Shelled Sea Slug! A small mystery solved.
Here’s a bit of a mystery that took me more than a year to sort out.
On April 27th, 2016, I found this egg mass while diving in Browning Pass with God’s Pocket Resort. This is to the north of where I live and is somewhere I only have the joy of diving a couple of times a year.
Mystery sea slug egg mass among horseshoe worms. (From Neil McDaniel re. worm ID – “they are Phoronids most likely Phoronopsis harmeri” – April 2016. ©Jackie Hildering.
I recognized it was likely a sea slug egg mass but did not know the species.
More than a year passed. On May 7th, 2017, I had a chance to dive the same site again and so hoped to find the species who laid the eggs. We quickly swam to where I had found the egg mass the year prior, into the shallows (~5m), and hovered over the ocean bottom strewn with bits of shell remains.
And I found these . . .
Tiny snail-like animals, plowing through the bits of shell and urchin remains. One, two, three . . . six of them!
I tried to calm myself down, to get photos, and to watch how, despite their soft bodies and the sharp bits of shell, they were able to even push under the surface.
They were Stripe Barrel Shells (Rictaxis punctocaelatus) with shells only to 2 cm long)!
A “Striped Barrel Shell” beside an urchin spine, giving a sense of how small these animals area. ©2017 Jackie Hildering.
These are often mistaken as being a marine snail (prosobranch) like a whelk but they are a type of “bubble shell” sea slug. They are also not nudibranchs. They have a thin shell and do not have “naked gills”. Therefore they do not belong in the “nudibranch” sub-group of sea slugs (opisthobranchs). For the classification super nerds, see this link or the graphic at the end of this blog for my attempt at offering clarity.
Plowing down into the shell debris! ©2017 Jackie Hildering.
Please know that I am not suggesting that this is a rare species. Rather, they are hard to find. Their size makes them hard to see; divers often do not target the sand or shell-covered bottoms where they live; AND . . . . they are often just under the surface.
I was incredibly fortunate therefore to find them out and about – maybe feeding on algae and/or trying to smell where a mate might be (and we think WE’RE challenged in finding a partner!)
And how about those eggs? Are they a match?
Yes, they are! I was able to confirm this thanks to the knowledge and brilliant documentation of Jeff Goddard on the Sea Slug Forum (see below).
Source: Sea Slug Forum; Jeff Goddard.
Another little mystery solved.
Another big influx of wonder about the life in the NE Pacific Ocean! 🙂
My attempt at summarizing the cassification of the group to which sea slugs belong.
Last updated 2020-04-17. Source: World Register of Marine Species.
For an additional blog about another bubble shell sea slug in the NE Pacific Ocean see – “Slugs that Fly? The Great Winged Sea Slug”
The Marine Detective 
6 Responses to “Shelled Sea Slug! A small mystery solved.”
Always fascinating! Thanks for sharing the wonders under the waves.
Nice find Jackie… Note that those other worm-like critters in the sand/shell substrate are Phoronids (horseshoe worms): most likely Phoronopsis harmeri.
Thanks Neil! Correcting the worm ID.
Hello Neil,
Is Phoronopsis harmeri a different species from the phoronid called Phoronis vancouverensis here in Southern California? I noticed that the phoronids in Hoody Nudi bay were peach-colored as compared to our bright white P. vancouverensis. Or has P. vancouverensis been reclassified?
Quick look on WoRMS – They are phoronids of different species (and genuses) but Phoronis vancouverensis is an unaccepted name. It’s Phoronis ijimai.
Differences include that Phoronis ijimai is indeed is white coloured and Phoronopsis harmeri is peach coloured.
Thank you for your continued explorations and posts on matters that matter Jackie; they are very much appreciated!