Population structure and reproductive biology of the marbled crab, Pachygrapsus marmoratus (Fabricius, 1787) in the south-eastern Mediterranean Sea, Alexandria, Egypt

Pachygrapsus marmoratus is one of the most prevalent intertidal crab species in the south-eastern Mediterranean Sea. It is an indicator of a stable and biodiverse ecosystem due to its contribution to ecological balance through its diverse feeding behaviours, bioturbation, and regulation of prey populations. However, its population characteristics have not been studied to date in the Egyptian Mediterranean waters. The present work aimed to assess some biological characteristics of the P. marmoratus population from Camp Caesar shore, Alexandria, Egypt. Crabs were collected monthly from July 2023 till June 2024. The sex ratio was not significantly different from 1:1. Pachygrapsus marmoratus exhibited an 8-month reproductive season from February to September, with the highest spawning peak in April. The average carapace width of females was significantly larger compared to males. The size-frequency distribution showed a unimodal pattern for both sexes, with the maximum peak in the 20.1-25.0 mm size class. Significant differences have been revealed between the two sexes with respect to the carapace width-body wet weight, carapace width-carapace length, and carapace width-claw length relationships. The number, weight, and average size of eggs were positively correlated with the ovigerous females' carapace width and wet weight. The obtained results shed some light on the population structure of P. marmoratus, revealing its stability in the Egyptian Mediterranean Sea and offering baseline information for making comparisons with other studies, thereby enhancing the understanding of how variations in the biological and ecological characteristics of this species influence its extensive distribution along the Mediterranean and Atlantic regions.