"A milky liquid-like thing leaked from it."
The abyssal depths of the ocean harbor slippery secrets. Take what researchers in Japan just dredged up: mysterious, jet black eggs so smooth and shiny they're like gross little marbles of darkness.
As detailed in a study published in the journal Biology Letters, the eggs — technically egg capsules, or cocoons — were discovered at some 20,000 feet deep in the Kuril-Kumchatka Trench in the northwest Pacific, one of the deepest oceanic trenches on the planet.
The egg capsules belong to soft-bodied invertebrates known as flatworms, and their discovery is the first clear evidence of these otherwordly creatures living at such an extreme depth, overtaking the previous but uncertain record of around 17,000 feet.
Study co-author Keiichi Kakui at Hokkaido University said that he was initially stumped by what he found.
"When I first saw them, as I had never seen flatworm cocoons (and I didn't know what cocoons look like), I thought they may be protists or something," he told IFLScience.
"Under a stereomicroscope, I cut one of them, and a milky liquid-like thing leaked from it," he continued. "At that time, I didn't know how rare this finding was."
Using a remotely operated vehicle, the researchers found the eggs attached to two rock fragments. Though each was barely a tenth of an inch in diameter, they contained between three to seven flatworm embryos at varying stages of development.
From there, a DNA analysis of two of the specimens revealed they belonged to a suborder of Tricladida flatworms known to inhabit far shallower depths. At their developmental phase, the researchers said the deep sea specimens appeared virtually indistinguishable at a superficial level to their surface-dwelling cousins.
This means that flatworms that live in the depths aren't born to be all that different from the ones that live in the shallows. That suggests, the researchers concluded, the shallow-dwellers may have gradually colonized deep waters over time.
"This similarity in development between the relatively benign shallow-water and the extreme abyssal environments suggests that triclads adapting to the latter faced primarily physiological and/or ecological adaptive challenges, rather than developmental ones," the authors wrote in the study.
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