Creature Feature: Hooded Nudibranch! Melibe leonina.

Six hooded nudibranchs, Melibe leonina, clinging to kelp to feed on plankton in the currents.

You may have seen meadows of eelgrass laying flat on a sandy beach at low tide, looking kind of boring. But, the eelgrass blades rise with the tide, flowing with the currents and teeming with life! Some of the residents in the eelgrass habitat look like aliens from another planet. Perhaps the most alien of all is the hooded nudibranch, Melibe leonina.

Naked-gilled lion in a hoody?

Nudibranchs are sea slugs, some of the most wildly colorful and varied invertebrates in the sea. They are molluscs, soft-bodied, mostly shelled animals like clams, oysters, and even octopus, but most like, well, slugs. The name, pronounced noody-brank, is Latin for “vulnerable” (nudi) and Greek for “gills” (branch). In other words, nudibranchs have exposed, uncovered gills for absorbing the dissolved oxygen they need for respiration from the water. The original SeaDoctor, Joe Gaydos, shows just a few of the nudibranchs found in the Salish Sea in this video clip:

The scientific name of the hooded nudibranch, Melibe leonina, refers to the lion’s mane-like structure around its mouth, called an oral hood. Sometimes they are even called lion nudibranchs. Can you imagine what that oral hood might be for?

Fishin’

Hooded nudibranchs attach their rear ends to a blade of eelgrass or kelp, face into the current with their hoods wide open, and, like any good fisher, wait. Sometimes they line up side by side, like the ones in the photo above. When some delicious zooplankton, like copepods, amphipods, or mollusc larvae, float by, they bend back and lunge forward to catch the prey in their hoods like a net. They close up the hood, stuff the prey into their mouths and swallow it whole. Not only do they get to wear their hoods at the table, but they don’t even have to chew their food. Some manners! The young catch their food from the sea floor. Perhaps their fishing skills are not yet up to catching food on the fly. Check out this formidable predator at work in this awesome video, taken last month by my friend, Chandler, at the Shaw Center for the Salish Sea in Sidney, BC:

Double duty

Hooded nudibranchs contain both male and female reproductive organs on the right sides of their bodies. They still need to mate with another slug to fertilize as many as 30,000 eggs. The eggs are glued together in a long, gelatinous ribbon that gets stuck in coils to a blade of eelgrass, kelp, rock, or other substrate. When the larvae hatch out, they will be plankton, floating in the currents until they gain enough weight to sink out of the water column and rest on the eelgrass or kelp below.

A hooded nudibranch laying ribbons of thousands of eggs in Bear Cove, BC. Photo by diver, Jackie Hildering

Close-up of individual eggs in a hooded nudibranch egg mass. Photo by diver, Jackie Hildering

What’s a rhino for?

Smelling! Nudibranchs have structures on their heads, in this case, on top of their oral hoods, called rhinophores. ‘Rhino’ is from a Greek word for nose and ‘phore’ just means to bear. Now you may be able to guess what rhinoceros means, too. Rhinophores can “smell” chemicals in the water. What types of information might they detect with these antennae-like organs?

How do they swim?

Not graceful, but effective, the hooded nudibranch thrashes and bends itself back and forth to paddle its way through the water. The flaps that stick out from a hooded nudibranch’s back are called cerata. Cerata, like gills, actually help the nudibranch to absorb oxygen and get rid of unwanted gases, like carbon dioxide. When under attack, they can actually drop these cerata, like a gecko can drop its tail. This gives the predator something to chew on while the sea slug makes a quick exit. A little less drastically, they can ward off many predators by producing a repelling smell using a special gland. To more than a few Melibe-lovers, this smells just like watermelon!

EXPLORE

Next time you head to the beach during a low tide, wade out into the eelgrass and take a look between the blades. You may find a whole new world of weird and wonderful plants and animals who need this meadow to survive. And if you’re really lucky, Melibe leonina may be among them. Watch them for awhile. Are they attached or swimming? Feeding or chillin’ in the hood? And remember, any day you come home salty is a good day!