Re: Recognize this youngster

May 22, 1998
From: Dr Richard Willan

Dear Bill,
I was browsing through past messages on the Sea Slug Forum yesterday when I read the correspondence between you and Bernard Picton on Micromelo.

In 1984 I wrote an article for "Shells and Sea Life" (reference below) on this genus because Steve Long was publishing a colour photo on the front cover of each number and I had taken some nice photos of a living animal from off Cape Byron at that time. As I said then, the beauty of a living Micromelo is a never-to-be-forgotten sight.

Whilst doing the research for that article it became evident that there were no differences in shell shape between Pacific Ocean and Atlantic Ocean popoulations and the differences in colour in the lines on the shell - either red or black - appeared to be merely intraspecific variation (differences between different animals). I said then that the genus Micromelo has only one biological species which is widespread in the tropical Pacific and Atlantic Oceans and although I have seen many living animals of all sizes (none as tiny as 2.5 mm though) over the intervening years nothing has caused me to alter that opinion. Incidently, another member of the same family, Hydatina physis, has this same distribution.

There is one point to make about the formation of the specific name though and it involves the technical point of agreement of the generic and specific names. The name Micromelo is feminine and, because the specific name is an adjective meaning wavy, it must agree with the gender of the genus. So the correct name is Micromelo undata (Bruguiere, 1792).

Willan, R.C. 1984. The Guam Bubble Shell Micromelo undatus (Bruguiere, 1792) in Australia. Shells and Sea Life 16(5): 49-50.

Dr Richard Willan
Museum & Art Gallery of the Northern Territory
Northern Territory

Willan, R., 1998 (May 22) Re: Recognize this youngster. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

I am going to sit on the fence with this one. Since I provided some of the evidence (Rudman, 1972 - See No 14 on publications list) to show that Hydatina physis was a circumglobal species, I guess I should agree that the Micromelo guamensis / Micromelo undata situation is the same. But I still would like some further anatomical comparisons....Bill Rudman

Rudman, W.B., 1998 (May 22). Comment on Re: Recognize this youngster by Dr Richard Willan. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


Micromelo undata

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