How many species of Hydatina are there?

February 10, 2004
From: Bill Rudman

PHOTO: Drawings from Pilsbry, 1893. 7. Bulla velum Gmelin, 1791 [Pilsbry Pl.44, f. 7]. 29. Hydatina albocincta van der Hoeven, 1839 [Pilsbry Pl. 45, f. 29].

In an earlier message I suggested that many species of Hydatina seemed to be just shell colour forms of one species, Hydatina zonata. With Debbie Lange's photos of Hydatina physis and Hydatina zonata together I think it appropriate to share some unpublished notes I have on the question. Her photo of H. zonata and earlier photos of Nishina Masayoshi's from Japan [see #4976] illustrate the 3 main colour variants.

Basically there seems to be 3 species, Hydatina physis, Hydatina amplustre, and Hydatina zonata. This third species seems to have been given many names based on colour variants

Hydatina zonata (Lightfoot, 1786)

Synonymy.
Bulla amplustre Born, 1780: 204, Pl.9, fig.1 (NOT B. amplustre Linnaeus, 1758).
Bulla zonata Lightfoot, 1786: 164.
Bulla velum Gmelin, 1791: 3433.
Bulla cinctoria Perry, 1811: Pl.40, fig.1.
Hydatina albo-cinctum van der Hoeven, 1839: 246, Pl.10.
Amplustrum velum: Bergh, 1901a: 242-6, Pl.20, figs 45-50, Pl.21, figs 1-10., Vayssiere, 1906: 29-33, Figs 52-5.
Aplustrum albo-cinctum: Bergh, 1901a: 251-2., 1901b: Pl.22, figs 18-23.
Hydatina (Amplustrum) velum: Eales, 1938: 79-80, Fig.1, Pl.1, fig.8.
Hydatina (Hydatoria) zonata: Habe, 1950a: 18, Pl.3, fig.6.
Hydatina (Hydatoria) albocincta: Habe, 1950a: 17-18, Pl.3, fig.7.
Hydatina cinctoria: Rudman, 1972b: 136-7, Pl.1D, E.
Amplustrum amplustre: Catala, 1979: 105, Fig.75 (NOT Bulla amplustre Linnaeus, 1758).

While Hydatina physis is characterised by many thin brown spiral lines colouring the shell, there are a number of names which apply to Hydatina shells in which brown axial lines predominate. Usually the brown lines are split into a series of spiral zones by white spiral bnads. In one form, (`H. cinctoria'), the brown zones are separated by five white bands, while in `H. velum' there is an upper, a lower, and a thin median white band, each of which is outlined with a thin dark brown line. The rest of the shell has pale brown pigmentation. These colour combinations and a gradation of intermediate patterns have been given specific names which are listed above. No anatomical differences have been observed and the colour and shape of the animal varies from a pale translucent almost white form to a dark pinkish brown edged with white. I suspect all these forms are one species.

Bill Rudman

Rudman, W.B., 2004 (Feb 10) How many species of Hydatina are there?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/12170

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