“It’s something never seen before,” marine biologist Cherisse Du Preez told Science Post France, when her team’s submersible lights fell upon more than two million golden eggs strewn across the volcanic flanks of a seamount off Vancouver Island. In the dark underworld two miles below the surface, where sunlight never reaches and temperatures are near freezing, it is a rare and revelatory sight a testament to the genius of life in Earth’s most inhospitable realms.
The find, which was accomplished by a team of Canadians with the aid of state-of-the-art deep-sea submersibles, provides the first direct confirmation that the Pacific white skate (Bathyraja spinosissima) deposits its eggs in the hot hug of a seething submarine volcano. The eggs, roughly the size of a flattened football, cluster tightly into rows on the mineral-saturated seafloor, filling a volcanic depression with a sea of eggs. Submersible cameras filmed the first-ever video of a Pacific white skate laying an egg in this harsh habitat, a sight that marine scientists have awaited but never previously seen in situ thousands of large, pale skate eggs.
Why this location is so remarkable is not only the number of eggs approximately 2.6 million but also the contribution of geothermal heat to the skates’ reproduction strategy. The volcano, previously believed to be extinct, is indeed alive and well, its hydrothermal vents spewing plumes of hot, mineral-rich water into the otherwise frigid depths. For the Pacific white skate, which lives at a depth range of 800 to 2,900 meters, this heat is a valuable commodity. In the deep sea, where cold throttles metabolic rate, embryonic development may last as long as ten years. But in the absence of hydrothermal vents, even a small temperature increase can significantly speed up incubation. It may reduce the process from ten years to four or fewer proximity to the source of volcanic heat facilitates the growth of embryos.
This is not something that happens only to skates. In the Galápagos, researchers have recorded analogous behavior in deep-sea octopuses, whose brooding periods are reduced from five to eight years in ambient ocean waters to under two years in the heat of hydrothermal springs brooding periods for females are ~1.8 years, much quicker than predicted for abyssal octopods. As Dr. Pelayo Salinas-de-León, a marine scientist at the Charles Darwin Foundation, told National Geographic, “It takes longer for eggs to incubate in cold water, so the skates may be warming them up on the vents.” Such a tactic, reminiscent of behaviors among ancient dinosaurs and today’s megapode birds, demonstrates a profound evolutionary rationale: by tapping geothermal heat, these creatures can beat predation risks and environmental volatility that attend long incubation skates may be warming them up on the vents.
Technical achievement in filming this nursery is not to be underestimated. Exploration of the deep ocean at such depths is dependent upon a new generation of submersibles and remotely operated vehicles (ROVs) that can be outfitted with high-definition cameras, robotic arms, and advanced environmental sensors. The Canadian voyage used time-lapse cameras and genetic sampling equipment to track the nursery, with similar missions off the Galápagos Islands and off California’s Davidson Seamount using high-resolution mapping equipment to build detailed mosaics of seafloor habitats high-resolution subsea mapping system. The equipment enables researchers to follow not just egg case distribution but also the fine-grained temperature and chemistry gradients that characterize hydrothermal vent ecosystems.
Hydrothermal vents themselves are wonders of geology and biology. Created where tectonic plates move apart and magma ascends to encounter seawater, vents have the capability to emit fluids at over 400°C, though the water around them rapidly cools to almost freezing. The resulting chemical gradients sustain distinct communities, ranging from giant tube worms to endemically adapted bacteria that respire poisonous minerals as energy in chemosynthesis hydrothermal vents resemble geysers, or hot springs, on the seafloor. These life oases are biodiversity hotspots, frequently sustaining species that occur nowhere else on the planet hydrothermal vent ecosystems are more biologically productive and sustain endemic invertebrate and fish taxa.
The Pacific white skate eggs are a dramatic illustration of adaptation to this unforgiving environment. Up to 50 centimeters in diameter, they are some of the biggest from any fish, loaded with nutrients to see the embryo through years of growth. Their mere size is one of a wider trend called marine gigantism, an adaptation to the cold, high-pressure environment of the deep ocean. Skate embryo can take up to four years to hatch. The eggs’ hard, collagenous coverings often referred to as “mermaid’s purses” have to resist both physical wear and chemical adversity of vent waters.
Yet these nurseries are vulnerable. The volcanic slope off the coast of Vancouver Island, as with so many deep-sea ecosystems, is outside of national jurisdictions and exposed to the increasing dangers of deep-sea mining and trawling. As Sonja Fordham, president of Shark Advocates International, said to National Geographic, We hope that exciting new findings like those in this paper can help to spark greater interest in skates and, in turn, a greater constituency for conserving them. The global scientific community has urged networks of protected areas to help protect these unusual ecosystems, whose productivity and complexity are only just becoming apparent a precautionary strategy for biodiversity protection on mid-ocean ridges.
The upshot of this find resonates far from the nursery itself. In showing how geological processes and biological creativity intersect in the deep sea, the golden skate eggs highlight the enormity of what still lies beyond our comprehension. As Dr. Lisa Levin of UC San Diego noted, “We know so little about the functions of these systems that finding a new function, like being a nursery habitat, is very important.” With under 20 percent of the ocean floor mapped, every dive in a submersible is venturing into the unknown, led by the light of science and the hope of discovery.
