Abstract
The larval development of reared European hake, Merluccius merluccius (Linnaeus, 1758), during the lecithotrophic phase, from hatching until 5 days post-hatching (dph), and throughout the endo-exotrophic feeding phase (6-10 dph) was studied by histology and histochemistry. Many crucial morphological, cellular and tissular changes were observed during both feeding phases, mostly those related to digestive and visual ontogenetic events, such as differentiation of buccopharyngeal cavity and eye development (at hatching); pigmentation and differentiation of cone-photoreceptors (4 dph); opening of the mouth and anus, appearance of intestinal valves (5-6 dph); presence of buccopharyngo-esophageal (5-6 dph) and intestinal (9 dph) mucous cells; folding of intestinal mucosa (6 dph); development of regional specific digestive musculature (6 dph); typical structure and functionality of the liver (sinusoids, biliary and pancreatic ducts, glycogen, protein and lipid reserves) (4-6 dph); and characteristic acinar distribution pattern of eosinophilic zymogen granules of the exocrine pancreas (6 dph). Between 9 and 10 dph, the hake larvae showed evident signs of lipid absorption within enterocytes of the anterior intestinal region and a remarkable process of pynocitosis and intracellular digestion was detected in the posterior intestine (supranuclear inclusions or acidophilic protein vesicles). In hake larvae at 10 dph, a proliferation of renal tubules, spleen differentiation and gill development, as well as the presence of the first thyroid follicle, were clearly distinguished. At this time, stomach gastric gland differentiation was not detected and endocrine pancreas and gill lamellae were not evidenced. However, and interestingly, swim bladder and eyes (developing rods) were well differentiated in larval development from 9 dph onwards. In summary, in European hake larval development during the endo-exogenous feeding phase and especially at 9 to 10 dph, most systems, organs and tissues were well differentiated. Particularly, digestive and visual systems were developed and physiologically functional some days before the start of the exogenous feeding phase.
Highlights
The European hake, Merluccius merluccius (Linnaeus, 1758), is an important commercial gadid species widely distributed in the northeast Atlantic Ocean and Mediterranean Sea
Owing to progressive decreasing catches since the 1960s (FAO 2005, Lloris et al 2005), and because this species is well adapted to warm climates and has high growing rates and competitive market prices (Casey and Pereiro 1995, Oliver and Massuti 1995, Recassens et al 1998, Arneri and Morales-Nin 2000, Olivar et al 2003, Morales-Nin et al 2005, Palomera et al 2005), the European hake is considered a promising species for diversification in marine aquaculture (Bjelland and Skiftesvik 2006, Iglesias et al 2010, Sánchez et al 2011)
The aim of this study is to provide an overview of the digestive tract morphogenesis and parallel development of eyes and other organs and tissues during the ontogeny and especially through the lecitotrophic (0-5 days post-hatching [dph]) and endoexogenous feeding phase (6-10 dph) of hatchery-reared hake larvae
Summary
The European hake, Merluccius merluccius (Linnaeus, 1758), is an important commercial gadid species widely distributed in the northeast Atlantic Ocean and Mediterranean Sea. In comparison with other important commercial species, knowledge on its reproductive biology in captivity (Groisson 2010), an important prerequisite for successful spawning and larval rearing, is still scarce. This is mainly because M. merluccius is very sensitive to handling and difficult to keep alive after capture. As described for other several cultured fish, high mortality rates during larval hatchery-rearing conditions, during the live prey exogenous feeding period, is a significant obstacle for industrial farming and commercial exploitation of this species (Bjelland and Skiftesvik 2006, Iglesias et al 2010, Sánchez et al 2011)
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