A contribution to the trophic ecology of the Adriatic Sturgeon, Acipenser naccarii: Morphological observations on mouth and head sensorial equipment

This study refers to morphological observations carried out on oral structures and head sensorial equipment of juveniles and adult of A. naccarii, taking into account the scarcity of bibliographic data pertaining to such structures in this species. SEM and histological examination were performed on a 9-year-old ( mature male, weight: 9 kg; SL: 108 cm) adult and a 180-day-old juvenile ( weight 10 gr; SL: 14.5 cm), from two different induced reproductions carried out in 1992 and 1997, respectively, at VIP fish farm ( Orzinuovi, Brescia, Italy) and reared at ENEL Thermal Aquaculture experimental plant "La Casella" ( Piacenza, Italy). The morphological and histological results can be interpreted as follows: i) a 180 day-old sturgeon should be considered still a juvenile, as revealed by the presence of some still persisting transitional structures ( e. g. teeth); ii) with the exception of olfaction ( not treated in this study), the most important sense organ in the external rostro-ventral region of the head are the ampullar organs, which are able to perceive electrical stimuli. These organs are similar to those described in Gymnotidae and Mormyridae by Szabo ( 1974) with both ciliated columnar and mono-ciliated pear-shaped sensory cells; iii) the sturgeon scans the surrounding environment in search of signals of the presence of food through the olfactory organ, which is considered to be the system with the greatest effective range ( more than 100 m) in many fish species, including sturgeons. Again, the head lateral line and ampullar organs ensure more precise and closer orientation of the fish towards the source of the signal. The inner and outer taste buds confer on juveniles a differential sensitivity to prey taste. External taste buds play a role in the preliminary determination of the suitability of food, intervening in the decision whether to seize the item or not. The taste receptors in the mouth, which are assumed to be more sensitive and to perceive a wider range of substances than the outer buds, are involved in the final verification of the quality of the prey seized ( swallow or reject it).