Organogenesis of digestive system, visual system and other structures in Atlantic bluefin tuna (Thunnus thynnus) larvae reared with copepods in mesocosm system

This study examines the anatomical development of Atlantic bluefin tuna during the first month of life at histological level with emphasis in the gut development and those organs required to establish the hunting behaviour characteristic of this species. The tuna larvae have been reared in mesocosm with copepods (Acartia and Trigriopus sp.) as primary live prey during the first two weeks. The larvae hatched with 3.4 mm average TL and exhibited a very fast growth reaching between 4 and 5 cm at the end of the first month. The yolk reserves were consumed rapidly in two days. As an altricial species the gut and most of the sensory organs were not developed at hatching. The transformation of the gut from an undifferentiated canal at hatching up to a complex and segmented juvenile-like digestive tract occurred in three weeks. At hatching, the digestive tract appeared as a straight tube dorsal to the yolk-sac. The opening of both mouth and anus occurred at 2 days post-hatching (dph). From 4 dph onwards, intestinal loops started to be visible and the primordial future stomach started to develop. Intestinal brush border and increased folds in length were detected in the intestine from 4 dph. Acidophilic protein supranuclear inclusions within the hindgut enterocytes and the first signs of lipid absorption within enterocytes of the anterior intestine were evident from 6 dph. First gastric glands were detected at 11 dph, and at 17 dph the stomach was practically developed with the apparition of the first pyloric caeca. Newly hatched Atlantic bluefin tuna larvae had unpigmented eyes. The eyes were pigmented and functional at the opening of the mouth (between 2 and 3 dph), and rods appeared at 17 dph. Nevertheless twin cones were not observed during the first month. The bluefin tuna larvae quickly acquired a degree of development that allows for efficient predation and digestion of more complex feeds. In fact, the digestive system, sensory and visual structures, thyroid gland, swim bladder, and the kidney and heart differentiated very early during the larval ontogeny of this species, and they were well developed and practically fully functional around 17-18 dph under our rearing conditions. At 21 dph with the change in the allometry of mouth size the post-larvae exhibited most of the juvenile anatomical characteristics.