Deep-sea anglerfishes’ unique sex strategy may have helped their ancestors move into a brand new habitat, enabling the surprising diversity of species that thrive today.
Chase Brownstein at Yale University and his colleagues reconstructed the evolution of the 160-plus species of deep-sea anglerfish, or ceratioids. Known for their large jaws and bioluminescent lures, ceratioids are a subgroup of the bigger order of anglerfish, which also include reef anglerfish and bizarre bottom-dwellers like monkfish, sea toads and batfish. Using genetic sequencing, the team found ceratioid ancestors walked on pectoral fins on the floor of the deep sea. But 55 million years ago, some began swimming in the ocean’s vast bathypelagic, or “midnight”, zone. There, they became more genetically diverse than their seafloor-dwelling cousins – all in a period of just 5 million years.
This is odd because the seafloor and reefs are usually home to a wider variety of species, says Elizabeth Miller at the University of California, Irvine. Environments with many topographical features help organisms specialise. But with constant temperatures and vast stretches of empty water, “the bathypelagic zone is truly the most homogenous habitat on Earth”, she says.
Miller and her colleagues constructed a family tree of anglerfish species late last year, and though her team and Brownstein’s used different methods, their work largely agrees on the timeline and scale of deep-sea anglerfish diversification.
Brownstein’s team found this surprising diversification may be due to ceratioids’ parasite-like reproductive strategy: a tiny male will use his jaws to attach to a much bigger female until she is ready to mate. In some species, the two even become permanently fused, including sharing a circulatory system. The unique mating technique theoretically benefits deep-sea anglerfish because they are so unlikely to encounter another member of their species in their lifetimes.
Brownstein’s analysis revealed that the traits necessary for this parasitism – size disparities between males and females and a weakened immune system that doesn’t attack attaching males – predated anglerfishes’ evolutionary jump to the deep ocean.
“The big parts of this complex trait were present before the ceratioids invaded the deep sea,” he says. “Instead of basically being the gas in the engine [of diversity], sexual parasitism was kind of like prepping the gas pedal.”
While Miller says it is “probable” the parasitic lifestyle helped anglerfish enter the bathypelagic habitat, how they then achieved so much diversification remains unclear.
Answering this question will be hard work, especially as retrieving intact anglerfish samples from the deep sea is difficult. But Kory Evans at Rice University in Texas says having two comprehensive family trees that agree will help other researchers investigate.
“I think over the course of the next six months, anglerfish enthusiasts are going to be eating good with these two papers,” he says.