Analysis of pigmentation characteristics on beak for Gonatopsis borealis in the Northwest Pacific Ocean

Gonatopsis borealis is a cold water cephalopod species which is extensively distributed in the northwest Pacific Ocean between 37°-55°N, and is rich in resources with a good development potentiality. The beak is an important feeding organ of cephalopods, pigmentation is one of the important changes in the growth of the beak, and its pigmentation characteristic is an important material to study the feeding ecology of cephalopods. In this paper, the characteristics of pigmentation stages in the beak were studied, which would provide the basis for the study of fishery ecology of G. borealis. We measured and identified the samples by basic fishery biology data including mantle length (ML), body weight (BW), sex and sexual maturity, divided the pigmentation stages based on Hernández-García's method, researched whether there were gender differences in pigmentation stages, and established the growth models between the beak pigmentation and ML, BW, sexual maturity and the external morphological parameters of 268 beaks of G. borealis collected in the Northwest Pacific Ocean from September to November of 2018 by Chinese jigging fishing fleets. The results showed that the stage 6 and 7 were the dominant pigmentation stages accounting for 91.11% of the total samples. There was no significant sexual difference in the relationships between beak pigmentation stages and ML, BW and morphological parameters including Upper hood length (UHL), Upper crest length (UCL), Upper rostrum length (URL), Lower hood length (LHL), Lower crest length (LCL) and Lower rostrum length (LRL) of beaks based on analysis of covariance (ANCOVA). The correlation analysis indicated that the pigmentation degrees of beak were highly significantly correlated with ML and BW. The beak pigmentation degrees had a positive relationship with ML, BW and morphological parameters of beaks, and the coefficient of association were 0.9800, 0.6715, 0.7615, 0.9165, 0.9188, 0.6772, 0.8071 and 0.9153, and there was also a positive relationship with sexual maturity. The pigmentation degrees of beak of G. borealis had a positive relationship with ML, BW, sexual maturity and morphological parameters of beaks, and increased with the growth of individuals and seemed to reach the maximum from September to November.