Influence of different mono, bi, and trispesific microalgal diets on the gonadal maturation of the carpet shell cLAm Tapes decussatus as a candidate for bivalve aquaculture in Egypt  

The present research studied the effect of different microalgal diets on the gonadic maturation of the carpet shell clam Tapes decussatus. The Chaetoceros calcitrans (Cc), Nannochloropsis oculata (No), and Tetraselmis suecica (Ts) were used to prepare seven diet regimes as follow: three monospecific diets (Cc; No; Ts); three bispecific diets (Cc + No; Cc + Ts; Ts + No at proportion of 1:1 in terms of dry weight of algae); and one trispecific diet (Cc + Ts + No at proportion of 1:1:1 in terms of dry weight).  The Clams fed the test diets were compared to unfed treatment as control. The results showed that all the animals attained the gonadic maturation but with different rates reflecting the different effects of the diets used. The clams fed Cc or Cc + Ts diets were the most developed animals and attained  the partial spawning phase with the largest proportion (62.5 % of sampled animals) for both, followed by animals fed Ts diet with (50%) after 48 days of conditioning. The first two diets led to spontaneous mass spawning after 47 days of conditioning and all of the three diets showed the highest weight gain and condition index values. Nannochloropsis oculata alone or in mixed diets demonstrated lower performance of gonadal maturation during the conditioning period.  The unfed treatment showed the lowest values for all examined parameter, suffered weight loss and did not show any sign of sexual maturation.  The results indicated that Chaetoceros calcitrans alone or mixed with Tetraselmis suecica can be recommended for Tapes decussatus broodstock conditioning in Egyptian hatcheries.