Nudibranch Physical Similarities

Ceratosoma gracillimum can be distinguished externally by the complete absence of the mantle edge between the head and the well-developed lateral lobes on each side. Ceratosoma trilobatum has the mantle edge, edged in purple, between the head and the lateral lobes. Both species have a similar range of colour variations, as does Ceratosoma tenue, but they differ from each other externally by the lack of a mantle edge in Ceratosoma gracillimum and the extra pair of small lateral lobes in Ceratosoma tenue.

Both have elongated bodyform, both have yellowish or orange-ish gills and rhinophones, both have similar mantle colouration and colour variation - pale to bright blue in the middle with black elongated lines bordering the blue area, and faded to bright orange mantle margin. Chromodoris elisabethina normally has more elongated black lines in the blue area, sometimes a median line, often some broken shorter lines. Chromodoris annae usually has no or less black lines, but some have been found to have multi black lines like Chromodoris elisabethina. So what's the telltale difference? Apparently it's the uniformity of the blue colour. If it's smooth and uniformly coloured then it's Chromodoris elisabethina, while if the blue area has tiny dark speckles hence not uniform (zoom your photos to see it!) then it's Chromodoris annae.

With their strikingly similar colours and patterns, it is quite tricky to distinguish Chromodoris willani, Chromodoris lochi, Chromodoris dianae and Chromodoris boucheti. Chromodoris willani is easier to differentiate from the other three. Apparently Chromodoris lochi and Chromodoris dianae have different number and shape of gills but this isn't always easy to notice by laymen. The 'easier' external characteristic differences to spot are as follows: Chromodoris willani has translucent translucent rhinophores and gills, with tiny white dots on them. Chromodoris boucheti has yellow tipped rhinophores, but with a special pattern in the yellow tipped gills: there is a vertical black line in the middle of each gill running from the base up. The mantle of Chromodoris boucheti is uniformly colored and smooth looking, without any white specks. Chromodoris lochi has a rather elongated body of the Hypselodoris genus. It has pink or yellow tinted rhinophores and gills which are sometimes translucent (but with no white dots like in Chromodoris willani). The mantle of Chromodoris lochi is uniformly colored and smooth looking, without any white specks. Its dark/black elongate band in the mantle is thin and unbroken. Chromodoris dianae also has pale blue based gills and rhinophores, with orange yellow or deep yellow tips, but has more of an oval body form of true Chromodoris genus. Its mantle has white specks or dots hence not smooth looking. The black band in Chromodoris dianae are thicker and discontinuous, especially around the rhinophores.

Chromodoris reticulata has more than 14 gills and no rows of blood-red spots on the outer flanks of the mantle that are possessed by the real Chromodoris tinctoria.

The Doriopsilla sp. from Western Australia and Doriopsilla aureopunctata are identical externally, but there are internal differences especially in the reproductive system. And then there is the vast geographical distance separating them as well.

Flabellina exoptata. Its consistent characters are the vivid orange rhinophores with numerous papillae on the posterior side and the central purple (or bluish violet) ring on the cerata. Flabellina rubrolineata also has papillae on its rhinophores, but it has 3 longitudinal stripes on its body. Flabellina delicata also has papillae on its rhinophores, but it has a purplish body and oral tentacles, and the tips of the cerata are dark purple. Flabellina macassarana has lamellate rhinophores (i.e., numerous plate-like folds). It has a translucent pinkish to purplish white body, and the tips of the cerata are burnt orange.

These three species can be distinguished by visually observing the mantle margin. In Glossodoris cincta, the mantle margin bordered by well-defined turquoise blue, black and gold concentric bands. While in Glossodoris hikuerensis, they are bordered by concentric bands of brown, brownish white, dark greyish black and then brownish white. In Glossodoris pullata, it is simply bordered by dark blue mantle margin. (Summarized from Undersea Jewels book)

Halgerda carlsoni is most similar to Halgerda batangas. However, Halgerda carlsoni has microscopic orange speckles between the ridges (very rarely these speckles join up to form lines) and even when they are lines, they never form into a network of intersecting lines as in Halgerda batangas. Halgerda malesso has much lower pustules, yellow spots and yellow lines. There are far fewer yellow lines than in Halgerda batangas. Halgerda bacalusia has the summits of the ridges with strong dark orange lines and microscopic orange speckles between the ridges. Here a description from Dr. Dave Behrens: Halgerda bacalusia: broad orange lines follow the ridges to the orange tipped tubercles Halgerda batangas: network of orange lines between the tubercles, rather than a series of spots Halgerda carlsoni: the are lines of minute orange spots between the tubercles Halgerda malesso: no lines between the tubercles, only along the margin Halgerda stricklandi: orange tubercles arranged randomly, not in ridges. Orange spots not connected

There are still some debates as to whether these two Hypselodoris nudibranch with the same body form are separate species or just colour variations of the same species. The main difference is in colouration. Hypselodoris apolegma has a darker and deeper purple colour that grades towards the mantle margin into purple dots and finally into solid white along the edge of the mantle. The base of the gills and antennas (rhinophores) are also rich purple in colours, but changed to deep yellow at the ends. Hypselodoris bullocki has several pale purple shade variations, from very pale, almost white, whitish purple to purple, but never as dark as the purple found in Hypselodoris apolegma. The colour is uniform over the whole mantle, with a narrow band of white or dark purple at the mantle edge. The gills and antennas are yellow with purplish base or pale purple all over.

To distinguish between Hypselodoris kanga and similar looking Hypselodoris infucata is by looking at the gills. If they are triangular in cross section, and edged in red (blue in other parts of the world) and with a row of yellow spots up the broad outer face, then it is Hypselodoris kanga. Otherwise it is Hypselodoris infucata

Hypselodoris maculosa has 2 orange rings on the rhinophores, white longitudinal lines, purple spots, and white speckles while Thorunna australis has ring of purple spots around the mantle margin.

Hypselodoris emmae: The body is a pale cream or yellow with a purple or bluish border to both the foot and the mantle. there are three purple longitudinal lines on the mantle and four brownish lines which vary in intensity in different individuals. The gills and rhinophores are a brilliant reddish orange and in Pacific specimens there is a white tip to the rhinophores. Hypselodoris whitei has a whitish background color with a reddish purple submarginal line around the mantle edge and a series of five longitudinal lines covering the rest of the mantle. These lines are usually very crooked and joined by a series of lateral connections, breaking the mantle into rows of slightly raised whitish regions. The rhinophores are orange to orange-red with a distinctive white tip and the gills are similarly coloured with white on the inside and usually at the tip of each gill. Hypselodoris maridadilus is yellow, with a purple border to the foot and the mantle, and a series of five longitudinal purple lines. the gills and rhinophores are bright orange-red. The lines in Hypselodoris nigrostriata are more or less oblique. In the very similar species, Hypselodoris zephyra the lines are parallel.

Quoted from Sea Slug Forum: The example I illustrate here are the phyllidiid dorid nudibranchs such as Phyllidiella pustulosa. The phyllidiids exude very noxious and toxic chemicals from their skin and appear to be avoided by most fish. They are often the only sea slugs which blatantly crawl out in the open during the day. Other nudibranchs, flatworms, and even a holothurian are known to mimic phyllidiid nudibranchs.

Noumea romeri<ul> <li>Always has a distinct white mantle marginal band</li> <li>The mantle is pink</li> <li>The gills are always orange-red. Most all of the gills in the circlet are red right down to the gill pocket</li> <li>The rhinophoral clavus is most often completely orange-red. But sometimes the lower half of the clavus is white. The stalk is always white</li> </ul> Noumea simplex <ul> <li>Never has a white mantle marginal band. It can have an orange band, or orange dashes, or orange spots but there is invariably some orange pigment somewhere on the mantle margin. The tip of the tail is usually orange too</li> <li>The mantle colour can be pink or white</li> <li>The gill are always white with only the very tips of the anteriormost 3 gills being red</li> <li>The rhinophores have a red-tipped clavus. The majority of the clavus and the stalk are always white. The individual rhinophoral lamellae are transparent white</li>

Durvilledoris pusilla always has an opaque mantle. The pattern on the central area of the mantle is always scalloped with 'lobes' or 'tongues' of colour that almost reach the margin; one pair of these lobes is immediately behind the rhinophores and the other pair is in front of the gills. The midline has 2 white circles. The colour pattern on the tail repeats that of the central mantle. Thius is definitely a good species. Noumea varians can have either an opaque or a semi-transparent mantle. The pattern on the central area is not actually lobed, but it often looks that way in photos because of the way the body is deformed when crawling over irregular terrain. The midline has 3 white streaks or a single continuous narrow stripe. The tail is uniformly purple. There is evey indication that what we presently call Noumea varians may, in fact, cover several different species.

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

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).

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 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).

Roboastra luteolineata: Feeds voraciously on other nudibranchs; has very large oral tentacles; two pale stripes on the head between the rhinophores, neither of them expanded; no colour band around the rhinophoral sheath. Tambja affinis or Tambja victoriae: Feeds on bryozoans; has relatively small oral tentacles; one pale stripe on the middle of the head between the rhinophores that is expanded; pale colour band around rhinophoral sheath. Since they are so similarly coloured overall, perhaps some mimicry is going on. The ways to separate Tambja affinis, Tambja victoriae and other Tambja species are set out nicely and clearly on <a href="http://www.seaslugforum.net/factsheet.cfm?base=tambafficg">Tambja affinis colour group</a> in the Sea Slug Forum

At the present time Tritoniopsis elegans is known from the Red Sea and eastern coast of Africa. It is closely related to Tritoniopsis alba from the tropical western Pacific Ocean. In fact, some people have speculated they may be the same species, in which case the name would have to be Tritoniopsis elegans because it is older. However, until a comparative anatomical study, possibly reinforced with molecular data, has been undertaken and published it is advisable to consider them as separate species.