Sexual Dimorphism and Age-Related Polymorphism in the Endemic Karst Fairy Shrimp Chirocephalus croaticus (Crustacea, Anostraca)

The karst fairy shrimp Chirocephalus croaticus (Steuer 1899) is a stenoendemic species restricted to temporary aquatic habitats in Slovenia. Focusing on the population from seasonal Petelinje Lake, this study investigates sexual dimorphism, age-related polymorphism, and reproductive traits, and provides insights into the evolutionary pressures shaping C. croaticus morphology, reproductive strategies, and adaptation to unstable environments. Sixteen morphological traits were analyzed across juveniles, adult males, non-ovigerous females, and ovigerous females. Results showed significant male-biased sexual dimorphism (average sexual size dimorphism (SSD) = -0.15), especially in antenna length, with males possessing longer antennae and slightly larger body size. Morphological differences between ovigerous and non-ovigerous females were subtle, despite greater variation in ovisac volume among non-ovigerous females (33.5% vs. 84.0%). Age-related polymorphism was evident, with juveniles (mean body length 7.66 ± 0.50 mm) distinctly separated from adults, while adult males (12.80 ± 0.32 mm) and females (non-ovigerous: 11.91 ± 0.32 mm; ovigerous: 13.12 ± 0.25 mm) overlapped in size and morphology. Body size correlated positively with ovisac volume, but not with egg number, suggesting larger females invest more in egg size than their quantity. Compared to other fairy shrimps, C. croaticus females produce relatively few eggs – ovigerous female carried 24.5 ± 1.31 eggs in the ovisac. Juveniles exhibited greater age size dimorphism from adults (average age size dimorphism (ASD): -0.50 males, -0.40 females) than adults differed between sexes (SSD = -0.15), indicating stabilizing selection in mature individuals. These findings emphasize pronounced male-biased sexual dimorphism, high variability in female reproductive traits, and challenge age group distinctions based solely on size. The study contributes to understanding reproductive adaptations of C. croaticus in temporary karst habitats subject to environmental fluctuations.