Sexual dimorphism in the beaks of Moroteuthis ingens Smith, 1881 (Cephalopoda: Oegopsida: Onychoteuthidae)

Abstract

Sexual dimorphism in size and morphology of the lower beak of the warty onychoteuthid squid Moroteuthis ingens is analysed. Beaks of maturing males exhibit a band of weak, clear cartilage across the shoulder region, while mature males exhibit a pronounced excavation of this area; the hood remains intact. Female beaks attain greater size, but relatively shorter lower rostral lengths (LRL) than those of males; they display neither the shoulder cartilage nor later erosion, but the hood is consistently eroded in mature specimens. The angle ridge in females is considerably longer than in males. Due to the difference in LRL relative to overall beak size, M. ingens beaks from predator stomachs should be sexed prior to calculating prey size using LRL; for both sexes, the LRL‐mantle length (ML) relationship is linear while the LRL‐weight relationship is exponential. Sex‐specific equations are provided for reconstructing ML and weight using LRL. Based on several incidences of male‐female pairs collected with beaks interlocked, M. ingens is postulated to mate in a head‐to‐head position, with both individuals incurring beak damage during the event. © The Royal Society of New Zealand 2006.