Members of the cephalopod genus Idiosepius Steenstrup, 1881, are currently mainly identified based on the arrangement of suckers on the tentacular club (i.e., in two or four transverse rows) and the number of suckers on the male hectocotylized ventral arms, as stated by Nesis (1982). However, the discovery of a further species, I. thailandicus, raises questions about the validity of the present systematic characters. Alternative morphological features, such as the mantle length and peg arrangement on the tentacle suckers, were proposed by Chotiyaputta et al. (1991) to be suitable for species classification.
To determine whether these attributes are more reliable diagnostic characters valid for all Idiosepius species, we re-evaluate all the above taxonomic characters as well as others features, such as the number of suckers on the other extremities, the shape of the hectocotyli, radula morphology, and beak size.
Based on the present analysis, the systematic positions of I. macrocheir and I. thailandicus are uncertain, although they both correspond morphologically to I. biserialis.
The current evaluation supports a revision of the systematic key of Nesis (1982). The data presented here indicate that the species should be characterized by the shape of the hectocotylus and its appendages rather their sucker number. All other evaluated morphological attributes—sucker and its peg arrangement, mantle size, beak and number of suckers in the first three arm pairs — do not provide good characters to discriminate the species of Idiosepius.