Temperature and salinity as key drivers of eggs hatching success in sibling species of the Contracaecum rudolphii (s.l.) complex from European waters

Egg hatching is a critical stage in the life cycle of parasitic nematodes and is strongly influenced by abiotic factors. This study investigates, under in vitro condition, the effects of temperature (5°C, 10°C, 20°C, 30 °C) and salinity (0–70 psu) on egg hatching success in the two sibling species Contracaecum rudolphii sp. A and C. rudolphii sp. B, which have been hypothesized to be adapted to brackish/marine and freshwater environments, respectively. Hatching was completely inhibited at 5°C in both species. At temperature of 10 °C and above, both taxa showed successful hatching with largely overlapping thermal profiles; however, C. rudolphii sp. A achieved a slightly significantly higher success, with maximum hatching observed at 30 °C—a value chosen to simulate a potential heatwave scenario. Temperature also influenced developmental timing, with faster hatching occurring at higher temperatures. In contrast, significant marked differences were observed along the salinity gradient: C. rudolphii sp. A hatched across a wide range (0–70 psu); while C. rudolphii sp. B was restricted to 0–20 psu, with a steep decline above 10 psu. The observed species-specific hatching dynamics, primarily driven by salinity factor, support differential ecological adaptation of the two taxa in their respective aquatic habitats. These findings also provide a basis for predicting parasite responses to environmental change, including rising temperatures and salinity shifts in aquatic ecosystems.