Abstract
Species of the genus Illex (Family Ommastrephidae) are distinguished morphologically mainly based on differences in the hectocotylus, the modified arm that males of many cephalopod species use to transfer spermatophores to females during mating We examined the developmental details of the hectocotylus in Illex coindetii , the species of Illex with the most highly modified hectocotylized arm, to determine whether it has an ontogeny that passes through stages that are similar to the less modified hectocotyli of sympatric I. illecebrosus and I. oxygonius The development of modifications of one of the ventral arms is essentially synchronous on all portions. The hectocotylus of I. coindetii is therefore morphologically distinctive even at early maturity stages, and therefore should not be easily confused with that of its sympatric congeners.
Highlights
The hectocotylus is a modified arm on males of many cephalopod species
Hectocotylization occurs in all ommastrephid species, and the characteristics of the hectocotylus are important for identifying species as well as evaluating phylogenetic relationships among the members of the Ommastrephidae (Roeleveld 1988)
Animals at all maturity stages, the smallest of which were around 75 mm mantle length (ML), were compared to determine how the hectocotylus develops as the animals mature
Summary
The hectocotylus is a modified arm on males of many cephalopod species. Hectocotylization occurs in all ommastrephid species, and the characteristics of the hectocotylus are important for identifying species as well as evaluating phylogenetic relationships among the members of the Ommastrephidae (Roeleveld 1988). Species within the commercially important genus Illex are differentiated based largely on hectocotylus morphology (Roper et al 1998, Martinez et al 2002). This can cause difficulties in identifying specimens that are not mature males and has contributed to controversy about how many species of Illex exist. Several authors (e.g. Roper et al 1969, Roeleveld, 1988) consider the genus Illex to comprise four spe-
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