On this slightly translucent specimen, the whitish ground colour is darkened centrally by the internal dark amphora-shape mark. Elsewhere the white is tinted greenish by the mantle within.
The specimen was found stationary on the side of a boulder lacking encrusting red algae.
T. testudinalis feeds preferentially on red calcareous algae encrusting bedrock and boulders but during daylight, when it would be easily seen on a red substrate, it is often found stationary on bare upright rock surfaces where its brown and white pattern is less obtrusive and, in this case, resembles neighbouring fucoid algae, a piece of which is growing on its shell.
APPENDIX re range advance/retreat: flic.kr/p/2jW74ig