As it cruises along a kelp frond, a red-gilled nudibranch (Flabellina pellucida) seems to emerge straight out of its cosmic backdrop. But this sea slug is very much ocean-bound, navigating waters just off the Norwegian coast near Gulen with the help of a pair of frosted antenna-like appendages, called rhinophores, that detect chemical signals from the surrounding environment.
Nudibranch means “naked gills,” a reference to the collection of swaying appendages this species—one of some 3,000 sea slug varieties—carries on its back. But these brilliant raspberry hues, beautiful as they are, aren’t meant to attract. Rather, the vibrant color serves to warn predators of chemical defenses stored inside the nudibranch’s tissues. In a stunning biological feat, red-gilled nudibranchs are able to ingest the stinging cells of their prey—tiny jellyfish-like invertebrates known as hydroids—and then transfer the cells to their gill tips for use in their own defense. Scientists think further study of this unique chemical arsenal could lead to advances in pharmaceuticals to treat everything from cancer to HIV infection.
Photographer Alex Mustard created this otherworldly scene with no fewer than 77 separate photographs merged into a single image. One photo captured the nudibranch slinking along the edge of a kelp frond, while dozens more captured Norway’s night sky. With his camera pointed toward the North Star, Mustard took a series of long exposures, resulting in star trails that trace the rotation of the Earth.
Dr. Alex Mustard has been taking underwater photographs for more than 30 years and has worked as a full-time underwater photographer for the past 12. His work has been displayed in exhibitions around the world and has won numerous awards, including seven wins in the BBC Wildlife Photographer of the Year competition and four category wins in the British Wildlife Photography Awards. His 2007 book “Reefs Revealed,” won the International Grand Prize for the best book of underwater photographs.
Katie Jewett is a science writer at the California Academy of Sciences, where she loves learning something new about our planet every day. Previously, she spent winters in the Colorado Rockies and summers living and working on the water.