Two hormones control bioluminescence in pygmy sharks, according to a study published online today (April 26) in The Journal of Experimental Biology. The sharks, which are about 8 inches in length, likely evolved their glowing stomachs as camouflage from predators. They swim hundreds of meters deep in the ocean while hunting jellyfish closer to the water’s surface. A lighter belly would help them blend into the light shining down on them from above by masking their otherwise obvious silhouette.
The light in pygmy sharks comes from bioluminescent photophores in their stomach, which are revealed when melatonin changes the pigment distribution in overlying skin cells, according to the new research. Prolactin, in turn, expands the pigment, blocking the luminescence.
Co-author Julien Claes of the Catholic University of Louvain in Belgium hypothesizes that pygmy shark illumination predate the luminsence of its cousin, the latern shark, in which melatonin turns on a dull glow, but prolactin stimulates a brighter strobe-light effect. "The pygmy shark, in that sense, is the missing link in the evolution of shark luminescence," he told ScienceNOW.