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On safari for the king of the underwater jungle

My recent dives have taken me on an expedition to lion country. Not Africa but the submarine canyon off La Jolla Shores. The aquatic leonine version is a sea slug (nudibranch) named Melibe lionina, which measures no longer than 2 inches. Paw prints won’t lead you to this lion but a slime trail might.

I find it easiest to survey kelp fronds or eelgrass strands to track down the beast. Mostly translucent with a whitish or pale-yellow tint, the lion nudibranch is seasonal so if you find one, you are likely to find a bunch. The “lion” part of the species name alludes to its large, expandable oral hood, which is fringed with sensory tentacles along the opening edges. The Melibe‘s gaping maw does somewhat resemble the open mouth of MGM’s iconic, roaring mascot. Either that or PacMan. To hunt down prey, the Melibe remains in position and extends the oral hood outward and downward like a net, not unlike a shore fisher casting a net into the water. When the hood’s surface touches a small animal (crustacea, jellyfish and sometimes fish), the hood locks shut, and the fringing tentacles overlap. Now that part of the victim is essentially behind bars, the rest of the animal can be brought into the mouth.

Large, flattened lobes (cerata) that project from the back are far from frivolous decoration. They house the respiratory apparatus and are also employed in the Melibe‘s personal Homeland Security system. Here the cereta function as decoy parts that can be thrown off if grabbed by a predator, though a Melibe doesn’t need a direct threat to shed cerata; any stress may provoke the behavior. One or more lobes may be detached but no matter because they regrow. How a Melibe survives with no lobes until then is a mystery to me.

As with other nudibranchs, a Melibe’s main way of getting around is by crawling but the lion can also “swim.” I use the word broadly because it’s not so much swimming as alternately flexing its body from side to side while upside down (more aerodynamic), like it swallowed something repulsive and is trying to eject it. The swimlike behavior must be confusing or alarming for a potential predator. I know I’d think twice after seeing my hoped-for dinner take off in a cloud of wiggling fleshy flaps or flaps chucked off and floating hither and thither behind an apoplectic slug. Given that a Melibe’s environment is often surgy or current driven, having any kind of motor ability must come in handy if the nudibranch is knocked off its roost or decides it’s time to scout out a new food venue.

Technically called “lateral flexion movement,” the thrashing swim motion may look painfully awkward but is effective in moving a Melibe across long distances. For the past couple of decades, the Melibe has been an important model in behavioral neuroscience research, such that much is known about the complex series of movements that entail flexion behavior. Its chemistry has also been studied. For example, like many nudibranch species, the Melibe secretes chemicals from glands in the skin’s surface that likely serve to deter predators. One known chemical, an aldehyde, is at least somewhat responsible for the honeydew melon scent it emits. Interestingly, this chemical is known to be concocted by the Melibe, as opposed to the slug co-opting it from prey as is commonly done by many other nudibranchs.

While detailed information is known about the Melibe’s central nervous system and chemistry, very little is known about the ecology of the animal. That it produces a scent is interesting but how it is involved in natural predation is unknown. I often wonder at the single-mindedness of the way research funds are doled out. Might we draw more accurate conclusions and gain a better understanding of systems and processes by including big-picture research studies along with small-scale work? The neuroscience research may have more meaning when expanded to include understanding the nudibranch in its environment. And aside from simply using the nudibranch model to ascertain clues about the workings of the human brain and how it has evolved, combining such studies with natural history studies of this marine creature would further aid in understanding the complex, yet little known, web of life in the ocean.

— Judith Lea Garfield, naturalist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. Send comments to