Berthella strongi

Photos couresty of Ken Ashman
San Miguel Island, Channel Islands, California

Berthella strongi (MacFarland, 1966)

Originally named Pleurobranchus strongi in the posthumously published - MacFarland's Memoirs, this side-gilled sea slug is not particularly a common find. Berthella strongi is very similar to Berthella californica (Dall, 1900). B. strongi and B. californica differ in the texture of the notal surface, B. califonica being tuberculate, B. strongi is smooth. Also, B. californica has a thin white line along the margin of the mantle, which is lacking in B. strongi . Two color variations of each species are shown in Pacific Coast Nudibranchs .

Concerning the differences in the various genera, a quote from Bill Rudman's Sea Slug Forum-- "In Berthella, species do not grow as large as Pleurobranchus, the mantle is relatively smooth, the anterior mantle margin is weakly cleft, and the gill rachis is smooth. The reproductive system is triaulic and there is a penial gland. I don't think the tips of the rhinophores pulsate, and I think a unique feature of Berthella is the anus opening 'in front of the middle of the gill's suspensory membrane."

Berthella agassizi is a similar species but lacks the white specks, while Berthella stellata has an opaque white transverse bar across the dorsum.

It is interesting to note that the egg ribbon of B. strongi is a cylindrical coil , while B. californica's is wide and flat. Although rather rare to come across this species is has been recorded from central California to Punta Rosarito, Baja California, Mexico. Rudman reports one record from British Columbia.

Ken's shot here is amazing showing the rolled rhinophores, the eye spot, and if it were taken from the other side of the body, we'd be able to see the gill rachis.

Thanks Ken.

Dave Behrens
Danville, Calif
Sept. . 2003

Ken and Rita Ashman

"...My goal in doing nature photography is to tell the animal's story. I want to show the creature as artfully as I can, within its natural setting. There's a well-known maxim: "photographer's are artists who paint with light". Techniques I learned doing weddings and portraiture work just as well in U/W nature photography. Sharply angling the highlight-light creates a tactile effect - makes it so you can almost feel the animal's skin. Diving at San Miguel Island, I found this Berthella strongi positioned on the edge of a large rock - allowing me to frame from below. Looking up at the pleuromorph, the perspective is a slug's eye view of the world..."

Ken Ashman

Equipment notes: Nikon CoolPix 5MP digital camera in Light & Motion Tetra housing, flat port, dual Sea&Sea YS90 strobes on 20" UltraLight arms & pivot grip. Image was taken 7/22/03 - ISO 100, F7.6, 1/500th

Ken Ashman is an avid diver & photographer on the Central Coast of CA. Along with his wife, Rita, he is owner of California Digital Diving and runs photography-orientated dive-trips to the Channel Islands. He invites your questions and feedback. Email him at
Taxonomic information courtesy of:

David W. Behrens

Author: Pacific Coast Nudibranchs
Co-Author Coral Reef Animals of the Indo Pacific
Proprietor of Sea Challengers Natural History Books !

Send Dave mail at

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