Eco-morphological patterns of sagittal otolith in European anchovy from northern and middle Adriatic Sea.

Teleost fishes are able to perceive their environment through the sense of hearing and particularly through the sagittal otoliths, calcareous structures in the inner ear, that are involved in the capture and transduction of sound. An eco-morphological study was carried out on the sagittal otolith of E. encrasicolus (L., 1758) from northern (Conero-Istria) and middle (Conero-Tremiti) Adriatic Sea in order to identified the relationship among otolith morphology, environmental factors and population structure for fisheries management strategy. 220 anchovies were collected by MEDITS trawl survey in 2015. A representative subsamples was analyzed by scanning electron microscopy (SEM). Crystalline arrangement of external proximal surface of otoliths showed a similar specie-specific profile but different features between sites. In all samples, along the sulcus acusticus linked to auditory system, a similar texture was found: bigger and more defined morphological units were shown in the posterior region (cauda) arranged in a circular pattern, while in the anterior area (ostium) the microstructures were smaller and grouped with preferential direction. The otoliths of two sites showed prismatic crystals of similar dimension, but morphological differences. All the northern sub-samples showed: 1) well defined acicular crystals in the caudal area (length ≅ 100 μm); 2) well defined crystallographic planes, crystal faces and habits (length ≅10-20 μm) in the ostial area. Instead middle samples showed: 1) a less evident morphological structures in the caudal area, with morphological units joined together; 2) roundish crystals with no well-defined habits, embedded in an amorphous substance, in the ostial area. These differences in otolith microstructures between sites, confirmed those suggested by shape analysis (Montanini et al., 2016) and could be related to metabolic and environmental influences. The results suggest that environmental conditions influences the morphology of fish otoliths, the efficiency of otolith morphology for stock identification and support management strategy.