Larval ontogeny of redbanded seabream Pagrus auriga Valenciennes, 1843 with special reference to the digestive system. A histological and histochemical approach

Abstract

The larval ontogeny of redbanded seabream Pagrus auriga was studied histologically and histochemically from 0 until 30 days after hatching (DAH). According to the source of food and the structural changes in the digestive tract, larval development was divided into four stages: (1) lecitotrophic (0–2 DAH), (2) lecitoexotrophic (3–4 DAH), (3) exotrophic I (5–15 DAH), and (4) exotrophic II (16–30 DAH). During the first three stages, larvae underwent an intense organogenesis, this being particularly intense from stage 2 (mouth and anus opening) to early stage 4 (appearance of gastric glands). Subsequent development during stage 4 was characterized by the proliferation and growth of pre-existing structures. During stage 1, the mouth and the anus were closed and the digestive tract undifferentiated (straight tubular segment), as well as the majority of the organs (observed as undifferentiated cells groups or primordial structures). As resorption of endogenous reserves proceeded (3–4 DAH) larvae acquired initial absorptive and digestive equipments necessary for first feeding (enterocytes brush border, zymogen pancreatic granules, and ducts connecting accessory glands to the gut). During stage 2, the digestive tract started to differentiate and buccopharyngeal cavity, oesophagus, incipient stomach, and anterior, mid and posterior intestine were distinguished. During stages 2 and 3, prey capture became guaranteed (early development of jaw, fins, teeth, and taste buds) and the digestive and absorptive processes continued developing (appearance of the gut mucosa folds and protein supranuclear inclusions and lipid infranuclear vesicles in enterocytes). The endocrine elements (Langerhans islets and thyroid follicles) except corpuscles of Stannius appeared from 3 to 5 DAH (mouth opening and total yolk resorption). During stage 2 and early stage 3 (3–7 DAH), the circulatory and excretory systems became functional, with the compartmentalization of the heart and the development of renal corpuscles, tubules, and collecting ducts. The beginning of stage 4 was marked by the appearance of gastric glands (16 DAH), which subsequently proliferated in association with the increase in size of the accessory glands. Such event ensured the development of gastric digestion, which around 30 DAH became fully guaranteed (transition from larval to juvenile stage). During stage 4, gill filaments and lamellae proliferated, the heart completed its compartmentalization, the pronephric and mesonephric regions in the kidney developed, and endocrine elements proliferated. The structural information presented here constitutes an initial step towards the determination of the functional systemic capabilities of P. auriga larvae, and thus the physiological requirements needed for optimal welfare and growth.