Burrowing behaviour of estuarine clam Rangia cuneata outside its native range

Abstract

Rangia cuneata, an invasive species originating from the Atlantic coasts of North America, has rapidly established itself in various European brackish habitats over the last two decades. Despite its significant ecological, commercial, and invasive implications, understanding of the species' burrowing behaviour remains limited. This study investigates the influence of seasons (winter at 8 °C versus summer at 22 °C) on the burrowing patterns of two clam size classes (small, ≥16 mm to <25 mm, and large, ≥25 mm to ≤36 mm) in two distinct substrates (intact cores of hard sandy versus soft silty sediment). The specimens and sediment were sourced from the recently invaded brackish Vistula Lagoon in the southern Baltic. The analysis revealed that larger clams from soft sediment exhibited thinner shells and a higher soft tissue energy value than those from hard sediment. While the condition index remained unaffected by the substrate, burrowed clams moved vertically several millimetres daily. In winter, clams generally buried deeper, although small clams consistently reached greater depths in soft sediment during summer. Three temporary clam behaviours were identified: shells protruding above the sediment (mainly in winter, indicating compromised physiological conditions), shells buried with an extended, visible siphon (most frequently observed in both seasons, especially during winter in soft sediment), and shells buried with a retracted, invisible siphon (more prolonged in summer, independent of sediment type). Interestingly, neither burial depth nor vertical mobility showed a clear association with the organisms' sex. The study's findings contribute to the understanding of the adaptive processes of R. cuneata, providing insights into how it adjusts to recently colonised, cold-water habitats.