Diet of Armases rubripes (Rathbun, 1897) (Brachyura: Sesarmidae) in subtropical salt marshes of southern Brazil: an omnivorous species dependent on two key plant species of the local flora

Armases rubripes (Sesarmidae) is a small, semiterrestrial brachyuran crab found in salt marshes, mangroves, and other environments along the western Atlantic, from Nicaragua to Argentina. It spends most of its time climbing vegetation and sheltering in mud cavities. Its diet is omnivorous, including arthropod larvae, invertebrates, algae, grasses, leaves, flowers, and organic matter. However, it is considered a functional herbivore due to the predominance of vegetation in its diet. This study aims to describe the diet of A. rubripes in subtropical salt marshes, an ecosystem recently distinguished from tidal marshes, and compare its herbivorous diet to the local plant abundance. We documented the species’ diet through opportunistic observations of vegetation and objects at heights from 0.30 to 2.0 m using binoculars. Observations took place monthly, over 3–30-day campaigns from 2006 to 2026, across 36.5 ha of herbaceous or arboreal-dominated marshes in Guaratuba Bay, Paraná, southern Brazil. We recorded A. rubripes consuming leaves, bracts, floral parts, and/or sporangia from 20 plant species across 12 families. Fifteen of these species were herbaceous, primarily from Cyperaceae and Poaceae. The crab fed on six of the seven abundant and 10 of the 12 common plant species. It also consumed body parts or eggs from seven animal species, including decomposing bird nestlings. Our results confirm an omnivorous diet and functional herbivory of A. rubripes . Since Crinum americanum (Amaryllidaceae) and Schoenoplectus californicus (Cyperaceae) were abundant year-round in the studied areas, we propose that folivory of these plants forms the local diet’s basis for A. rubripes .