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A smaller bluet, with adults ranging from 28-34 mm in total length and 16-21 mm in hindwing length. Males are predominantly blue (andromorphic), while females are polymorphic and range from light blue to yellow-green, the former being more common. Thoracic stripes are typical of most bluets in both sexes. Black abdominal banding is present in both sexes. In males, the first three black bands occurring on abdominal segments 3 to 5 increase in width towards the end of the abdomen. Male abdominal segments 6, 7 and 10 are mostly black on the dorsal surface, segments 8 to 9 are predominantly blue. Males have a large apical spot on the dorsal surface of abdominal segment 2. In females, the abdomen is mostly black due to broadening of abdominal bands on the dorsal surface. Male eyes are dorsally black and ventrally blue, while female eyes are dorsally brown and ventrally tan to pale green. Postocular spots are prominent and rarely separated, forming a dumbbell shape in both sexes. In certain populations, abdominal banding may be wider in marsh bluets than in Hagen’s bluets (Enallagma hageni); however, over much of its range, marsh bluets are virtually indistinguishable from Hagen’s bluets except when viewed under magnification. Male marsh bluets and Hagen’s bluets differ in morphology of cerci. The large size of the mesostigmal plates, or “shoulder pads”, of female marsh bluets differentiate them from all other female bluets save Hagen’s bluets; however, the mesostigmal plates of female marsh bluets lie flat towards their posterior end, while the mesostigmal plates of female Hagen’s bluets are raised above the thorax. Compared to marsh bluets, northern bluets (Enallagma annexum), boreal bluets (Enallagma boreale), and familiar bluets (Enallagma civile) may be similar in colouration, but all 3 are notably larger (Acorn 2004; Paulsen 2009).

Males congregate near the edges open water, perching on emergent and shoreline vegetation or algal matts, alert to passing females (Paulson 2009). Males are rarely seen over open water. Mating pairs form adjacent to water bodies, generally around midday and into the evening. Females may oviposit on floating and emergent vegetation while still in tandem, or may break tandem for submerged oviposition. During submerged oviposition, females may descend 30 cm or more beneath the water’s surface (Paulson 2009). Nymphs presumably hatch anywhere from a few days to a few weeks after eggs are laid and continue to grow through the late summer and fall (Kormondy and Gower 1965). Little larval growth occurs during the winter season (Kormondy and Gower 1965). In early spring, larval growth resumes until the last instar is reached in June (Kormondy and Gower 1965). Shortly after nymphs will climb out of the water onto a favorable perch where they will undergo their final molt, revealing a winged adult marsh bluet (Acorn 2004). After emergence, adult marsh bluets mature in approximately one week (Kormondy and Gower 1965). The entirety of the marsh bluets life cycle is completed within one year (Kormondy and Gower 1965).

Adults feed on a diverse assortment of small soft-bodied winged insects (e.g., mosquitoes, other dipterans, mayflies, small moths). Nymphs feed on a variety of mobile prey, primarily aquatic invertebrates (e.g., mosquito and other dipteran larvae, mayfly larvae) (Lung and Sommer 2001).

Marsh bluets are common throughout north-central North America, ranging from central British Columbia to the Atlantic coast of Newfoundland. Can be found as far south as Tennessee and West Virginia. Does not extend as far north as the Yukon, Northwest Territories, or Nunavut (Acorn 2004; Paulsen 2009).

The effect of ectoparasites, specifically water mites (Arrenurus spp.), on marsh bluets have been reported in numerous studies. Water mites may appear as small red spheres on and around damselfly leg bases (Acorn 2004). Forbes and Baker (1991) show that the number of eggs produced by females is depressed with increasing levels of parasitism by water mite larvae. Forbes (1991) found that the mating success rates of marsh bluets is negatively related to the degree of parasitism, resulting in a mate selection bias by females towards males with fewer parasites present. Acorn (2004) proposed that American bluets (Enallagma) arrived in America in three waves; the first being northern and boreal bluets, the second being marsh and Hagen’s bluets, and the third being tule bluets (Enallagma carunculatum) and alkali bluets (Enallagma clausum). This second wave of bluets containing marsh bluets is supported by the fact that Francis C. Whitehouse did not detect marsh bluets when writing his booklet on the dragonflies and damselflies known to occur in Alberta in 1918 (Acorn 2004).