Morphological and population changes of the cockle Cerastoderma glaucum in Lake Qarun, Egypt, may be driven by accelerated environmental deterioration

Abstract

The cockle Cerastoderma glaucum represents a significant element of the benthic community in Lake Qarun, Egypt. The population of this species has severely declined in the lake. For stock position assessment, cockles were collected in the winter of 2008 and 2018. The body size (length-frequency) distributions considerably varied among the collections. A single peak at 14 mm was present in 2008, and two peaks appeared ten years later (one at 13mm and another at 19 mm). The total weight, shell weight and flesh weight of individuals collected in 2008 exhibited isometric increase with shell length (SL). For samples collected in 2018, flesh weight increase was slower than shell length, indicating negative allometric growth. When 50% of the population achieved maturity (SM50), their sizes were 7.7 and 7.4 mm of SL for 2008 and 2018 collections, respectively, consistent with the sexual maturity being related to size rather than age. The overall sex ratios did not significantly differ from unity (1:1). The results of this study suggest that the species is sensitive to salinity changes and water pollution, with larger individuals being more resistant than the smaller ones. This may have implications for species’ long-term reproduction and density.

Keywords

• lake qarun
• cerastoderma glaucum
• population structure
• condition index
• onset of sexual maturity
• sex ratio