It’s a fish that hides out inside a squid. And for this creature, researchers have found it: a one-celled embryo.
Since the 1980s, researchers have seen one pregnant trout embryo living in the ovaries of a deep-sea rock squid, according to Jean-Luc Ingrow, a professor of evolution and marine ecology at University of California, Santa Barbara. The last one to hatch, in 2017, was a female named Maddy; the offspring she’s bringing to the surface will likely later eat her.
Maddie and her baby are from an isolated group of blue and brown trout called yellowscales, which live in deep water far from the sun. In total, the fish are only about 15 years old, according to Ingrow.
“They’re beautiful, full of active eyes, that can be at the surface, they can be way below the surface, they can be off the ocean floor. And they’re only 10-12 inches long,” Ingrow said in a phone interview. “That kind of species, these are rare.”
But while Ingrow has seen these fish eggs in nature before, he’s not even sure where to start the research.
So in September 2017, Ingrow, along with an undergraduate student, Jennifer Manley, set out to help them find out more about the species, thinking they might find more eggs in the water.
But when they went out, Ingrow said, they noticed something: a kite-tailed squid was up close to them and “acting in an unusual fashion, wagging its tail,” he said. So they watched from afar as the squid “spent three or four minutes catching fish” in the area. “It was almost like they were trying to fatten up.”
Eventually, the researchers found the squid’s “regular egg sac,” but didn’t catch a glimpse of any yellowscales in the process. Still, Ingrow was pleased with the oddity.
The next year, Ingrow, Manley and another student, James Peek, set out to find out more about yellowscales. Again, they found a kite-tailed squid, but this time, the researchers saw a silvery egg sac on the surface, this time flapping to and fro in the water. They were happy to find the egg sac, but weren’t yet ready to proclaim the species had had its last spawn.
“If you were to try to have a live embryo, you would never get the reproductive failure,” Ingrow said. “There’s this giant band that doesn’t die for that same reason, you try for five months, or six months or seven months, and then your last embryo, that one you’ve been trying to create for over a year, there’s only that one in the ocean.”
Still, they were happy to have caught this glimpse of what their long-running search had uncovered: a single larval trout egg in the ocean — and a video of its plastron embryos, too.
Scientists often test whether or not fish eggs are fertile by allowing them to swim in the wild, which makes them more open to the sun and nutrients. But since fish eggs don’t travel this far — the eggs were only 32 feet out — Ingrow said it’s hard to know what happened to them.