Population structure, temporal allometry status of Neotrapezium sublaevigatum (Lamarck, 1819) (Family: Trapezidae) in the Vellar estuary, India

Abstract

Seasonal variation in growth and condition of Neotrapezium sublaevigatum was investigated in the Vellar estuary from August 2024 to July 2025. A total of 926 individuals were examined to evaluate shell morphometry, length–weight relationships, size–frequency distribution, and condition indices across pre-monsoon, monsoon, and post-monsoon seasons. Shell length ranged from 4.70 to 41.10 mm, with the highest mean length and weight recorded during the pre-monsoon. Length–weight analysis revealed consistent negative allometric growth (b = 2.5299–2.7875; p < 0.05) in all seasons, indicating that shell length increased proportionally faster than biomass. Stronger morphometric correlations were observed during the monsoon (r² up to 0.9164), suggesting enhanced growth consistency under nutrient-enriched conditions. Size–frequency distribution showed dominance of the 16.0–20.9 mm size class, with peak abundance during the monsoon, reflecting active recruitment and favorable growth conditions. Condition indices exhibited clear seasonal trends, with higher relative condition (Kn) during the monsoon and elevated Fulton’s condition factor (K) during the post-monsoon, indicating dynamic energy allocation in response to environmental variability. The bivalve, N. sublaevigatum, demonstrates adaptive growth strategies characteristic of tropical estuarine bivalves, with population structure and physiological status closely linked to monsoon-driven hydrological cycles. These findings provide baseline ecological information for future monitoring and management of estuarine bivalve populations.