Identifying Favorinus pacificus

April 27, 2005
From: Bill Rudman

A number of recent messages have raised the identity of Favorinus pacificus.  Although I have discussed most of the following points while discussing those messages it is probably worthwhile to summarise them all into a single message.  Favorinus pacificus, was unfortunately based on a single specimen, which means that no matter how good the description was, it gives us no idea of variability within the species - does it chnage with size? is its colour pattern variable? do the rhinophores vary in degree of lamellation etc?.

Photos posted by Rie Nakano [#13535] and Dong Bum Koh [#13627] both fit many aspects of Baba's original description but they are not 'perfect fits' . For example, Baba described the cerata as being arranged in 'oblique rows' and yet if you look at Koh's photo, the first 3 rows of cerata are clearly in arches. You will see 3 pinkish arches on the left side of the animal. What has happened here is that the animal has dropped the cerata [autotomy] from these 3 rows - perhaps through being handled. Another point is that Rie Nakano's photo, and Koh's, show a white tip to the oral tentacles and a white line continuing down towards the base. Baba does not mention any colour on the oral tentacles.

In Atsushi Ono's 1999 book (Photo 276, p.164) there is a Favorinus sp. which also fits Baba's description quite well, but that animal has translucent yellowish oral tentacles, with no trace of white, and has a yellow median line on the head between the rhinophores. Since these features were not described by Baba could this animal be the real Favorinus pacificus? In a recent message from Singapore [#13607], Lim Han Peng has posted a photo of an animal very much like Atsushi Ono's, with a yellow line between the rhinophores. The cerata are pinkish rather than yellow, but that colour is due to the colour of the eggs they have been eating so is variable. The Singapore animal has much more pronounced lamellae on the rhinophores than your animal, but that suggests the degree of lamellae development is variable. Some animals indentified as Favorinus mirabilis [see Scott Johnson's message #1639] have a yellow median line. Does this mean that F. mirabilis has variable development of the rhinophore lamellation?

This brings up the question of Favorinus mirabilis, which as I have discussed separately, is probably two species? The 'form' I have called Favorinus cf mirabilis seems to me to fit Baba's description of Favorinus pacificus, at least on external features, extremely well. They have brownish rhinopores, bright yellow cerata with a white cap and a purple mark below. The only difference is the lamellae on the rhinophores. Can Baba's description 'indistinct constrictions' fit? Again the problem of not knowing how variable these characters are in each 'species' makes interpretation impossible.

What we definitely need is more information on all these 'forms' or 'species'. In the meantime I will tentatively identify Rie Nakano's and Dong Bum Koh's animals on the Forum as Favorinus pacificus.

Best wishes,
Bill Rudman

Rudman, W.B., 2005 (Apr 27) Identifying Favorinus pacificus. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/13616

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