Some deep-sea skates use the volcanic heat emitted at hydrothermal vents to incubate their eggs, the first time such behaviour has been seen in marine animals, reveals a study this week in Scientific Reports. With deep-sea skates having some of the longest egg incubation times, lasting years, the researchers believe the fish are using the hot vents to accelerate embryo development.

Among the least explored and unique ecosystems, deep-sea hydrothermal fields are regions on the sea-floor where hot water emerges after being heated in the ocean crust. In their study, Pelayo Salinas-de-Leon and colleagues used a remotely operated underwater vehicle (ROV) to survey in and around an active hydrothermal field located in the Galapagos archipelago, 45km north of Darwin Island. The authors found 157 mobile-phone-sized, horned, yellow-to-brown skate egg-cases, and collected four with the ROV’s robotic arm. DNA analysis revealed the cases belonged to the skate species Bathyraja spinosissima. The team found that 58% of egg-cases seen lay within 20 metres of chimney-like ‘black smokers’- the hottest kind of hydrothermal vent, which emit dark, sulphurous plumes. Furthermore, over 89% of the egg-cases had been laid in places where the water was hotter than average. Similar incubating behaviours are also seen in some land animals. Nesting eggs in volcanically-heated soils is a behaviour seen in both the modern-day Polynesian megapode - a rare bird native to Tonga - and fossil of certain Cretaceous-era sauropod dinosaurs.

In comparison with other marine creatures, deep-water skates have a longer life and slower rate of development; this may translate to a higher risk of extinction, the researchers note. Fostering a better understanding of the skates’ development and key habitat needs is vital for developing effective conservation strategies, as well as identifying and protecting specific egg nurseries from the expansion of fisheries into deeper water, the researchers argue.