Cutest slug that you ever did see! Ft. The Northwest Onchidella

Northwest Onchidella, Onchidella borealis
- Where: Pescadero, California
- When October, 2020
- iNaturalist Post

I found the cutest creature in the gosh darn ocean this weekend while tide-pooling near Pescadero, California.

Nudibranch? Snail without a shell? I wasn’t too sure what it was. The lowest point of the tide was in the evening, so we kept exploring with low light and into the darkness. I thought the low light may be confusing me, but I wasn’t too far off!

It’s a Northwest Onchidella (Onchidella borealis), and it’s an AIR-breathing, shell-LESS, SEA SLUG! How neat! This beauty is a member of the Onchidiidae family, the Tectipleuran Sea Slugs.

For a quick size reference, it was about the size of my thumb nail, or about 1.5cm in length. Macro-photography can be deceiving sometimes. During my research I noticed multiple sites referring to them as “limpet-like sea slugs,” and I can’t disagree, I thought the same thing as I watched it “zoom” across a piece of Oar Weed, Laminaria farlowii.

The intertidal, specifically in the middle and high zones is where you’ll find this creature, on the shores of the Eastern Pacific. Since it breathes air, it traverses above the water line munching on diatoms as well as algae. The Inververtebrates of the Salish Sea website from Walla Walla Uni states they are found on a couple types of algae and holdfasts such as “sea cabbage, Laminaria [kelp] and red algae such as Odonthalia floccosa [Sea Brush].”

**Other true Nudibranchs I saw on the same trip: Colorful DIrona (top row left and right), Stubby Frond Dendranotus (top center, and bottom row)**

It’s also been known to inhabit caves, I imagine the kind of caves that exists in the intertidal, but they could be terrestrial caves too! Onchidella’s have been found in the high-elevations rainforests of Borneo or the Philippines. Wow!

For such a small blob, it’s got to have some sort of defence against predators. In order to repel sea stars it produces a special viscous secretion from its tubercles and papillae which are both small projections along the outer margin of the body (aka dorsum) when physically stimulated. Young et al. wrote all about these secretions in their 1986 paper, titled “The Ecological Role of Defensive Secretions in the Intertidal Pulmonate Onchidella borealis.” While the fluid it produces repels seastars, other critters such as “ gastropods, polyclad turbellarians, nemerteans, or fishes” aren’t bothered. The sea star L. hexactis occupies the same areas as O. borealis so it makes sense these cuties have developed a defence method against their much larger predator. Consequently, the researchers determined the presence of O. borealis has an effect on the distribution of L. hexactis, wow! I love learning about these predator-prey interactions, and will definitely be paying attention to their distribution during the next tidepooling adventure.

I actually saw another species of this family while tidepooling in Ecuador. We found the O. binneyi in Puerto Lopez, Ecuador it was stationary and resembled a lil brown hedgehog.

I’m looking forward to the next low tide to see if I can re-find one, or multiple! Young et al., suggest overcast days are when these dudes are crawling the most. As the days turn colder and wetter here in California, I too am hoping for more overcast days.