Chthamalus proteus sp. nov. is described as a distinct species based on its clear separation by starch-gel electrophoresis of enzymes. A redescription is given of C. fragilis, a related species, which differs absolutely from C. proteus in 3 out of 16 enzymes. Given the distinction of C. proteus from C. fragilis by enzyme methods, it has been possible to devise morphological criteria, involving simple measurement of the opercular valves and first maxilla, which can separate the two species in 99% of individuals. Although the morphological characters overlap slightly, the enzyme results show no evidence of hybridization. A species previously confused with C. proteus, C. bisinuatus from Brazil, differs considerably in morphology and is briefly redescribed.

A recent review of polychaete diets (Fauchald & Jumars, 1979) considered aphroditids to be slow-moving carnivores, taking microscopic prey if nothing else is available but specializing in slow-moving or sessile prey. This opinion was based on remarks by Day (1967) and on two documented gut analyses of Aphrodita aculeata (Blegvad, 1914; Hunt, 1925). Other reports in the literature were either derived from these few original statements or else gave unsubstantiated accounts of the type of food taken.

In 1964 the late Gunnar Thorson told me that he had seen a specimen of A. aculeata swallowing a large Nereis virens and likened it to a hedgehog eating a snake. This vivid analogy puts Aphrodita in a rather different light, as an active predator capable of dealing with relatively large, powerful prey. In fact Hunt (1925) had recorded remains of ‘Pectinaria, Lumbriconereis, Polynoidae and Nereidae’ in 24 out of 26 specimens containing food. Very young crabs and hermit crabs were recorded in five guts and a nemertean in one. Blegvad (1914) recorded the chief food to be other worms, especially sabellids and terebellids, besides nemerteans, from examination of 50 specimens, 35 of which were empty.

I have kept A. aculeata, obtained from Millport and from Plymouth, in bowls containing several centimetres depth of beach sand in a closed-circuit sea-water system. Worms without sand took no interest in items offered as food, neither did worms which had emerged from the sand and were wandering over the surface, but buried worms fed readily. A variety of living animals was given as potential food including Macoma balthica, Corophium volutator, Nephtys hombergi and Nereis diversicolor.