Cercyon marinus Thomson, C.G., 1853
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POLYPHAGA Emery, 1886
HYDROPHILOIDEA Latreille, 1802
SPHAERIDIINAE Latreille, 1802
Cercyon Leach, 1817
Cercyon Leach, 1817
This very widespread species is native across the Holarctic region; it is locally common across the northern United States, southern Canada and northern Asia, extending south into the Near East but absent from North Africa, in Europe it extends to the far north of Fennoscandia but to the south occurs sporadically and is absent from the Balkans and the Iberian peninsula. It is generally common throughout England and southern Scotland and more local across Ireland, it is generally absent further north but there are records from Orkney and Shetland, and despite occurring throughout Wales, including Anglesey, it is absent from the West Country although it does occur on Scilly. Adults are present year-round, peaking in abundance in late spring and early summer, they are associated with wetland habitats, usually among moss or decaying litter on densely-vegetated margins of permanent water bodies, in the UK usually on heavy soils although on the continent also in wet sandy and silty habitats, and in Canada both adults and larvae have been recorded in abundance in saline lakes. Breeding occurs in the spring and the predaceous larvae develop among aquatic algae or in permanently wet marginal situations, pupation occurs in a cell out of water and new-generation adults occur from early summer. Adults are detritivores although they are also thought to take occasional live prey, they cannot swim and when submerged will crawl about stems or under the water surface, they spend most of their time among marginal soil and plant remains but they disperse by flight and occasional swarms have been observed in the summer when individuals regularly occur at light traps. They may be sampled by working through marginal litter etc. and sometimes appear in the pond net, through the winter they often appear in extraction samples or among flood-refuse, and might be found at any time among decaying organic material in wetland bird nests or among accumulated waterfowl droppings etc.
Adults are small, 2.2-3.4mm, and may be recognized by the distinctively coloured elytra and narrow mesosternal field; they may be identified in the field with a lens but C. bifenestratus Küster, 1851 is closely similar, differing in the extent of the pale elytral colour and in the shape of the mesosternal process. Broadly elongate-oval and smoothly rounded apically, in side view the pronotum and elytra form a single smooth curve c.f. C. ustulatus. Body glabrous and shiny black but for the pale anterior margin of the labrum, lateral pronotal margins and apical and lateral areas of the elytra, legs pale to dark brown, palps dark brown, usually with the terminal segment darker, and antennae pale brown with a dark brown or black club. Head transverse and smoothly convex, without impressions or fovea, surface finely and evenly punctured throughout, eyes moderately convex and continuous with the outline, clypeus narrowed in front of the eyes and truncate anteriorly, palps slender and about as long as the antennae, the last two segments almost equal in length. Antennae 9-segmented, with a long and curved scape and a compact club. Pronotum broadest across the base and narrowed to rounded anterior angles, the lateral margin curved or weakly angled about the centre and evenly narrowed (not sinuate) to obtuse posterior angles, the basal margin gently sinuate, surface evenly convex and finely punctured throughout. Elytra with distinct striae consisting of finely impressed lines with rows of larger punctures, the interstices are finely punctured and without microsculpture (although very weak lines may be present at high magnification), apical area yellow, this continuing along the lateral margin almost to, or reaching, the humeral angle-in C. bifenestratus it extends to about half-way along the margin. Mesosternal process flat and narrow, 3.5-3.5X longer than wide, in C. bifenestratus it is slightly concave and much broader, at most 2X longer than wide. Legs short and robust, all tibiae with stout external spines and simply rounded or slightly truncate apically. Tarsi 5-segmented, the basal segment of the middle and hind tarsi longer than the second.