• Knife fish live in tropical waters, growing up to a foot long (30 cm). They emit electrical signals to communicate with each other and to identify prey. The six-foot (1.8 m) electric eel lives in the same waters, and eavesdrops on the electrical signals given off by knife fish in order to home in on their location and get an easy meal. The knife fish customarily communicated in very low electrical frequencies, which the larger eel could easily hear, but have now evolved to communicate in higher frequencies that are beyond the electric eel’s ability to sense.
• The bobtail squid lives in shallow coastal waters off Hawaii. It spends the day covered in sand, coming out at night to feed. The problem is that it generally feeds not far beneath the surface, while its predators are hunting in deeper waters. The predators are able to look up at the surface and clearly see the tasty squid outlined against the sky. To protect itself, the squid harbors a colony of bacteria capable of producing light. The bacteria multiply inside the squid during the day. At night, when the squid goes out to hunt, the bacteria sense how much moonlight is filtering through the water, and determine how brightly the squid’s body should glow to be perfectly camouflaged against predators that lurk below.
• Sharks boast some enviable and terrifying features, like their sleek design and razor-sharp teeth. And while glow-in-the-dark sharks sound like something you’d see in a sci-fi film, they’re totally real, as noted in a 2019 study. Researchers were already aware that some shark species produce a glow that only other sharks can see, but now they have discovered that “previously unknown small-molecule metabolites are the cause of the green glow,” according to research. This glow helps sharks identify each other and even fight against infection on a microbial level.
SINGING FISH
• Marine biologists were studying a specific species of fish off the coast of Washington state. The fish is called the midshipman because its coloring resembles the uniform of a naval midshipman’s dress coat. They were interested in the way it communicates: the male hums to attract a female. It sounds like the drone of an airplane propeller, the thrum of a motorboat, or a foghorn in the distance.
• Using his tail fin, the male excavates a cave-like nest in a tidal pool near shore, then hums to invite a female to come and lay her eggs so that he can fertilize them. The low-frequency drone carries for long distances through the water, attracting potential mates from more than a mile away.
• The mating ritual always happens after nightfall during high tide. The females gather a short distance away from the humming males, then choose their preferred mate, laying up to 200 eggs in his cave, which he then fertilizes. When she leaves, he continues to hum, attracting mate after mate, until he has up to 3,000 eggs in his cave, laid by perhaps 15 different females.
• Only the best singers get mates. The biologists wanted to figure out what constituted the best songs. They recorded the songs of 30 different midshipman, taking the recordings to their lab. They remixed the songs and played the synthesized sounds to the females with an underwater hydrophone. The females flocked to some tones, but not others. The researchers found that the females were attracted to a hum of a particular pitch; that they liked the loudest hums; and the longer a song carried on, the better: “low, loud, and long.”
• Some male midshipman who are not the biggest fish or the best singers have evolved to resemble females, allowing them to get close enough to nests of eggs to fertilize them before being driven off.
