Williamia gussoni (O. G. Costa, 1829)

Salvador, Xavier, Fernández-Vilert, Robert & Moles, Juan, 2022, Sea slug night fever: 39 new records of elusive heterobranchs in the western Mediterranean (Mollusca: Gastropoda), Journal of Natural History 56 (5 - 8), pp. 265-310 : 301-302

publication ID

https://doi.org/ 10.1080/00222933.2022.2040630



persistent identifier


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scientific name

Williamia gussoni


Williamia gussoni (O.G. Costa, 1829)

( Figure 6 View Figure 6 (h))

Material examined

Cala Maset caves, Sant Feliu de Guíxols ( Spain), 41°47 ʹ 10”N, 3°2 ʹ 44”E, 30 September 2020, 1 m depth, 4 spcs, adults and egg masses, L = 2–5 mm; 31 October 2020, 1 m depth, 2 spcs, juveniles, L = 2 mm; 12 December 2020, 1 m depth, 1 spc GoogleMaps ., juvenile, L = 2 mm; 22 December 2020, 1 m depth, 2 spcs, juveniles, L = 2 mm; 31 December 2020, 1 m depth, 2 spcs, juveniles, L = 2 mm; 14 January 2021, 1 m depth, 2 spcs, juveniles, L = 2 mm; 27 February 2021, 1 m depth, 2 spcs, juveniles, L = 2 mm; Barceloneta Beach , Barcelona ( Spain), 41°22 ʹ 49.5”N, 2°11 ʹ 43.3”E, 30 April 2021, 4 m depth, 1 spc GoogleMaps ., juvenile, L = 1 mm.

External morphology

Shell external, patelliform, covering entire body; protoconch on the posterior part, farther from the head; with white longitudinal lines, background red. Body background beige to white with concentrations of red marks in the front.


All specimens found in communities of the red alga Peyssonnelia rosa-marina Boudouresque and Denizot, 1973 , grazing and laying the egg masses at night.


Throughout the Mediterranean and the eastern Atlantic , in the Azores, Cavo Verde , the Canary Islands and the north-eastern African coast ( GBIF.org 2021) .


Williamia gussoni could be confused with the sympatric Tectura virginea (O.F. Müller, 1776) , from which it can be distinguished by the absence of long oral tentacles typical of Tectura .Also, in Tectura the apical part of the shell is in the anterior segment of the body, near the head. The pattern of the shell ribs is also different, with a greater number of vertical and more irregularly distributed white lines. Remarkably, the photograph herein is, to our knowledge, the first one taken of live W. gussoni specimens in their habitat. This species has been widely cited based only on shells found on sediments,with scarce data on living specimens,related to red algae of the genus Peyssonnelia Decaisne, 1841 . In our study, we observed a specimen from 31 October 2020 until 27 February 2021, always on the same algal frond of P. rosa-marina .