Sea slug mystery solved?

The mysterious, translucent sea slug I photographed at Gulen Dive Resort last April may be closer to giving up its secret identity. The evidence suggests it to be a Pleurobranchus species.

Posted on May 26, 2011

This strange sea slug was feeding on a Corella parallellogramma sea squirt. This is semi-consistent with Pleurobranchus membranaceus, which is reported to feed on other ascidians. Berthella feeds on a variety of sponges.
Ever since I found this strange sea slug I have been trying to find out what species it is - but it has proved very difficult to identify.

» Read more: Unknown sea slug

Initially, the characteristics pointed towards a Berthella species, but closer scrutiny and a little help from friends and online resources suggests this is not the case.

Fresh eyes

Nudibranch Safari participant Henrik Gram Rasmussen from Denmark told me he thought he had seen this slug in a book by danish zoologist Henning Lemche, and this spurred me to review the case with fresh eyes.

So, let's have a look at what we are seeing, and try to come up with an answer:

Mantle and foot

The foot is large and extends well beyond the mantle on the sides. This is consistent with Pleurobranchus, while the mantle on Berthella species most often seem to cover the foot on the sides of the animal.

Also, the foot is reported to have an indentation at the anterior (front) end of the animal in Pleurobranchus. Both species may present the siphon seen at the back of the animal.

Rhinophores and tentacles

The rhinophores (head tentacles) are rolled, which is consistent with both species.

However, in Berthella, the rhinophores appear to extend below the mantle, whereas in Pleurobranchus they protrude through a notch in the front end of the mantle, giving the appearance of a sheath.

Also, Berthella most often display a large oral veil, which is not present in my specimen.

Feeding tube

On the image below what could be (and most likely is) a feeding tube or proboscis is clearly visible, going inside the ascidian.

A note at Seaslugforum states that Pleurobranchus membranaceus is a "specialist feeder on simple and compound ascidians. They drill circular straight-sided holes through the body wall of the prey, by the combined action of the jaws and the radula, which are carried on the tip of a long extensible proboscis (feeding tube)".

According to Seaslugforum this species feeds on the ascidians Ascidia mentula, Ascidiella aspersa and Botryllus schlosseri. The specimen I came across was feeding on a Corella parallellogramma.

From what I can gather on the internet, Berthella seems to be feeding on a variety of sponges rather than ascidians.

Enlargement of the images shows that the rhinophores does not extend from a sheath, but rather a notch in the mantle. This is consistent with Pleurobranchus. It also clearly shows the proboscis (feeding tube) inside the ascidian. I have not found any evidence to support that Berthella species even have such a feeding appendix.


Jumping to a conclusion at this stage may prove premature, but the characteristics points in the direction of a Pleurobranchus, possibly a Pleurobranchus membranaceus.

The color is however completely wrong, as this species is reported to be (and I have observed this in situ on many occasions) pale brown. Little research has been done on this sea slug family, and it is possible that we could be looking at a previously unknown pleurobranch.

Feedback from Seaslugforum guru Bill Rudman also supports the Pleurobranchus theory. Bill wrote me this:

My first thought on seeing the photo was that it was a lamellarid snail, but the rhinophores are quite wrong as they appear to be 'rolled' like typical pleurobranchs. From the photo however the rhinophores seem to be in a sheath like a species of Aphelodoris. If so, the animal can't be a pleurobranch because they dont have sheaths like this.

Perhaps it's just the angle the photo is taken at? Perhaps what I think is the sheath is the front edge of the mantle? If that is the case then it's more likely your animal is a species of Pleurobranchus rather than Berthella because in Pleurobranchus the front edge of the mantle has special 'notches' for the rhinophores.

If that is the case then your animal could be a juvenile Pleurobranchus membranaceus. I can't say I'm 100% sure but I think it more likely than a Berthella."

Awaiting further information

As previously mentioned, the specimen was collected and handed over to nudibranch experts Jussi Evertsen and Torkild Bakken at the University of Science and Technology (NTNU).

They run a nudibranch project, and at some point they will hopefully be able to study the internal shell and do a DNA sample of the animal, confirming its true identity.

Until then, I think it is safe to say that it is not a Berthella but rather a Pleurobranchus species.

Sources: Seaslugforum, MarLIN, British Marine Life, Marine Species identification Portal, WoRMS, Encyclopedia of Life.

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