Colourless interior shell-layers may be stained orange by digestive gland when feeding on red algae in P. vulgata as well as P. ulyssiponensis. These orange P. vulgata were found feeding alongside orange P. ulyssiponensis on surface covered in red encrusting algae and Corallina in north Yorkshire. In this case, they also have the oft quoted P. vulgata silvery-grey “head scar” (vertex patch), but not consistently present; pallial tentacles should always be checked.

OTHER SPECIES ALBUMS
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