Ontogeny of the digestive system and eye of reared flathead grey mullet, Mugil cephalus (Linnaeus, 1758), and evaluation of lipid deposition in the liver according to the feeding protocol

Abstract

Ontogenic development of the digestive system and eye was studied in flathead grey mullet (Mugil cephalus) from hatching to the juvenile stage (69 days after hatching, dah), in rearing conditions. Moreover, the liver lipid content (area covered with lipid vacuoles, ACLV%) was evaluated according to the feeding protocol in association with the stomach content, using histological methods.Larval growth rate was slow until tail flexion was achieved (~14–15 dah), subsequently the growth was very rapid until reaching the mean total length ± SD of 22.27 ± 2.79 mm at the end of the rearing. The main structures of the digestive system were completely developed by 17 dah (395 degree days). Photopic vision began by 3 dah with the appearance of cones, while the ability for scotopic vision was acquired at 17 dah, when the larvae started gained the ability to see and catch prey in reduced light conditions.When the feeding protocol included both rotifers and Artemia nauplii, the mean ACLV remained high (15.66 ± 3.90%), as did the occurrence of lipid vacuoles in the intestine. ACLV in the liver started to decrease significantly during the weaning phase (until 40 dah, when it was 3.80 ± 2.51%). This was probably linked to the decreased total energy received by the larvae, because of the gradual reduction of the delivered Artemia coupled with the low acceptance of the artificial food. However, larvae accepted early artificial feed which was found in the stomach content a few days after the first administration. Consumption of growing amounts of artificial feed resulted in a growing ACLV, which rose again up to 20.38 ± 6.05% at 62 dah. The transition to a different artificial food (with half the lipid dose compared to the first one) led to a non-significant drop in ACLV in the liver, decreasing as much as 13.66 ± 7.19% on the last day of the rearing period.These results provide important information for the biology of this promising species for aquaculture, indicating that flathead grey mullet larvae rapidly develop the basic structures of the digestive and visual systems required to successfully overcome the most critical stage of commercial rearing. The study also confirms that histological evaluation of liver lipid content using the ACLV is a valuable tool in commercial aquaculture in order to identify the weaknesses of a feeding protocol and improve larval rearing protocols, and therefore production efficiency.