming a permanent organ for producing sperm.
“We find that a suite of traits, including those necessary for sexual parasitism, enabled anglerfish to invade the deep ocean,” Chase D. Brownstein, lead author of the study and a graduate student in Yale's Department of Ecology and Evolutionary Biology, said in a news release.
Today, there are more than 300 species of anglerfish, making them the most diverse family of vertebrates that live in the ocean's aphotic zone, a region that begins about 1,000 feet below the photic zone, where there is enough sunlight for photosynthesis and most seagrass to grow, and stretches for several miles. The team's study was published last week in the journal Current Biology.
Finding companions in the deep seas is extremely difficult due to the unfathomably vast environment. By one estimate, the dark zone makes up more than 97 percent of Earth's inhabitable space, mainly because the oceans sink to depths of up to about seven miles. In contrast, terrestrial habitats make up less than 1 percent of Earth's biosphere, because the life zone is so narrow and its volume so small.
The anglerfish's odd mating habits are thought to hinder its slim chances of finding a mate in the world's largest ecosystem. Anglerfish are the only known vertebrates to practice sexual parasitism, which has given them an evolutionary advantage.
Over time, the male will physically fuse with the female, connecting to her skin and bloodstream. Eventually the male will lose all internal organs except for his eyes and testes. Females can house multiple males within their bodies.
The Yale team documented how the immune system, which attacks foreign threats, changes over time to prevent female hosts from rejecting parasitic males.
Anglerfish are fish-catching fish, so named because females use shiny, rod-shaped appendages to lure prey with their needle-like teeth. Their mouths are so large and their bodies so flexible, they can swallow prey up to twice their own body size.
Using fossil and genetic data from more than 100 living anglerfish species, Yale researchers trace a sudden diversification that occurred during a period of global heatwave between 50 and 35 million years ago, when the world's oceans were thrown into turmoil and the anglerfish's floor-walking ancestors began to explore the wider marine world.
“It happened in the blink of an evolutionary eye about 50 million years ago,” Brownstein said in an interview. “It was like whales coming back into the ocean. It was amazing.”
At the same time, the team discovered that the fish had developed an unusual reproductive ability, though they were unable to determine whether temporary or permanent attachment came first.
Some female anglerfish can grow to over three feet in length, but most are smaller. Free-swimming males are usually only a few inches long.
How do they find females in the eternal darkness of the deep sea?
Brownstein said males have enlarged noses that are thought to enable them to find mates by following the faint trail of female pheromones in the dark.
“They literally sniff out their peers,” he said.