Nudibranch Similarities

Phyllidia varicosa group

Author: Nila Murti

To untrained eye, Phyllidia coelestis can easily be mistaken for Phyllidia varicosa and vice versa. Understandable, as these two posses very similar basic form, coloration and pattern. Both have tubercles with blue bases and yellow caps.

To differentiate the two look at the dorsal pattern. If the nudibranch has a median ridge it's a Phyllidia varicosa. If it doesn't have a median ridge and the ridge forms a Y shape arrangement instead, it's a Phyllidia coelestis.

Phyllidia coelestis also shows a broader mantle margin with smaller tubercles that has no yellow caps.

And also juvenile form of sea cucumber, Pearsonothuria graffei, mimic Phyllidia varicosa

Phyllidia coelestis from Perhentian, MalaysiaIdentification reviewed by Nathalie Yonow
(195 photos)
Phyllidia varicosa from Oahu, Hawaii, USAIdentification reviewed by Scott Johnson
(420 photos)
 

Phyllidiella pustulosa group

Author: Dr. Richard C. Willan

The main differences between Phyllidiella and Phyllidiopsis are internal – to do with the anatomy of the foregut. Externally these three species are indeed quite similar in colouration and in having compound tubercles, and all three species vary a great deal between individuals. The main external differences are:

Phyllidiella pustulosa and Phyllidiopsis burni have entirely black rhinophores (bicoloured in Phyllidiopsis krempfi);

Phyllidiella pustulosa and Phyllidiopsis krempfi have narrow black lines (very broad black areas in Phyllidiopsis burni).

 

Phyllidiopsis pipeki group

Author: Dr. Richard C. Willan

In Phyllidiopsis shireenae, if the rhinophores were extended they would have been completely red. By contrast, Phyllidiopsis pipeki has half pink/half black rhinophores and compound pustules (the pustules are simple in Phyllidiopsis shireenae).

 

Reticulidia halgerda group

Reticulidia fungia apparently does not attain the size of Reticulidia halgerda. Reticulidia fungia is separable from Reticulidia halgerda by having: broad based ridges which are fewer in number, the bases being finely bordered in white; a broad blue-grey mantle margin (narrow and orange in Reticulidia halgerda); paler coloration ventrally, a black line around the side of the foot just above the gills; and rhinophores with 14-18 lamellae on the clavus (24-28 in Reticulidia halgerda).

Reticulidia fungia from Maratua, IndonesiaIdentification reviewed by Richard Willan
(95 photos)
 

Risbecia imperialis group

Author: Richard Willan

I would have identified this nudibranch as Risbecia godeffroyana too if I didn’t know where it was from. But because it comes from Hawaii and because nobody has studied the comparative anatomy of that species and compared it with specimens from the western Pacific Ocean (presently called Risbecia godeffroyana) and hence concluded they are separate or different species, then we are obliged to call it Risbecia imperialis for now. In fact, if they are the same species, then it will have to take the name Risbecia imperialis anyway because imperialis was published in 1860 and Risbecia godeffroyana in 1877.