Intra-specific and inter-specific variability of the sulcus acusticus of sagittal otoliths in two gurnard species (Scorpaeniformes, Triglidae)

Abstract

Morphology, morphometries and compounds of sulcus acusticus of sagittal otoliths of two gurnard species (Aspitrigla cuculus and Eutrigla gurnardus) from north-central Adriatic Sea (north-eastern Mediterranean) were investigated. The study objectives were to find morphometric variables and shape variability of sulcus acusticus of these two closely related sound producers species linked to developmental changes from juvenile to adult stage and species-specific pattern which influenced ecomorphological adaptations of the auditory system. The method used was a combination of multivariate techniques, X-ray diffractions pattern and ultrastructural investigation by SEM. Multivariate analysis (PCA, LDA and PERMANOVA) performed on standardized morphometrics of a total of 202 individuals, significantly separated four groups: A. cuculus juveniles, A. cuculus adults, E. gurnardus juveniles and E. gurnardus adults due to different life history characteristics. At intra-specific levels higher mean values of sulcus area/otoltih area (S:O) ratio of adults than the juveniles were observed within A. cuculus and not for E. gurnardus. At inter-specific levels significant differences were seen between adults and not between juvenile groups. Considering all the groups, the main variability on sulcus area were observed for caudal part of the sensorial macula. Although no differences in compounds were shown by X-ray diffraction at intra-specific level, the crystals on sulcus acusticus were different in shape, size and arrangement between juveniles and adults of each gurnard. The crystals of juveniles were quite enmeshed into otolith matrix and showed a rod/smooth-shape. The crystalline arrangement of adults became more complex with larger, rod/rectangular-shape and well-formed faces. Ontogenetic and specie-specific differences might be related to specie-specific physiological factor (i.e sexual maturity), depth distribution and feeding patterns that change during gurnards growth. These changes, relating to both endogenous and exogenous factors, were marked by the sulcus acusticus which provided a useful tool for fish ontogenetic, physiological and ecological studies.