Argonauta argo Linnaeus, 1758

Finn, Julian K., 2018, Recognising variability in the shells of argonauts (Cephalopoda: Argonautidae): the key to resolving the taxonomy of the family, Memoirs of Museum Victoria 77, pp. 63-104 : 95

Argonauta argo Linnaeus, 1758

An image referenced in the original description of A. argo Linnaeus, 1758 , is considered a paralectotype, with designation of a lectotype by Moolenbeek (2008); Rumphius, (1705) plate 18 figure A (fig. 28b). Shells of A. argo can be recognised by an extremely narrow keel of consistent width. The keel tubercles are paired and the lateral ribs are continuous (i.e. they are not broken into separate tubercles).

Shells of A. argo are extremely consistent in dimensions and sculpturing. The area that has caused the most confusion for naturalists defining the species has been the aperture edge. A. argo can display huge variation in the shape of the aperture edge near the axis. Note the variation in the aperture edge of the two shells presented in fig. 32. Unlike ear formation, this variation occurs on the edge of the lateral wall parallel with the longitudinal axis of the shell; it is not a lateral extension. The expression of the lateral ribs can vary slightly from fine to coarse, suggesting the presence of two varieties (fig. 32). Transition between fine and coarse shell formation on a single shell is extremely rare. An illustrated shell from Monterey, California (81.9 mm ShL, USNM 61374) displays a shift from finer to coarser shell formation at the point of earlier damage (fig. 33). Small A. argo shells can also display laterally protruding ears. A shell from Venezuela (51.4 mm ShL, USNM 122208) highlights the plasticity of this character, displaying an ear on only the right side (fig. 34). The varied size and shape of the keel tubercles on the opposing sides of this shell demonstrate the range of variability of these structures in this species.