Photo of Melibe leonine, copyright of Monterey Bay Aquarium
Sites and Critters

The hooded nudibranch

Two things are occurring right now that justify a posting about my favorite of all sea slugs, the hooded nudibranch (Melibe leonina). The first is that they have been all over Seattle waters in recent months.  The second is that they are the inspiration for my Halloween costume, which interns will have to put up with tomorrow in the lab.

Rhoda Green, my diving partner in crime, recently posted this great medley of Melbe leonina video footage:

In Rhoda’s video, you can see their interesting movements when they’re swimming.  It is common to see Melibe in doing these side to side movements in the water column, but eventually they settle in seagrass or kelp and get to their real purpose in life: feeding and mating. Melibe doesn’t have a radula like most other sea slugs. Instead, their large oral hood, which is lined with tentacles, closes around food particles like you see in the video. Like many gastropods, Melibe is hermaphroditic. Mating Melibes will reciprocally fertilize one another and then each lay coils of cream-colored eggs on the surface of eelgrass or kelp.

One of my favorite things about Melibe is that when you pull them out of the water and smell them, they have the undeniable scent of watermelon.  It’s not even an “essence” of watermelon or a “hint” of watermelon.  It’s like chewing Bubbalicious watermelon gum. Oddly, this smell at least in part results from the production of 2,6-dimethyl-5-heptenal and 2,6-dimethyl-5-heptonic acid, two chemicals the organism produces for defense purposes. I’m not sure why smelling like Bubbalicious would deter predators from gobbling you up, but apparently predators of the marine environment have different tastes than I do.

Photo of Eliza's Melibe leonina Halloween costume

My Halloween costume…

Photo of noble sea lemon
Sites and Critters

The noble sea lemon

I took the photo above recently at Alki Pipeline, in West Seattle.  It’s a noble sea lemon, Peltodoris nobilis, and it’s larger than any other specimen I’ve ever seen, at almost 20 cm in length. According to Andy Lamb and Bernard Hanby, who wrote every Pacific Northwest Diver’s go-to companion, Marine Life of the Pacific Northwest, noble sea lemons can actually grow to be up to 25 cm long.

Photo of noble sea lemon eating a sponge

Photo by Marquis McMurray. Source:

Noble sea lemons are a type of nudibranch, or sea slug. Closely related to snails and terrestrial slugs, nudibranchs come in a striking array of beautiful and vibrant coloration patterns. Many have frilly (functional) adornments, such the darker-colored, fuzzy gill rosette you see here on the noble sea lemon’s back.  (The word nudibranch actually means “naked gills.”) In the front, it has two rhinophores to detect odors. Because nudibranchs have lost their shell, they have developed alternative methods of defense, including blending into their surroundings and harboring chemical toxins, which they may produce themselves or harvest from their prey and reuse.

Nobles sea lemons generally feed on sponges (and sometimes detritus), but individuals apparently have quite specific preferences when it comes to their favorite sponge species. This one on the right appears to have found its prey species of choice! I love this photo from Marquis McMurray – the noble sea lemon there is chowing down so intensely on a sponge that its face is almost completely is buried in it! From what we can tell, noble sea lemons gain chemical defenses (toxins) from the sponges they eat. When they’re eating sponges, they contain doridosine, a toxin that was found to be lethal when injected into shore crabs and mice.

We took the above photo at about 25 ft. While I didn’t see a lot of sponges in the vicinity, I’ll certainly be looking out for potential prey items of the noble sea lemon on future dives.