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 ofmouth size the post-larvae exhibitedmost
of the juvenile anatomical characteristics.