Cafius xantholoma (Gravenhorst, 1806)
This strictly coastal rove beetle is generally abundant, except where human development has destroyed its habitat, throughout the Black Sea, Mediterranean, and Atlantic coasts of Europe north to mid-Scandinavian latitudes; in the UK it occurs throughout, including Isle of Man, Anglesey, Western Isles and Orkney although records become more scattered in the north and it is most abundant in the southwest. They occur under debris and accumulated seaweed for a few metres from the strandline, both among decaying seaweed and in the few centimetres of organic rich sand below, they inhabit areas of high humidity, abundant organic matter and narrow temperature variation, are quick to establish in new environments and may occur in great abundance among deeply piled rotting seaweed thrown up by the tide. Both adults and larvae have been found year-round, although adults are generally active between March and October, and as the larval development is rapid; from egg to adult in a few weeks, the species is thought to be multivoltine. Both stages are predatory on small beach-hoppers (Crustaceae) and insects and their early stages but especially on seaweed flies (Coelopsidae) and other diptera which usually also occur in great abundance. They are typically abundant among well-decayed, almost fluid, and powerfully pungent seaweed where they are protected from sunlight, desiccation and wave action, but if this is flooded the adults will rapidly appear and take flight, always away from the sea and very rapidly so they are very illusive; they are extremely resistant to wetting and can withstand immersion, usually curving the abdomen over the body and holding an air bubble which makes them buoyant, and can stand, run or take flight from the water surface. The larvae are typical of Staphylininae and do not appear to be particularly modified for such a lifestyle but unlike most marine staphs, in which they are exarate, the pupae are obtect. They usually occur along with other halobionte coleoptera e.g. various staphs including other Cafius, and several hydrophilids and tenebrionids etc. and are quite tolerant of human interference; we found them in abundance among seaweed on a busy West Country beach, and among decaying seaweed collected from beaches and piled on rocks by a roadside.
Distinguished among our species by the form of the pronotum and the yellow reflexed-elytral margins. 7-9mm. Entirely black to dark brown (immature specimens are pale brown), usually with paler legs and variegated abdominal pubescence. Head quadrate with long straight or weakly dilated temples and a wide neck, large oval eyes, beside each of which is a large foveate puncture, and long slender mandibles bearing distinct teeth internally. Antennae long and rather slender with segments 4-11 near quadrate and less shiny than 1-3, terminal maxillary palpomere elongate and at least as long as the penultimate segment. Pronotum elongate, generally about as wide as the head, broadest just behind rounded anterior angles and quite strongly narrowed to the base (a distinguishing feature of the genus in the UK), surface with linear microsculpture and a series of four foveate punctures (or rarely with an extra one on one or both sides) either side of the middle, the anterior pair variously offset, and that near the apical margin usually smaller than the others. Lateral margins with a series of long erect setae as well as a few splayed laterally near the anterior margin. Scutellum large, triangular and densely punctured and microsculptured. Elytra elongate and usually wider than the pronotum, with strongly micro-rugose and evenly pubescent surface, and dark with pale reflexed lateral margins. Abdomen strongly bordered laterally, with dense variegated pubescence, in some specimens with pale markings to each segment; in life they seem iridescent and to flash with yellow spots etc. as they move. Basal pro-tarsal segments dilated in both sexes.
Generally larger (9-13mm)
Elytra lacking yellow margins (fig. 1)
Pronotal microsculpture granular, if present.
Mandibles with two teeth on inner margin.
Fig. 1 - Elytral margins of C. xantholoma (above) C. fucicola (below).