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The objective of the present study was to describe histological development of the European long-snouted seahorse Hippocampus guttulatus, to increase understanding of the biology and physiology of the species. Most vital organs were present in juveniles by the time of their release from the male's pouch. Digestive tract specialization occurred at 89 effective day-degrees (D° ), corresponding to 15 days post partum (dpp), with development of the first intestinal loop and mucosal folding. At 118 D° (20 dpp), lipids were being mobilized from the liver and oocytes attained the perinuclear stage. The fovea emerged at 177 D° (30 dpp), contemporaneous with the shift from pelagic to benthic behaviour in juveniles. At this stage, the most interesting feature was the formation of the second intestinal loop. Male gonads were never observed during the study (from 0 to 354 D° ; 0-60 dpp), but the first oogonia were present at 30 D° (5 dpp). In 354 D° (60 dpp) juveniles, oocytes were observed in a cortical alveoli stage, indicating maturity. Low digestive efficiency was observed at early stages, which was due to a poorly developed gastrointestinal tract and an immature digestive tract prior to 89 D° . The present study demonstrates that approximately 89 and 177 D° represent two important transitional stages in the early development of H. guttulatus. At a temperature of approximately 19 ± 1°C and an age of 1 month (177 D° ), main organs were fully functional, suggesting that the adult phenotype was largely established by that age, with females becoming mature at the age of 2 months (354 D° ).

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