Dear Doug,

This does look quite like Stiliger or one of the other sacoglossans which look like aeolids. It belongs to an interesting genus of sacoglossans which are most easily identified by having their eyes very close together in the middle of the head. I have included two close-ups from your photos which show this. The genus is Costasiella, and this species is what I am calling Costasiella sp. 3.

One other characteristic is that species of Costasiellaappear to specialise on living on, and eating the fan-shaped green alga Avrainvillea. It would be interesting to know if this is the type of algae your animals were found on

Best wishes,
Bill Rudman

Dear Vishal,

Thanks for the feedback.

Best wishes,
Bill Rudman

Dear Vishal,

Although the cerata remind us of an aeolid, this is in fact a herbivorous sacoglossan,Costasiella sp. 3. One clue to its identity is the position of its black eyes - very close together in the dorsal midline between the head tentacles. Species of Costasiella have usually been found associated with species of the green algal genus Avrainvillea

Best wishes,
Bill Rudman

Dear Art,

When you see aeolid-like sea slugs on this type of green algae then they are most likely to be a species of the sacoglossan genus Costasiella. The algae, with a post like 'stalk' and a flattened plate-like 'leaf' is likely to be a species of the green algal genus Avrainvillea.

I am pretty sure your animal is one I am calling Costasiella sp. 3. If you look carefully you can see a number of their spiral egg strings stuck to the algae, including one along the edge of the algae just behind the Costasiella.

Best wishes,
Bill Rudman

Dear Ann,

This is a species of Costasiella, but unless they show an incredible amount of colour variation I don't think this is C. kuroshimae. I am calling this one Costasiella sp. 3 on the Forum because I don't think it has a name at present.

Best wishes,
Bill Rudman

Dear Jeff,
This is a sacoglossan, but not a species of Placida. In the lower photo you can see that the two large back eyes are close together between the rhinophores. This is only found in the genus Costasiella, and you animal is one we are calling Costasiella sp. 3 at present.

Best wishes,
Bill Rudman

Dear Jean-François,

This is what I am calling Costasiella sp. 3.  Species of Costasiella seem to feed on green algae of the genus Avrainvillea, which could well be the one your animals are on. One easy way to recognise species of Costasiella are their eyes. No other group of sacoglossans have their eyes close together in the middle of the head like this.

I found this is species in Noumea in 1988 on the green algae  Avrainvillea nigricans.

Best wishes,
Bill Rudman

Dear Linda,
This species seems to be quite variable in colour, but I suspect the difference in 'greenness' - if there is such word - may be to do with how much algal material is in the digestive gland branches in the cerata.

Best wishes,
Bill Rudman

Dear Ian,
You've done a good job firstly finding this animal, and secondly photographing it. It is indeed a sacoglossan, but although it looks very like a species of Stiliger, it is a species of Costasiella, an unnamed species I am calling Costasiella sp. 3 on the Forum. One sure way to identify a species of Costasiella is to look at its eyes, which are very close together - almost touching each other. Another clue in this case is the strange green algae it is on. Quite a few species of Costasiella specialise in feeding on species of the green alga Avrainvillea, which has a broad fan-like 'leaf' and a solid cylindrical stalk, anchoring it in the sediment.
Best wishes,
Bill Rudman

Dear Nils,
This is a sacoglossan rather than an aeolid, and belongs to the genus Costasiella which can be distinguished externally by the position of the eyes, close together in the dorsal midline. This species is rather widespread and is on the Forum as Costasiella sp 3.
Best wishes
Bill Rudman

Thanks Erwin,
The algae is almost certainly a species of Avrainvillea, which Kathe Jensen says is the normal food plant of this genus. The biggest problem with identifying species of Costasiella is not so much difficulties with their anatomy, but difficulties with previous descriptions. It is just not possible to know how variable a species is when it is described from a single specimen, and that description gives a minimal amount of information on the colour, and a stylised black & white drawing. These photos, and your earlier ones, which I labelled Costasiella cf. kuroshimae, suggest that this species is quite variable in colour, sometimes having a reddish spot below the ceratal tip, and in some photos at least showing iridescent blue spots on the cerata. It also seems the upper half of the rhinophores are black, but can sometimes have a white tip. The head also has a broad brownish band from the rhinophores to the mouth and a second patch from the rhinophores back towards the pericardium.

I guess we may as well accept that the animal we are now identifying as Costasiella kuroshimae is correctly identified. I suspect there is no way of ever being sure. To prevent confusion it is probably best to amalgamate the Costasiella cf. kuroshimae messages and the Costasiella sp. 3 messages and call them all . It doesn't give the species a name, but the many photos we now have of it certainly give us a better idea of its colour variation than we have of most of the named species.
Best wishes,
Bill Rudman

Dear Erwin,
I guess its possible they are the same but as I said in my earlier comment, many of these species have been named from one or very few specimens so we have no idea how variable they are in colour. While the absence of a red spot and scattered blue spots on the cerata of your animal may be a variation, I would find it less likely that the difference in rhinophore colour is just a variation. In Takako's animal there is a black line along the upper half of the dorsal side of the rhinophore, while in your animal the whole of the upper half of each rhinophore is black.
Best wishes,
Bill Rudman

Thanks Kathe,
Bill Rudman

Dear Takako,
The best way to separate Costasiella from other sacoglossans with cerata is to look at their eyes, which in Costasiella are very close together. However I can't see them in your photo. However from the general shape of the body I am pretty sure your animal is a species of Costasiella. I have checked the known species of the genus and cannot find a description that fits your species. One problem is that many of the species have been described from one or very few specimens, so without any knowledge of the colour variation in each species it is impossible to know whether your photo represents a new species or a colour variation of a named species. At present I will call it Costasiella sp. 3.
Best wishes,
Bill Rudman

Note: Moved to Costasiella sp. 3. See relevant message. - 2 April 2003

Thanks Erwin,
Yes the eyes are clearly those of a Costasiella. It is possibly Costasiella kuroshimae Ichikawa 1993 but unfortunately the description is not detailed enough to be sure. We have no idea of how variable species of Costasiella are in colour so it is impossible to know whether differences between this animal and the original description are intra-specific or inter-specific.

Costasiella kuroshimae was described as having a white head with dark marks in the midline and a dark streak across the head just in front of the rhinophores. Also a dark grey-black streak behind the eyes in dorsal midline was described. The tips of the white rhinophores were reported to be black, but in Ichikawa's drawing the black did not extend halfway down as in these photos. The cerata were described as 'generally green with white tips'. Ichikawa described a number of species of Costasiella in a similar brief fashion. Of all the species described, this is the only one that come near to these photos.

• Ichikawa, M. (1993) Saccoglossa (Opisthobranchia) from the Ryukyu Islands. Publications of the Seto Marine Biological Laboratory, 36: 119-139.

Best wishes,
Bill Rudman

Note: Moved to Costasiella sp. 3. See relevant message. - 2 April 2003

Dear Erwin,
I am pretty sure this is a species of Costasiella but I can't see the position of the eyes in your photos to be sure. If it is that genus there are a couple of species described from Japan which are quite close but not exactly the same.
• Costasiella paweli was described with black rhinophores, head and tail white, cerata green with white tips and spotted with black specks.
• C. kuroshimae has white rhinophores with white tips and the cerata are generally green with white tips.
Both species were described by Ichikawa (1993) from a few specimens. At present we do not know how variable in colour species of Costasiella but at present I can't really identify your animals with any named species.

• Ichikawa, M. (1993) Saccoglossa (Opisthobranchia) from the Ryukyu Islands. Publications of the Seto Marine Biological Laboratory, 36: 119-139.

Best wishes,
Bill Rudman