Estimation of biometric characters, relative growth and sex ratio of cuttlefish, Sepia officinalis Linneus, 1758 (Cephalopoda: Sepioidea) off the coast of Lagos, Southwest, Nigeria
The biometric characters, relative growth and condition factor (K) with sex ratio were investigated of the common cuttlefish (Sepia officinalis) off the Lagos coast (Nigeria) between August 2018 and January 2020. In total, 1,082 cuttlefish were analyzed for biometric parameters, measured to the nearest 0.01 cm using a measuring board. To record the body weight, samples were blotted dry and weighed on an electronic balance up to the nearest gram. The correlation coefficient (R) for various biometric characters against mantle length ranged between 0.253 and 0.823, indicating a high degree of relationship among the characters compared. The highest value of R (0.823) and hence the highest correlation was between the dorsal mantle length and total weight. The strongest positive association was with the length of arms, mantle breadth, head width, cuttlebone and total weights. The linear regression equation for both sexes using a log-transformed length-weight relationship revealed negative allometric growth (b < 3), reflecting a comparatively slower growth rate in total weight than in mantle length throughout the species ontogeny. The highest K value (4.44) was recorded for the largest size group (24.98–27.60 cm), while females had a higher K value than males. Out of the 1,082 specimens, the incidence of males (799) was significantly (p < 0.05) much higher than for females (283), resulting in a sex ratio of 1:0.35. This statistic suggested a strong indication of skewed sex ratio within the cuttlefish population off the Lagos coast.