Gonadal cycle and spawning of date mussel Lithophaga lithophaga (L.) (Bivalvia: Mytilidae) in Egyptian water

Abstract This study describes the gonads of Lithophaga lithophaga (Linnaeus, 1758) inhabiting the coast of Alexandria City, Egypt, on the Southeastern Mediterranean Sea. Investigated samples were collected from five sites situated between Sidi Gaber and Montaza. Seven hundred and forty six individuals were collected through one year from December 2017 to November 2018. Two hundred and thirty eight specimens with shell length of 46.83 ± 12.28 and total weight of 7.29 ± 5.74 were selected representing the monthly abundance and were used for histological analysis. The sex of L. lithophaga is dioecious with no sexual dimorphism and distinct by the gonad color. However, differentiation of sex was accurately defined histologically. The investigated animals showed a statistically unbalanced sex ratio. The male to female ratio was 3.5:1. However, the results of the present study showed that the ratio of males decreased as the shell length increased. The developmental stages of gonads were identified as five stages. The spawning activity occurred during July and August, while the spent stage was found mostly in September. Furthermore, the results showed that L. lithophaga has one annual gonadal cycle. There is a clear correlation between the maximum values of condition index and meat yield of these studied date mussels with their spawning activity during July–August, followed by a decrease in these values from October to the following June. This result can be a more validating method to identify the reproductive conditions of L. lithophaga’s population. Finally, each gonad developmental stage was described microscopically in details for both sexes.