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Sthenoteuthis oualaniensis, commonly known as tobiika, purpleback flying squid or purple squid (among other names), is thought to be the most abundant large squid in the tropical and subtropical waters of the Indo-Pacific region (Young and Hirota, 1998; Dunning, 1998). Its congener, S. pteropus, replaces S. oualaniensis in the Atlantic Ocean.
S. oualaniensis has a complex population structure that consists of three major and two minor forms (Nesis, 1993): A giant form occurs only in the northern Indian Ocean in the region of the Red Sea, Gulf of Aden and Arabian Sea and reaches a maximum size of 650 mm ML; a medium form occurs throughout range of the species and is 120-150 mm ML in mature males and 190-250 mm ML in mature females (this is the "typical" S. oualaniensis); a dwarf form occurs in equatorial waters and lacks the dorsal mantle photophore patch characteristic of the species and is 90-100 mm ML in mature males and 90-120 mm ML in mature females (150 mm ML maximum). The medium form appears to consist of two minor forms based on features of the gladius (double or single lateral axes of the rhachis). One of the two medium forms occurs only in the Red Sea, the Gulf of Aden and the Arabian Sea north of 15°-17°N. Complicating this picture is a small form, similar to the medium form but maturing at a smaller size that is nearly the same size as the dwarf form and is found in the Western Indian Ocean and the eastern tropical Pacific. The dwarf equatorial form is found roughly within 10° latitude of the equator where it co-occurs with the typical S. oualaniensis.
The dwarf form has several morphological characters that separate it from the typical S. oualaniensis: Absence of the dorsal photophore patch, slightly different hectocotylus and slight differences in the spermatophore structure and in the gladius structure (Nesis, 1993). Paralarvae of the dwarf and middle forms cannot be distinguished (Nesis, 1993). Researchers have disagreed on whether or not the dwarf form is a distinct species (Clarke, 1965; Wormuth, 1976; Nesis, 1993). The most recent research on the status of the dwarf form was done by M. Roeleveld-Companyo and she considered it to be a separate species that can only be identified as an adult (personal communication). Snyder (1998) suggests that the giant form results from a plastic phenotype in the species.