Larval organogenesis of flatfish brill Scophthalmus rhombus L: Histological and histochemical aspects

Abstract

The present tabular overview of organogenesis in larvae of the flatfish brill Scophthalmus rhombus (L.) provides valuable information on its structural status during ontogeny, and it can be useful for establishing the functional systemic capabilities and physiological requirements of larvae for optimal welfare and growth. The organogenesis of the brill larvae was studied during the first month of larval life by means of zootechnical, histological, and histochemical approaches. Based upon its feeding mode, and analysing the main morphohistological characteristics of the organs and systems, larval development in brill was divided into four stages from hatching: Stage 1: 0–1 days after hatching (DAH); Stage 2: 2–9 DAH; Stage 3: 10–22 DAH and Stage 4 from 23 DAH onwards. As in most marine fish species, brill larvae at hatching had an undifferentiated digestive tract composed of monostratified epithelium cells, each containing a basal nucleus and evident apical microvilli. At the beginning of stage 2, both the mouth and the anus opened in conjunction with the differentiation of the digestive tract. During stages 2 and 3, the digestive tract differentiated and buccopharyngeal cavity, oesophagus, incipient stomach, and anterior, mid- and posterior intestine became distinguishable. At this time, prey capture began and the digestive processes continued to develop (e.g. gut mucosa folds, lipid infranuclear vesicles and protein supranuclear inclusions). The first taste buds appeared at the onset of the exotrophic stage, at 8 DAH, while the first pharyngeal and mandibular teeth were apparent at 4 DAH. Histological and histochemical observations suggest that digestive tract development of the brill larvae, involving the presence of functional liver (glycogen lipids), exocrine pancreas (proteins) and gall bladder, enabled early larvae (from 4 DAH) to ingest, digest, and assimilate the first exogenous food, even before endogenous reserves were completely resorbed. The appearance of the digestive mucous cells (from 3 DAH), containing neutral and/or acid mucosubstances, and the functional gastric gland differentiation (23 DAH) resembled such developments in other flatfish species. The most critical ontogenetic events occurred at stage 2. Primordial gills were detected at 2 DAH while gill filament, vascular structures, and lamellae differentiated from 6 DAH. At the beginning of stage 2, four defined cardiac cavities (sinus venosus, atrium, ventricle, and bulbus arteriosus) were discernible. The primordial swim bladder was differentiated from the dorsal wall of the digestive tube at 2 DAH and at this time renal tubules between interstitial kidney tissue were detected. Endocrine elements (Langerhans islets and thyroid follicles) were also evident at stage 2. From stage 3 onwards, most organs essentially exhibited an increase in tissue structure number and size.