https://www.livescience.com/deep-sea-anglerfish.html
Meet the fish that fish for other fish.
In 1833, an almost perfectly spherical fish washed ashore in Greenland and was taken to zoologist Johannes Christopher Hagemann Reinhardt in Copenhagen, Denmark. This fish — later known as the footballfish, Himantolophus groenlandicus, or the man-gobbler — was the first anglerfish known to science, wrote Ted Pietsch, a systematist and evolutionary biologist, in his book “Oceanic Anglerfishes” (University of California Press, 2009).
Today, there are about 170 known species in 12 families of deep-sea anglerfish, and a “huge diversity” within those families, Mackenzie Gerringer, a professor of biology at SUNY Geneseo in New York who specializes in deep-sea fish told Live Science. Common names for anglerfish hint at some of the wild forms they can take — snaggletooth sea devil, wolf trap and pugnacious dreamer (also known as the tyrannical toad), to name just a few. They sport a fantastic range of shapes and textures; some are squat and round (Melanocetus johnsonii), while others are flat and huge-snouted (Thaumatichthys binghami) or covered in whiskery filaments (Caulophryne jordani). But while these fish are found all over the world, they are fairly elusive, solitary creatures — par for the course for a fish that lives 1,000 to 16,400 feet (300 to 5,000 meters) below the surface. As a result, new species are still being discovered, each more strange than the last.
But no matter what it looks like, any deep-sea anglerfish is a small ocean-dwelling creature’s worst nightmare.
Related: Bizarre fish are deadly deep-sea predators (and Twitter stars)
Anglerfish are named for the glowing lure they use to attract the fish and crustaceans they eat. These fearsome hunters lurk quietly in the depths of the ocean. They’re ambush predators, Gerringer said, floating and waiting in the dark until prey comes near. Then, they use their built-in fishing rod to lure in the unlucky animal, wiggling, hiding and revealing their lure to tempt potential prey until they are close enough to be sucked up.
This feeding strategy explains anglerfish’s bodies: Because they don’t actively hunt, they haven’t evolved to be fast swimmers, which is why many are blobby, non-hydrodynamic shapes. National Geographic even called anglerfish “quite possibly the ugliest animal on the planet” (though the blobfish would like a word).
Related: This terrifying, toothy ‘monster’ is the world’s deepest-living predator.
In the deep ocean, meals are few and far between. Pietsch wrote in Oceanic Anglerfishes that most anglerfish stomachs that have been examined are empty. So when an anglerfish does come across a meal, they make it last. Anglerfish mouths are often the biggest part of their bodies, and if a meal “can fit in the mouth, it can fit in the body,” Gerringer said. Many anglerfish can stretch their stomachs to double their original size.
“They’ll end up with a bubble belly,” she told Live Science. “Sometimes they’re caught and they have whole fish in their stomachs. If you touch the stomachs, it’s quite squishy, for lack of a better term.”
But don’t worry too much about these deep-sea horrors: They’re far too small to hurt a human, making their oversized teeth and misshapen bodies… kinda cute? While some anglerfish can grow up to three or four feet (0.9 to 1.2 m) long (like Ceratias holboelli), the average size of an adult is 6 inches (16 centimeters) long — a little smaller than a volleyball.
Anglerfish lures glow in the deep ocean, at least half a mile (0.8 kilometers) below the sunlit surface, thanks to luminescent bacteria that take root in the fish’s lure. The lure, also called an “esca,” has a pore on the end that is designed to host these bacteria, many of which can’t live anywhere else, and many of which are unique to that species of anglerfish.
Related: Check out an image gallery of glowing fishes.
But where do the glowing bacteria come from? Anglerfish are born deep in the ocean as tiny, transparent larvae and float alone to the surface to feed and develop into their adult forms. They don’t grow an esca until later in life, so they have nowhere to nurture their bacterial colonies from birth, Gerringer said. “It’s a big research question right now,” she added. Of the anglerfish esca bacteria that have been studied, none have been found living freely in seawater, Pietsch wrote in his book, meaning that it’s unlikely the fish pick their glowing buddies up from their environment. Do they live on an anglerfish’s skin until the esca develops? Do they, as one study in the journal eLife suggested in 2019, come from adult anglerfish spewing bacteria into the water, to be immediately picked up by younger fish? “There’s a lot of open questions,” Gerringer said.
Related: More fish glow than researchers previously expected.
The diverse anglerfish don’t stop at a simple glowing lure, though. Some species, such as Phyllorhinichthys balushkini, have elaborate light guides protruding from their bodies, like biological fiber optic cables. Others, like Cryptopsaras couesii, have glowing spots on their backs called caruncles. Some, like members of the Thaumatichthys genus, have lures on the roofs of their mouths.
There is more in the original post…