[Morphological variations and discrimination of medium form of the purple flying squid Sthenoteuthis oualaniensis in the central and southern South China Sea].

From 3177 specimens of purple flying squid Sthenoteuthis oualaniensis collected in 2012 and 2013 in the central and southern South China Sea, the morphological indicators including mantle length (ML), arm length-I (AL1), arm length-2 (AL2), arm length-3 (AL3), arm length-4 (AL4), tentacle length (TL), fin length (FL) and fin width (FW) were measured. The morphological variations among the squid species were analyzed using principal component analysis. Results showed that the medium form could be distinguished into cohort I and cohort II which accounted for 69% and 31% of the total, respectively. The percentage of females of the two cohorts decreased first with increasing ML, then increased and reached the ratio of 100% finally. Using Logistic regressing, the MLs at first maturity for females and males of cohort I were estimated to be 164 mm and 112 mm, respectively, and those for females and males of cohort II were 158 mm and 109 mm, respectively. The cohort I concentrated in the waters of Xisha Islands and Nansha Islands, whereas the cohort II was located in the waters of Nansha Islands. Significant differences in all the 8 morphological indicators between either males or females of cohort I and cohort II were found (P＜0.05). Significant differences between cohort I and cohort II in ML, AL1, AL2, AL4, TL and FW were identified in females (P＜0.05), while significant differences between cohort I and cohort II in ML, AL1, AL2, AL3, AL4 and FL were found in males (P＜0.01). The analysis of coefficients of difference indicated that the morphological variations between cohort I and cohort II suggested a clear division within the species.