Gyrinus marinus Gyllenhal, 1808
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ADEPHAGA Clairville, 1806
GYRININAE Latreille, 1810
GYRININI Latreille, 1810
Gyrinus Geoffroy, 1762
Gyrinus Geoffroy, 1762
This is a widespread though local and sporadic species throughout Europe from northern Spain, Italy and the Balkans north to Scandinavia and the UK and east through Siberia to China and Kamchatka, in many northern areas e.g. Denmark and Sweden it is generally common while in Poland it is mostly coastal. It is sometimes quoted, in error, as occurring in the Nearctic region. In the UK it is locally common throughout England though more sparse in the West Country and the northeast, mostly coastal in Wales and widespread in southern and eastern Scotland north to the Moray Firth, it occurs on the Isle of Wight, Anglesey and Man but is generally absent from the Scottish Islands. In Ireland it is generally common though much less so in the south. The typical habitat is open areas and sparsely vegetated margins of large lowland ponds and lakes, in the UK it is usually the commonest species in such habitats but may also occasionally occur in slow-moving streams or canals etc, it avoids vegetated areas and may form large populations on calm open areas of large water bodies; in June 2009 we observed a huge population gyrating over the entire surface of a gravel pit some 200m by 300m outside Uxbridge in Middlesex, all specimens examined were the present species but they are known to form mixed schools with other gyrinids, most often G. aeratus Stephens, 1835. Adults occur year-round, they overwinter clinging to submerged vegetation and may be active very early, from early March, peaking in May and June and again later in the summer. Pairs may be observed mating from early spring, larvae have been recorded from late spring and summer and freshly emerged adults from July. This is generally not an easy species to record, adults avoid vegetated margins and while they may be present in large numbers within a few metres of the edge they are very easily disturbed and will dive or swim rapidly to open water, patience will be needed but they are attracted to floating dead insects and other carrion and this may provide a way of attracting them towards marginal areas.
4.5-8.0mm, females are generally larger than males and this difference may be obvious in the field so giving the impression of two distinct species. Broadly-oval and widest behind the middle, entirely shiny black with a bluish or bronzy lustre, red legs with dark claws, this last feature is present in only one other UK species, G. aeratus but here the elytral margin between the outer row of punctures and the edge is narrow throughout whereas in G. marinus it becomes wider from the middle before tapering before the apex. Dorsal surface glabrous although the terminal abdominal segments are clothed with dense long and pale pubescence. Head shiny, the cuticle finely wrinkled and with two or three deep impressions beside each eye, eyes convex and protruding, labrum transverse and strongly wrinkled. Pronotum transversely impressed in the anterior half but without a median ridge or impression, mesosternum with a fine median groove or suture in the posterior half which may continue anteriorly as an obscure impression. Scutellum flat and without a basal median ridge or tubercle. Elytra evenly curved to separately-rounded apical margins, with strongly punctured striae extending to a transverse semi-elliptical group of punctures before the apex, microsculpture narrow and slightly elongate, less often isodiametric. Legs yellow, contrasting against the mostly dark ventral surface, the middle and hind claws black or a little lighter towards the base. Aedeagus distinctive; median lobe slightly sinuate and tapering to a sharp or very narrowly rounded apex; in G. aeratus it is less strongly tapering and widely and distinctly rounded apically.