Skates [wikipedia.org] have been observed laying their eggs in lukewarm water near hydrothermal vents [uri.edu]:
A team of scientists from the University of Rhode Island and the Charles Darwin Research Station exploring the seafloor northwest of the Galapagos Islands in 2015 made an unexpected discovery. Large numbers of egg cases of a deep-sea skate – relatives of sharks and rays – were observed adjacent to the hot water emitted from hydrothermal vents, which the scientists said the skates use to accelerate the development of the embryos.
It is the first time such behavior has been recorded in marine animals. The discovery is published this week in the journal Scientific Reports.
[...] In total, 157 mobile-phone-sized egg cases were observed, which DNA analysis revealed to be from the Pacific white skate (Bathyraja spinosissima). About 58 percent of the egg cases were found within 20 meters of a black smoker, the hottest kind of hydrothermal vent, and 89 percent of the egg cases were laid in water that was hotter than the background temperature of 2.76 degrees Centigrade.
"The eggs weren't right next to the active vents, because the water can get so hot – hundreds of degrees – that it would kill them," Phillips said. "We found most of them in the lukewarm water not far from the vents and near some extinct vents.
"The kicker is that we showed our data to a bunch of shark experts, and they had seen anecdotal evidence of shark and ray egg cases near hydrothermal vents, but they never had the data to put the story together," he added.
Also at CBC [www.cbc.ca].
Deep-sea hydrothermal vents as natural egg-case incubators at the Galapagos Rift [nature.com] (open, DOI: 10.1038/s41598-018-20046-4) (DX [doi.org])