Population structure, condition index and age structure of the Mediterranean carpet shell clams Donax semistriatus and Donax trunculus (Mollusca: Bivalvia) in Idku, Egypt

This study investigates the population structure and shell morphometric relationships for the most common bivalve species in Egypt; Donax semistriatus and Donax trunculus. The clams were monthly collected from November 2008 to May 2010 from two sites on the Mediterranean coast, Idku city, Behira governorate, North Egypt. The most dominate size of the calm D. semistriatus is the individuals with class intervals of 12.1-16 mm. For D. trunculus, animals with size class intervals of 18.1-22 mm and 14.1-18 mm dominated site 1 and site 2 respectively. The population of D. semistriatus showed two periods of recruitments each year whilest that of D. trunculus recorded one period only. The results also showed apparent general tendency toward negative allometry in most relationship between shell length and each of shell measurements and body weigh parameters. Positive allometric growth exhibited in gonad weight / shell length and dry weight/shell length relationships. The growth rate at site 2 slightly higher than that at site 1 for the two clams. Gonad indices of both species were more or less coincided with the reproductive activity. In addition, digestive gland and condition indices were also discussed in the present study. Age determination using Bhattacharya s method revealed that D. semistriatus and D. trunculus recorded three age groups at both sites. Several ecological explanations were formulated for the two bivalve species.