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.
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.