In case you missed the earlier news of an unusual nudibranch showing up at the Oregon Coast Aquarium, there is already another that threatens to eclipse its limelight: A lion’s mane nudibranch (Melibe leonina) was recently found by the same aquarists during a related storage tank cleaning. While not strictly outside of its normal range, this ghostly sea slug is nonetheless uncommon to see in Yaquina Bay, the body of water from which the Aquarium draws in seawater for its exhibits. In fact, only three or four previous records exist for this species in Oregon, despite it being far more common both north and south of the state. One record, from 1857, was the very first nudibranch specimen ever officially documented in Oregon—and it was collected in Yaquina Bay.
“For whatever reason, Melibe larvae don’t seem to recruit [transition from their planktonic larval state to settle down as juveniles] into Oregon’s bays very often,” said Jeff Goddard, a research scientist at University of California, Santa Barbara’s Marine Science Institute, who specializes in Pacific nudibranchs. “The fact that we’re in an El Niño year—which is usually associated with less upwelling, less southward water movement, and more shoreward water movement—might have something to do with this one’s recruitment. At least three of the prior Melibe records from Oregon coincided with strong to moderate El Niño conditions.”
The six-inch nudibranch was discovered in a 40,000-gallon reservoir that had been drained to dredge out nearly 10 inches of accumulated mud and debris. As was the case with the sea hare, the Melibe likely found its way into the reservoir as a free-floating larva, sucked up by an intake pump and filtered through a slotted screen. “The interesting thing about the nudibranch and other larger organisms found in these reservoirs is that they come through a pipe with a ¼-inch by 3-inch-slot screen on it,” said Gary Smith, Curator of Life Support at the Aquarium. “The water flowing through this pipe spins at 1,750 revolutions per minute, so whatever animal survives the turbulent ride gets to grow in relative safety in the reservoirs.”
The lion’s mane nudibranch is a filter feeder and lacks the tongue-like “radula” that most nudibranchs use to graze on algae or soft-bodied invertebrates. It’s named for the tentacle-lined “oral veil” on its head that acts as a net, trapping plankton and other small organisms that drift by on the current. When it comes time to feed, the nudibranch finds an area with good water circulation and perches on a blade of eelgrass or kelp, unfurling its tentacled veil like a sea jelly turned sideways.
Lion’s mane nudibranchs do not have gills, instead relying on paddle-shaped extensions of their digestive tract, called “cerata”, for gas exchange. The flattened cerata of this species don’t just look like paddles; they actually function as them, propelling the animal to evade predation or to another perch if dislodged. If under extreme duress, the cerata can be “autotomized”, or cast off, in much the same manner as a lizard dropping the tip of its tail to distract a predator.
Curiously, the lion’s mane nudibranch can also exude a sweet, melon-like aroma when lifted out of the water, possibly to deter predators. “When we saw it in the mud after we drained the storage tank, we weren’t sure what we were looking at,” said Anna Welsh, the aquarist intern who discovered the nudibranch during the reservoir cleaning (and who also discovered the sea hare, incidentally). “But then we caught a whiff of it, and their odor is very fruity, and very distinctive.” For this singular characteristic, congregations of lion’s mane nudibranchs are known as a “bouquets”. Like most nudibranchs, this species typically lives no longer than a year. Visitors can view this strange, beautiful and ephemeral creature in the Rocky Shores “Close-up” exhibit!
Information and photos by Oregon Coast Aquarium