I am not at all sure what the artist was thinking when he added this little marginal painting in a manuscript, but I love it. An angry oyster protecting a pearl? unwilling to be eaten? or just ornery from a life time of an irritating parasite giving him a silvery ulcer. Maybe a painter frustrated in love since the oyster “exciteth Venus."
Jacob van Maerlant, Der Naturen Bloeme, Flanders ca. 1350 (Den Haag, Koninklijke Bibliotheek, KA 16, fol. 108v) This manuscript is rather interesting and features quite a few "angry" creatures and some even stranger pairings...like this one below: