Moult cycle and setal development of the Atlantic ditch shrimp <i>Palaemon varians</i> Leach, 1814 (Decapoda: Caridea: Palaemonidae)

Abstract The Atlantic ditch shrimp Palaemon varians Leach, 1814 is a common estuarine and brackish water species on Northern Atlantic coasts. Palaemon varians is an appealing model organism for studying arthropod developmental processes, such as moulting (ecdysis). Detailed morphological information on its moult cycle is still lacking, hence we have characterised the changes in the setal features corresponding to the moult stages of P. varians grown under laboratory conditions. The stages of the moult cycle were differentiated and described using microscopic analysis of the setae in the uropods of P. varians based on Drach’s classification system. Moult stages were defined as early and late post-moult (A and B), inter-moult (C), early-, mid- and late pre-moult (D0, D1, and D2), as well as ecdysis stage (E), the actual shedding of the exuvia. Average moult cycle duration was 8.7 days, where pre-moult accounted for the longest duration of 4.4 days on average. This study provides a morphological reference for determining the moult stage of P. varians without the use of invasive techniques, and thus it is well suited for repetitive observations of an individual to track the entire moulting process.