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Rise of the Aeolids

In all of my identification guides for marine animals of the Pacific Northwest there was a nudibranch labelled as a "common grey sea slug". I scoffed at this. How could it be common if I had never seen one in all my years of tide pooling and months looking for sea slugs whilst dock fouling? Perhaps I simply hadn't noticed one yet. Then, all of a sudden, they were everywhere.

The first place I saw them was at Bremerton Marina. This makes sense as Aeolidia papillosa, the common grey sea slug, feeds on plumose anemones. Bremerton Marina has the greatest abundance of plumose anemones that I have seen at any floating dock. I was surprised I hadn't seen these sea slugs appear here sooner. Although not as flashy as other nudibranchs, I still found the common grey sea slug endearing. I would describe them as "floofy" and "chonky", two words I would also use to describe my cat. I often observed them huddled together in pairs or groups of up to four individuals. Their rhinophores have a brightly colored tip, mimicking eyes that appear to stare at you. They look like a Jim Henson creation.

Common Grey Sea Slug Aeolidia papillosa

Don't be fooled by their appearance, the common grey sea slug is a vicious predator. The above image is one such individual chowing down on a plumose anemone. They seemed to range in size from perhaps 2" to 6" long. Most had a round, fluffy looking body shape while a few were the more traditional, elongated slug shape. Their egg masses were abundant. The images below show the egg ribbons.

I should mention that there are sea slug species that are incredibly difficult to distinguish from Aeolidia papillosa, namely Aeolidia loui. From my interactions with experts on iNaturalist I have learned that Aeolidia papillosa tends to have a range extending to Washington while Aeolidia loui does not stray much further north than Oregon. Visually, Aeolidia papillosa can be distinguished by its smooth rhinophores.

Aeolidia papillosa Rhinophores
Aeolidia papillosa Rhinophores

The sudden appearance of so many common grey sea slugs is a bit of a mystery to me. Not only were they at Bremerton Marina, but I saw them at Tacoma Narrows, Port Orchard and places where I have never seen sea slugs, such as Edmonds Marina and Don Armeni Boat Ramp. This makes me think that whatever caused them to suddenly dominate the docks was a pervasive phenomenon, such as the weather. This didn't make sense with what I know about sea slugs, but since I haven't been dock fouling a full year yet I am open to the possibility that it is a seasonal change. As their food source- anemones- doesn't have much seasonal variation it doesn't make sense that the slugs would also, but there might be a factor other than food source that shifts their habits. They are a delightful addition to the dock fouling landscape, I just hope it isn't at the cost of seeing other species of slug. Since I have noticed their dominance I have seen fewer of all other slug species in these locations.

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