Overview
Adorning the lush, sun-dappled kelp canopies of the eastern Pacific Ocean, the Hooded Nudibranch (Melibe leonina), affectionately known among marine biologists as the ‘Lion’s Mane Nudibranch’, is one of the most eccentric sea slugs in the world. Departing radically from the typical crawling, scraping habits of other nudibranchs, Melibe leonina is a specialized swimming predator that hunts like a living cast net. With a body structure resembling a translucent extraterrestrial flower, this charismatic creature commands attention along the coastal Pacific Northwest and California coastlines.
Biology and Physical Characteristics
Melibe leonina possesses a translucent, pale greenish-yellow or opalescent gelatinous body completely devoid of a shell. Its most defining evolutionary marvel is an enormous, expandable oral hood rimmed with two concentric rows of sensitive, tentacle-like sensory cirri. Unlike traditional gastropods that scrape algae using a rasping radula, Melibe has discarded its radula entirely in favor of this predatory funnel. Trailing along its dorsum are paired, paddle-shaped cerata that function in respiration, digestive nutrient storage, and predator defense. If severely attacked, the nudibranch can voluntarily cast off (autotomize) its cerata to distract assailants, rapidly regenerating them within weeks.
Behavior and Ecology
The Hooded Nudibranch spends its life clinging to giant kelp blades and eelgrass stems using a narrow muscular foot. When hunting, it raises its expanded oral hood into the tidal surge, sweeping it back and forth through the water like an open parachute. The instant an unsuspecting copepod or mysid shrimp touches the sensitive fringe cirri, the hood snaps closed rapidly, trapping the prey in a water-filled chamber and squeezing out excess water before ingesting the meal whole. Furthermore, when dislodged by strong waves, Melibe leonina demonstrates remarkable active swimming ability, flexing its body back and forth in rhythmic, lateral undulations to navigate between kelp fronds.
Conservation and Human Impact
While Melibe leonina populations are naturally robust and cyclic across their Pacific range, they are intimately reliant on thriving kelp forest ecosystems. Historic declines in giant kelp and bull kelp beds driven by marine heatwaves, ocean warming, and explosive purple sea urchin barrens directly restrict foraging habitat and nursery substrates for this species. Coastal marine protected areas along the coasts of Washington, Oregon, and California serve as crucial strongholds safeguarding the temperate canopy habitats that sustain high seasonal densities of these wondrous marine invertebrates.
Interesting Factoid
When handled or stressed, the Hooded Nudibranch secretes a remarkable natural defense chemical that smells surprisingly sweet and fruity—so much so that divers frequently report that they smell like sweet ripe watermelon or sugary candy!
Last updated: October 2026