Mouth morphological development of Hilsa (Tenualosa ilisha) larvae collected from Meghna Estuary, Bangladesh

Aim: In larval fish, food size is closely related with the length of the fish jaw, as well as mouth gape. So far, no information is available on the larval mouth morphology of Hilsa shad Tenualosa ilisha. Hence, observations on the mouth morphological development during different larval stages of T. ilisha were examined. Methodology: Samples of T ilisha larvae were collected weekly from Meghna estuary, Bangladesh between August 2016 and January 2017. The collected samples were transported to the laboratory and the mouth morphology of larvae and early juvenile stages were microscopicallyexamined. Results: The smallest mouth size was found at 45°, while was largest found was at 90°. There was no evidence of teeth, while the lower jaw was longer than the upper jaw. The four larval stages were observed in this study; viz. yolk-sack stage, pre-flexion stage, flexion stage, post-flexion stage. The mouth was closed at yolk sac stage while in the pre-flexion stage, larval mouth gape ranged between 177 and 367 pm. In the flexion stage, the range was 241 to 497 pm, in post-flexion stage it ranged between 307 and 621 pm and finally in early juvenile stage the range was 393 to788pm. Interpretation: There was a significant relationship with standard length (SL) and upper jaw length (R2 = 0.941, p < 0.05) as well as lowerjaw (R2 = 0.931, p< 0.05). Significant relationship was also found with SL and mouth gap at 90° (R2 = 0.938, p < 0.05). Overall, these findings can be considered a base data for mouth morphological development of T. ilisha.