All oviparous (egg-laying) animals have to incubate their eggs at the appropriate temperature to enable proper development of the embryo. Some birds sit on their eggs and use body heat for incubation. Others, like turtles, bury their eggs on beaches under a layer of sand. Researchers have recently noted that the deep-sea skate Bathyraja spinosissima lays its eggs close to deep-sea hydro-thermal vents which spew out hot water.
Popularly known as the Pacific White Skate, this deep-sea skate is known to have a very long incubation time of 4 years. It means that the eggs take 4 years to hatch once they are laid. The temperature of this incubation time has to be appropriate for proper embryonic development of the embryo growing inside the egg. As the waters in the deep seas are cold and mostly bereft of sunlight, it is difficult for marine egg laying fish to generate heat for incubating their eggs. Scientists believe that by laying eggs near hydro-thermal vents, these fish ensure heat for incubation, possibly accelerating embryonic development.
Reptiles and birds have been known to lay eggs in soil with optimum temperatures. Specifically, the Tongan megapode lays its eggs in soil heated by volcanic ash. However, this is the first time that such a behaviour has been observed in marine animals.
DNA analysis of skate egg cases
The researchers used remotely operated underwater vehicles (ROV) to inspect the unique landscape around the hydro-thermal vents located in the Galapagos islands. They were surprised to notice several layers of giant egg cases, also called as mermaid purses (see gallery above) near a black smoker, one of the hottest kind of vents. More than 150 egg cases were found and 4 of them were picked up for DNA analysis.
DNA analysis identified the species as Bathyraja spinosissima, one of the deepest living skates but not typically found near hydro-thermal vents. The Pacific White Skate is under threat of extinction because of the long incubation times, long lifespan, slow rate of development, deep water fishing, etc. So researchers speculate that the skates evolved this behaviour to probably speed up the development of the embryo. They also believe that more research about such previously unknown behaviours will help to develop better conservation strategies of under-threat species.