Stylocheilus longicauda & Stylocheilus citrina
Nomenclatural discussion

Superfamily: APLYSIOIDEA
Family: Aplysiidae


A. Aplysia longicauda Quoy & Gaimard, 1825 - original illustration. "..Green with red spots circled with blue, and other spots of white and blue. On floating Fucus off New Guinea" [translation]
B. Aplysia (Notarchus) citrina Rang, 1828 - original illustration. ".. Yellow with white spots. In equatorial mid-Atlantic on floating Fucus" [translation]
C. Aplysia nudata Rang, 1828 - original illustration. ".. Green with some pale spots. On floating Fucus .. Hawaiian waters" [translation]
D.Aplysia ocellata Ferussac in Rang 1828. [This is a photo from the Leiden Museum of van Hasselt's painting from Indonesia on which Ferussac based his species. - Yellow with blue spots with yellow ring].
E. Aplysia striata Quoy & Gaimard, 1832 - original illustration.

This problem concerns two species of Stylocheilus described by Quoy & Gaimard. The first, Stylocheilus longicauda, in 1825 (PHOTO A) and the second, Stylocheilus striatus in 1832 (PHOTO E). Unfortunately Quoy & Gaimard noted that S. striatus was probably the same as S. longicauda so beginning a confusion which has persisted until the present. Most workers consider that there are two species of Stylocheilus, one with dark longitudinal lines, a mottled colour pattern, and compound papillae, and the second with a uniform yellow or green colour, simple papillae, no lines, and an extremely long slender 'tail'. I have presented many of the original descriptions and illustrations here to show that Quoy & Gaimard's two species are not synonymous and represent the earliest names for each species.

In recent years the yellow species of Stylocheilus has been identified as Stylocheilus citrina (Rang, 1828) (Marcus, E. & Marcus, E., 1962; Marcus, E., 1972). It has a circumtropical distribution and has usually been found floating in the open ocean on drifting brown algae. On two occasions I found numbers of animals washed ashore in Tanzania, on drift algae. The body ranges in colour from bright yellow to bright green with scattered ocellate spots. In specimens I have examined, the spots are usually blue, often with a reddish edge. Surrounding the spot is a clear ring then a ring of orange. In some specimens the red edge dominates and the blue spots appear red. White and blue spots without a surrounding ring can also occur. There a scattered short papillae over the body. The body is very elongate and its most characteristic feature is the extremely prolonged narrow 'tail' which can make up to half the body's total length. The sole produces a very sticky mucus. This elongation of the posterior foot, and the sticky mucus, are probably an adaptation to life on floating weed.

It seems there is an important nomenclatural problem surrounding this species. Although the name Stylocheilus citrina (Rang), refers to it, the earliest name for it must surely be Stylocheilus longicauda (Quoy & Gaimard, 1825), which is at present used incorrectly for the common species with dark longitudinal lines down the body and a relatively short 'tail'. Quoy & Gaimard's description of Stylocheilus longicauda is of a 'pleasing' green coloured animal with spots of varied red and blue and an extremely long narrow tail, found on floating Fucus in New Guinea waters.

On the right I have reproduced the original figures of most of the early Stylocheilus names. The confusion seems to have arisen from a comment by Quoy & Gaimard (1832) in their description of Stylocheilus striatus where they suggest it may be the same as Stylocheilus longicauda. Stylocheilus striatus would seem to be the earliest name for the lined form which has usually been called Stylocheilus longicauda.

Apart from the dark lines on the mantle, the two species differ externally in body shape and background colour. Stylocheilus striatus has a much shorter 'tail', usually less than a third the total body length while in Stylocheilus longicauda, as its name inplies, the 'tail' is up to half the total body length. Stylocheilus striatus has a translucent body with patches of greens, browns and white, and often with many branched papillae. The background colour of Stylocheilus longicauda, on the other hand, is a uniform yellow or green, and papillae are sparse and seldom branched. Ecologically they are quite different, Stylocheilus longicauda having a circumtropical distribution as a pelagic animal drifting on floating algae and other floating objects, while Stylocheilus striatus is a circum-tropical shallow water bottom-dweller.

Two other names, Aplysia (Phycophila) euchlora A. Adams, 1861 [green…tail produced ..on floating Zostera - Japan] and Tethys (Phycophila) adamsi Pilsbry, 1896 [.. very long tail] also refer to Stylocheilus longicauda. A.Adams (1861) proposed the genus Phycophila for these two 'species', which he characterised by its long and narrow tail adapted for clinging to floating weed far from shore.

Relevant Stylocheilus references.
Go to Stylocheilus longicauda Page.
Go to Stylocheilus striatus Page.

Authorship details
Rudman, W.B., 1999 (August 3) Stylocheilus longicauda & Stylocheilus citrina Nomenclatural discussion. [In] Sea Slug Forum. Australian Museum, Sydney. Available from

Related messages

Stylocheilus's name - beating a dead horse?

March 16, 2001
From: Edwin Cruz-Rivera

With the risk of asking what to some might be obvious, I once again bring the question of the taxonomic status of Stylocheilus longicauda vs. S. striatus. I have recently submitted a paper on some aspects of the ecology of this sea hare and a few more are underway. Given the issues brought forth in this Forum, I am wondering what species name I should use.

For my article, I used Stylocheilus striatus and as reference I cited the website where this was discussed. However, a couple of problems with this have been pointed out to me in reviews:
• 1) Websites are very rarely permanent (addresses can change, lists die off, etc.) so citing a website might not provide a source for confirmation in the future.
• 2) The opinions in the website are not unilateral in favor for the change.
• 3) The website is not a "peer reviewed" source.
• 4) Other things pointed out like the fact so many people have used the presumed "wrong" name are already in the webpage and need not be mentioned.

Having seen both Stylocheilus "types" alive and kicking, I have little doubt in my mind they are two species. I also agree with Rudman's arguments on changing the name of what is currently called S. longicauda to S. striatus (based on his analysis of Quoy and Gaimard's treatise) and I have spoken to Clay Carlson about this more than once. However, I can not ignore the comments above. So I want to know what this Forum thinks about these issues. Should S. longicauda keep on being used until someone publishes a paper clarifying all this, or do we have enough to simply correct the missmatch in future publications?
Your thoughts are appreciated.

Cruz-Rivera, E., 2001 (Mar 16) Stylocheilus's name - beating a dead horse?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Edwin,
No you are not beating a dead horse. The nomenclatural complication in this case is that S. longicauda was being used for the wrong animal. This is not a case of resurrecting an old name for pedantic reasons. The old name was being used - but for the wrong species.

Let me address the 4 points you bring up.
Website permanence.
We are living in a time of change, where the web is becoming an important and growing source of information. Many funding bodies worldwide are supporting the development of online databases of various sorts, which are going to become an essential source of information. The challenge is not to ignore these databases but to find ways to refer to them. I have developed a standard citation for the Forum which if cited as published, will find the record as long as the Forum exists. Even if the citation system is not used and messages are cited by author, title and full date, they will be found. Which leads to the question about the permanence of the Forum. The Forum is part of the Australian Museum website and all the planning that has gone into it is based on it being a permanent site. Part of this planning includes how to organise the site so that it is self-sustainable and in the future will allow me to fade out from day to day participation. In general, website 'publications' have created a new problem for archivists, but we can't just wring our hands and ignore the advantages of this new technology. I am sure we will find a solution to the 'permanence' problem, especially, if by our use of it, we show we have something valuable to preserve.
Opinions not unilateral.
Basically science is not a democracy. People disagree for all sorts of reasons. In the end you have to make your own judgement based on the evidence available to you.
Not peer-reviewed.
What is peer review? In theory, it is an excellent system in which we try to keep our colleagues honest by checking their methods and alerting them to possible errors. Unfortunately what it often is, at least in taxonomy and systematics, is 2 or sometimes 3 overworked colleagues quickly scanning through a manuscript, picking up obvious errors, and some typographical errors etc. Some reviewers are incredibly diligent, and we should be grateful for their efforts, but they are unfortunately a minority. In taxonomy in particular, the author is often the first person for many years to have studied the group in detail so his reviewers are often only able to make a judgement on the methodology, the layout and the grammar. There is no way they can check the anatomy, which is the basis of the work. The other problem with print publications is that no journals have a 'Letters to the Editor' column, so mistakes, missed by reveiwers, go uncorrected for years. I think peer review is better than nothing but it is not perfection. And most colour photo guidebooks, which many use for identification, undergo no serious pre publication review.

The Forum has been denigrated as an unreviewed 'backdoor publication'. I am proud to be associated with any publication - backdoor or frontdoor - that has 7784 different readers every month [see Visitors to Forum]. I would be surprised if any peer reviewed taxonomic publications were read by 7784 readers in a 100 years let alone a month! So is the Forum peer reviewed? Amongst the Forum's 7,784 readers a month, are a good proportion of my collegues - our peers. I know that if I post a message which someone disagrees with, there is a response - and a correction if needed - within a few days. To me that is real peer review.
it's not worth mentioning
Clearly Stylocheilus longicauda has been used for the wrong species in quite a few ecological papers. I think it is worth a short paragraph saying why you are using S. striatus for what was previously called S. longicauda with a reference to the Forum. Unless you alert ecologists and non-taxonomists to the name change they will either think you are dealing with a different species or just become confused.
Wait for a publication.
You could be waiting a long time. This is a simple identification problem, not a taxonomic problem. There are too few taxonomists in the world for you to wait until every identification problem is resolved and published on paper. For that matter, I think it has been published. You know about, and at least 8000 others will read this and know about it as well. If that's not published I don't know what is. There is of course a distinction between what we normally mean by 'publish' and what the International Commission on Zoological Nomenclature define as a valid publication for nomenclatural purposes. I would not attempt to publish a new species description on the web at present. But we are not talking about a nomenclatural decision here, we are talking about a question of identification - and there is no International Rule telling us how we are to do that.

Sorry to be so long-winded, but your query gave me a chance to say a few things about the Forum that I have been forced to argue over quite a lot recently. Is it science? Is it worthwhile etc etc? My advice is defend and use the new technology - I wish I had it when I started my career,
Best wishes,
Bill Rudman

Rudman, W.B., 2001 (Mar 16). Comment on Stylocheilus's name - beating a dead horse? by Edwin Cruz-Rivera. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Stylocheilus nomenclature

June 11, 2000
From: Ángel Valdés

Dear Bill,

I was reading your discussion with Richard on Stylocheilus nomenclature with great interest. But I think you are talking about two different things here.

One problem is the whether the species names Stylocheilus longicauda and Stylocheilus striatus are synonyms or not. I agree with you that there are probably two different species. A good starting point to solve this problem could be to study the type material of both species. One syntype of Aplysia striata and one syntype of Aplysia longicauda still remain in the collections of the Muséum National d'Histoire Naturelle of Paris. Philippe Bouchet will the delighted to lend the specimens for study.

If we have two different species involved, both names should be maintained and used according to their original meanings (Article 49). The law or priority and the ruling of the Commission have nothing to do with this. The Code is very clear in this respect: names should be applied in the meaning of their name-bearing types. If the name S. longicauda was used for the wrong species, then you should correct the application of the name, as you did. There is no reason to conserve the misapplication of a name unless that misapplication was used as the type of a new genus (Articles 11.10, 67.13, 69.2.4 and 70). And this is not the case.

I agree with Richard that a complete taxonomic study should be carried out before reaching a definitive conclusion, and that study should include the related genus Bursatella. But if you are right (and I think you are), and Stylocheilus longicauda and Stylocheilus striatus are different, then you should go ahead and use the right name for the right species, regardless the consequences it might have.

All the best,


Dear Angel,
I agree that sorting out the identity of the two names and using them correctly, is an act of identification and as such has nothing to do with the Rules. However I also think that identifying the two species can be done independently of sorting out the phylogenetic relationships between Stylocheilus and Bursatella.

Concerning the study of the types in Paris. I wish I could be as confident as you that they would solve the problem. In cases where there is a key character which can survive 150 years of museum preservation then looking at old types is obviously useful. The shell of a mollusc is usually definitive, as are the internal hard parts of primitive opisthobranchs. Sometimes we can be lucky with an old nudibranch type which has a species or genus specific radula, but equally often, as I have found with Risbec material, past preservation and mounting techniques have destroyed the radula.

From the published accounts of the anatomy of the two? species of Stylocheilus, there are no easy characters to use in the genus. The radular morphology of aplysiids, at least at the genus level, is not a very precise character, and in the case of Stylocheilus the gizzard plates show no distinctive differences, and Marcus' early suggestion that the penial spines of S. longicauda (as S. citrina) were smaller than in S. striatus (as S. longicauda), she later decided were a function of age (size) not species.

Personally, I think the available illustrations and descriptions quite clearly define the two species. If the debate is whether there are two species, then we might as well use more recently collected specimens, in which the anatomy is easier to study, and multiple specimens can be compared. If after such an investigation, one or more distinctive characters are discovered, then I think it would be appropriate to borrow the types from Philippe Bouchet and confirm their presence in the types. If distinctive characters can't be found in more recent material then the types may as well rest in peace.

References to anatomical studies can be found on the following pages: 'S. citrina' References  and
S. striatus References.

Best wishes,
Bill Rudman.

Rudman, W.B., 2000 (Jun 11). Comment on Stylocheilus nomenclature by Ángel Valdés. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Stylocheilus nomenclature

June 6, 2000
From: Richard Willan

Dear Bill,

Thanks for pointing me in the direction of your change to the name for the common widespread Stylocheilus. You have really opened a can of worms!

Taxonomically, Stylocheilus is not as distinct from Bursatella as you might think and the external characters you mention as distinguishing between the two supposed species of Stylocheilus are not clear cut - juvenile S. longicauda do have very long tails; the papillae are always compound on S. longicauda, and I have slides of a S. longicauda from Daydream Island, Queensland, with no brown lines on the body at all. The colour and size of the ocelli on the body are extremely variable too. It is timely to undertake a study comparing the anatomy of all the "woolly" sea hares and another one investigating the phylogeny of the resulting species.

Nomenclaturally, your change will cause confusion now that both longicauda and striata are in usage. It is good you raised the matter, but I'm not sure if a change in name is actually appropriate at this time, regardless of the biological conclusions. A case for retention of longicauda to the International Commission on Zoological Nomenclature, though lengthy, would have succeeded because it could have been easily proven: (a) the name longicauda was in general usage for the species in question and (b) longicauda has been used as the presumed valid name, in at least 25 works, published by at least 10 authors in the past 50 years, and encompassing a span of not less than 10 years. In other words the case would have been argued as one of reversal of precedence of synonymous names to achieve stability which is explicitly the overarching principle embodied in the present International Code of Zoological Nomenclature which came into effect on 1st January 2000.

Unfortunately I think you need to change your name once again. Quoy & Gaimard originally used the name Aplysia striata. But the gender of Stylocheilus is masculine, so the specific name striatus, -a, -um ,which is a Latin adjective meaning striped or lined, needs to be written in the masculine form, striatus, to be in agreement with the genus.

Richard Willan

Willan, R., 2000 (Jun 6) Stylocheilus nomenclature. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Richard,
Thanks for your comments on Stylocheilus. You raise a number of interesting points.

Gender matching
Firstly the need to change the ending of the name to match the gender. You are of course correct. Personally if we are interested in stability of nomenclature, the need to follow the arcane grammatical rules of Latin, which none of us learn, every time a species is moved to another genus is somethinh many of wish had been changed in the new addition of the Rules. With the increased use of computerised lists and searches, the prospect that the spelling of species names may never be fixed is absurd. This is a case in point. The spelling of striata changed to striatus when it changed genus from Aplysia (feminine) to Stylocheilus (masculine). Without a background of latin grammar you would think that the spelling of longicauda would change to longicaudus but it doesn't, because longicauda meaning 'long tail' is treated as a noun in apposition and so is independent of the gender of the genus name.

Identification of species.
Anyone who has seen true Stylocheilus longicauda has no difficulty distinguishing it from the striate form. It is true that juvenile S. striatus often have a long tail. It is interesting to speculate that this is an adaptation in juveniles to cling on to small weeds so they are not washed away, which is lost in adults. Perhaps Stylocheilus longicauda which is always found in association with floating algae has evolved neotonously from a juvenile benthic species. Whatever its origin, the feature mentioned by all the early describers of this species was not the branched or unbranched papillae but the association with floating algae. In my mind there is no difficulty in identifying Stylocheilus longicauda. There may be more than one benthic species, but if so, that will initially be a question of comparative anatomy and only later a question of nomenclature. You say that I should have applied to the Commission as a case could have been argued "as one of reversal of precedence of synonymous names". In my opinion (and Terry Gosliner's and Clay Carlson's) the names are not synonyms. This is a case of misidentification not nomenclature.

The 'Rules' and the Law of Priority.
You mention that changing the names of these species will cause confusion. Name changes always cause confusion, but it is usually short lived except in the few cases where a name applies to a economically important organism. Much more confusion would occur if the Law of Priority* which underpins all of nomenclatural stability, became something which we could easily be overrule at the urging of a few hidebound taxonomists who don't want to change. Unfortunately taxonomy is the only science where you can't ignore the work of incompetent or sloppy workers. But I don't see why we need to reward them as well. I greatly admire some of the early naturalists who suffered great privations and sometimes, like poor van Hasselt, an extremely early death, but still produced descriptions and illustrations which far surpass many of those that came later from workers who worked with the preserved collections of other people. I don't see any reason to displace the valid identifiable names of diligent field naturalists, just to accommodate incorrect nomenclatural decisions produced in the comfort of Europe and North America years later.

You seem to suggest we have a duty to suppress early names to protect later mistakes. I am afraid I don't agree. Any weakening of the Law of Priority, except in the most exceptional circumstances, would weaken the whole system of nomenclature and lead to endemic instability.

Bill Rudman.

* Law of Priority. The rules on naming organisms is underpinned by one basic tenet, which is that the earliest valid name given to a species has priority over all subsequent names given to the same species.

Rudman, W.B., 2000 (Jun 6). Comment on Stylocheilus nomenclature by Richard Willan. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

The true identity of Stylocheilus longicauda

August 5, 1999
From: Bill Rudman

Betsey Hansen's photo of a bright yellow Stylocheilus from the Caribbean was the stimulus I needed to present my ideas on the true identity of Stylocheilus longicauda and Stylocheilus citrina. Since finding bright yellow and green animals of Stylocheilus in Tanzania in 1976 I have always thought that it fitted Quoy & Gaimard's description of Stylocheilus longicauda much better than the lined species which usually bore the name.

I have presented my ideas, with photos of all the original illustrations in a separate page. I propose that the yellow species is the true Stylocheilus longicauda and the lined species should be called Stylocheilus striata. I realise it will be inconvenient to change the usage of a name attached to such a common animal but I can see no good grounds to ask the International Commission to act. The only solution would be to suppress the name Stylocheilus longicauda which would still mean that the common lined species would have to have a new name. I think in this case we just have to admit that past workers have got it wrong. I have discussed this with Clay Carlson and Terry Gosliner who have both independently reached the same conclusion.
Bill Rudman.

Rudman, W.B., 1999 (Aug 5) The true identity of Stylocheilus longicauda. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from