ASPECTS OF THE BIOLOGY OF THE GIANT FORM OF STHENOTEUTHIS OUALANIENSIS CEPHALOPODA: OMMASTREPHIDAE) FROM THE ARABIAN SEA

An investigation was carried out on the recently discovered‘giant’ extra large (XL) form of the squid Sthenoteuthisoualaniensis from the Arabian Sea. The sample consisted of 2males, which have not been previously described, and 13 females.Diet composition, parasite loading, sucker ring dentition, biolumi-nescenceand sexual dimorphism were examined and compared to known parametersof the medium (M) form. Reproductive strategy, potential fecundity,egg size distribution in the ovary and oviducts were examinedin mature XL females. Evidence of multiple spawning in the giantform was also investigated. Overall body shape, bioluminescentstructures and coloration of the giant form were similar tothe M form, though the XL form had a smaller fin angle thanthe M form. The mature female XL form has a dorsal mantle lengthabout twice that of a mature female M form. Adult females ofthe XL form have a dorsal mantle length about twice that ofadult males of the same form. Differences between males andfemales were found in arm sucker ring dentition and parasiteload, suggesting a difference in diet. This could be linkedto size differences between the sexes. A strong correlationbetween ovary mass and mantle length was found (r² = 0.64).Poor correlation was found between mantle length and oviductmass (r² = 0.128) and potential fecundity (r² = 0.07). Potentialfecundity ranged between 2–5 million eggs and the holdingcapacity of the oviducts was approximately 300, 000 eggs. Thiscombined with the presence of spermatangia and the presenceof food in the stomach suggest that the XL form is a multiplespawner. S. oualaniensis appears to have a plastic phenotypeand has adapted to the Arabian Sea conditions by evolving thecapacity to grow to a giant size. (Received 6 November 1996; accepted 15 February 1997)