Beak microstructure and growth characteristics of Sthenoteuthis oualaniensis in the Northwest Indian Ocean

Beak is one of the important hard tissues of Cephalopod, which is widely used for age and growth estimation, population structure analusis and resource assessment of Cephalopod. The microstructure and the morphological growth characteristics of beak of Sthenoteuthis oualaniensis were studied with morphometric parameters of 260 pairs of beaks from 1009 samples collected in Northwest Indian Ocean by a Chinese lighted cover net fishing vessel during survey from March to May 2019 in combination with their ages. The result indicated that there were obvious growth increments in the upper beak rostrum sagittal section (RSS), and increments in hood region were more clear. The analysis of covariance indicated that there was no significant difference between sex and age in the morphological growth characteristics parameters and relative size of beaks. The results of Akakchi Information Criterion (AIC) showed that the linear functions were found to be the optimal for the relationships between all the morphological characteristic parameters and age except UHL, and the logarithmic function was the optimal for UHL growth. The relative size decreased with the increase of age, and the variation trend was basically the same. The absolute and relative growth rates of the six morphological characteristic parameters showed a decreasing trend, and the absolute growth rate of ULWL and LWL reached the peak at 181-210 d but the absolute growth rates of UCL、UHL、LLWL and LCL reached to the peak at 211-240 d, while the instantaneous relative growth rates of all the parameters peaked at 181-210 d. This study laid a foundation for further study on the age and growth, population structure and life history of S. oualaniensis based on the microstructure of the beak, and provided a scientific basis for scientific exploitation of this resource.