Argonauta nodosus

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 : 91

Argonauta nodosus [Lightfoot], 1786; the A. nodosus / tuberculatus complex

The original description of A. nodosus [Lightfoot], 1786, refers to a single image in Rumphius (1705): plate 18, figure 1 (fig. 28a), designated as a lectotype by Moolenbeek (2008) in the absence of type material. Shells of A. nodosus can be recognised by the presence of lateral ribs composed of separate tubercles.

Two types of A. nodosus shells exist in collections: a finer shell with more ribs and small rib tuberculations (fig. 29a), and a coarser shell with fewer ribs and larger rib tuberculations (fig. 29b).

This variation has previously been used as justification for splitting A. nodosus into two species. Kirk (1885), in recognising the two forms, generated a new species name for the fine tuberculated and earless form ( A. gracilis ) to separate it from the coarse tuberculated and eared form (known to Kirk, 1885, as A. tuberculata Shaw ). Robson (1932) recognised the two shell types as varieties, not separate species, stating: “Though the shell of this species is clearly distinguished from its fellows by the rough tuberculations, there are evidently two well marked varieties – one with very large carinal knobs and coarse sculpture, the other with low knobs and fine sculpture” (p. 200). Dell (1952) called this the “ nodosa - tuberculata complex” 2 and described it as follows: “Group 1. The shell is eared laterally and the tuberculations on the ribs are comparatively large – this is what has been called nodosa . Group 2. The edge of the lip comes off the previous whorl in an even curve without trace of an ‘ear’. The tuberculations are much finer and more numerous than in Group 1 – tuberculata ” (p. 54). Dell (1952) considered both forms to belong to a single species.

While both shell varieties are common, individual shells displaying an obvious shift between fine and coarse shell formation are extremely rare. A single shell from Moreton Bay, Queensland (109.1 mm ShL, QM Mo14232) displays a transition from fine shell formation to coarse shell formation at a point of previous damage (fig. 30). While the later component of the shell possesses ears, it is not clear whether the earlier component was eared or earless. No obvious changes were noted in shell thickness, curvature of the keel or relative heights of sequential keel tubercles.

Examination of a large number of A. nodosus shells found no examples displaying a marked change in keel tubercle height or ears that had been formed or subsumed. While eared and earless forms exist, transition between the two types appeared more gradual than the sudden transformation documented in smaller species. A shell in the British Museum (109.0 mm ShL [P], BMNH unreg., locality unknown, “B395, e.”) displays an ear on only one side, clearly demonstrating the plasticity of this character in this species (fig. 31) .

2 Following Finn (2013) it is necessary to correct the original spelling of A. nodosa to A. nodosus . In accordance with the International Code of Zoological Nomenclature, Article 34.2 “the ending of a Latin or latinized adjectival or participial species-group name must agree in gender with the generic name with which it is at any time combined [Art. 31.2]; if the gender ending is incorrect it must be changed accordingly (the author and date of the name remain unchanged)” (I.C.Z.N., 1999). As Argonauta is masculine “from the final noun nauta (a sailor)” (I.C.Z.N., 1999, p. 34) the species-group name must be changed from the feminine nodosa (- a feminine) to the masculine nodosus (- us masculine).