Morphometric and reproductive divergence of deep-water shrimps Plesionika martia (A. Milne-Edwards, 1883) and P. heterocarpus (A. Costa, 1871) along a depth gradient in the eastern Aegean Sea

This study provides a comparative analysis of two deep-water pandalid shrimp species, Plesionika martia and P. heterocarpus, based on monthly samples collected over one year in the Eastern Aegean Sea. Specimens were obtained from species-specific depth zones (550 m for P. martia and 350 m for P. heterocarpus) to examine morphometric traits, sexual dimorphism, and reproductive patterns. A total of 4419 individuals were analyzed. Females were significantly larger and more abundant than males (K–S test, p < 0.05), indicating pronounced sexual dimorphism. Length–weight relationships revealed negative allometric growth in both species, confirmed by t-tests (p < 0.05). Reproductive activity differed markedly: P. martia showed a short and distinct spawning period peaking in late autumn, whereas P. heterocarpus displayed a prolonged, bimodal cycle with peaks in spring and summer. The gonadosomatic index (GSI) and the proportion of ovigerous females varied significantly by season and species. A strong positive correlation between carapace length and gonad weight was found, with a more linear relationship in P. martia and greater variability in P. heterocarpus. Size at first maturity (L₅₀) was estimated at 22.3 mm CL for P. heterocarpus but could not be reliably determined for P. martia due to limited small size data, likely reflecting ontogenetic depth segregation. The consistent bathymetric separation between these species—also reported in the Western Mediterranean—suggests ecological niche differentiation. Species-specific management measures, including depth-based fishing restrictions and temporal closures during reproductive peaks, are recommended to support the sustainable exploitation of deep sea shrimp stocks in the region.