Only one subspecies of Ludwigia peploides (Onagraceae) in Croatia?

Abstract

The aquatic species Ludwigia peploides (Kunth) P.H. Raven (family Onagraceae ) is an invasive alien species of Union concern, i.e. included on “the Union list” since 2016 (European Commission 2016), according to the European Union Regulation No. 1143/2014 (European Union 2014). It is native to Americas, Australia and east Asia, with four subspecies recognized today (POWO 2026). In Europe, the species is associated with the formation of dense stands in slow-flowing and stagnant freshwater habitats, where it can alter habitat structure, reduce native biodiversity and affect water quality (Di Lernia et al. 2025). Its invasion success is largely driven by high ecological plasticity and multiple reproductive strategies. The species was recorded in Croatia eight years ago (Buzjak and Sedlar 2018), yet its distribution and infraspecific identity remain insufficiently resolved. In 2025, several new populations were discovered that are morphologically different from the one first reported. One recently discovered population was analysed in detail and can be unambiguously assigned to Ludwigia peploides subsp. montevidensis (Spreng.) P.H.Raven (Fig. 1. This determination is based on a consistent set of morphological characters. In contrast, at the first reported Croatian locality, population was re-sampled and re-examined in the field and subsequently in the laboratory. This population shows morphology predominantly like L . peploides subsp. glabrescens (Kuntze) P.H.Raven, particularly capsule length (up to ~40 mm) and number of seeds per locule (15–17), both of which correspond to diagnostic traits in existing identification keys. However, it also exhibits a mixture of characters, including sparse indumentum on upper stem portions and occasional pubescence on fruits and tepals (Fig. 1). Herbarium specimens deposited in the herbaria ZA and ZAGR, representing previously published records identified only at species level, show similar glabrescens -like morphology. This combination of traits suggests that key diagnostic characters traditionally used to delimit subspecies within L. peploides may not be fully stable. The incongruent morphological pattern may reflect either a higher degree of infraspecific plasticity than previously recognized, or a partial overlap of diagnostic traits among subspecies.