Understanding the early ontogenetic stages of Mugil liza (Mugilidae): Morphological traits and digestive/metabolic profile of pre-juveniles after recruitment.

The family Mugilidae consists mainly of diadromous species, whose reproduction occurs in offshore waters. Pre-juveniles shift their diet in the surf zone (zooplanktophagous to iliophagous). Later, during their recruitment into estuaries, huge changes take place in their digestive system. However, digestive and metabolic characteristics and some morphological traits at recruitment are unknown for Mugilidae. We performed comparative studies on early and late pre-juveniles of Mugil liza recruited in Mar Chiquita Coastal Lagoon (37°32'-37°45'S, 57°19'-57°26'W, Argentina). We determined digestive enzyme activities (intestine), energy reserves (liver/muscle), total/standard length, total weight, intestinal coefficient, hepatosomatic index and retroperitoneal fat. Pre-juveniles exhibited amylase, maltase, sucrase, lipase, trypsin and aminopeptidase-N (APN) activities, which were maintained over a wide range of pH and temperature, and exhibited Michaelis-Menten kinetics. In late pre-juveniles, amylase (422 ± 131 µmol maltose min-1 mgprot-1 ), sucrase (86 ± 14 mg glucose min-1 mgprot-1 ), trypsin (84 ± 9 µmoles min-1 mgprot-1 ) and APN (0.58 ± 0.08 µmoles min-1 mgprot-1 ) activities were higher (42%, 28%, 35% and 28%, respectively) than in the early stage. Also, the intestinal coefficient was higher in late (3.04) compared to early (2.06) pre-juveniles. Moreover, the liver appeared to be a main site of glycogen and triglyceride storage in late pre-juveniles, muscle being the site of storage in early pre-juveniles, exhibiting higher glycogen, free glucose and protein concentrations (92%, 82%, 32%, respectively). The results suggest that pre-juveniles of M. liza exhibit an adequate digestive battery to perform complete hydrolysis of various dietary substrates, availability of energy reserves and morphological characteristics to support their feeding habit and growth after recruitment. Our results represent an important contribution to knowledge of the ecology and digestive physiology of pre-juveniles of Mugilidae in the wild.