[3] The South American Classification Committee of the American Ornithological Society (SACC), the International Ornithological Committee (IOC), and the Clements taxonomy recognize two subspecies of marsh antwren, the nominate F. a. acutirostris (Bornschein, Reinert & Teixera, 1995) and F. a. paludicola (Buzzetti et al., 2013).

[6] A subsequent molecular phylogenetic study showed that it belongs in genus Formicivora and by the end of 2021 worldwide taxonomic systems had made the change.

[11] The SACC, IOC, and Clements soon recognized the new taxon but as a subspecies of acutirostris rather than as a full species.

[12][3][4][5] However, the fourth edition of the Howard and Moore Complete Checklist of the Birds of the World and BirdLife International's Handbook of the Birds of the World (HBW) have retained the two taxa as species, using "Parana antwren" for acutirostris and "marsh antwren" for paludicola.

Adult males of the nominate subspecies have an ill-defined pale supercilium and a whitish crescent below the eye on an otherwise lead gray face.

Their throat, breast, and belly are lead gray and their flanks and crissum dark olive brown.

Adult females, like males, have an ill-defined pale supercilium and a whitish crescent below the eye, but on an otherwise black and white streaked face.

Subspecies F. a. paludicola is found further north, at fewer than 20 sites in the upper reaches of rios Tietê and Paraíba do Sul in São Paulo state near the city of that name.

[14][15] The marsh antwren is a year-round resident throughout its range though there appears to be some local movements between nearby sites.

It forages singly, in pairs, or in family groups while staying within dense marsh vegetation.

[14] The breeding season of the marsh antwren's nominate subspecies spans from August to February.

That of F. a. paludicola is "a sequence of rapidly but evenly repeated double-note phrases (the first note slightly descending, the second ending in a small upward and downward modulation)".

[14] The IUCN follows HBW taxonomy and therefore has separately assessed the "Parana" (acutrirostris) and "marsh" (paludicola) antwrens.

Some patches of habitat, including the type-locality, are under constant human pressure and have been reduced by fires, allotments and landfills."

"Current threats arise from anything affecting the tiny patchwork of marshes that form the entire range of the species.