Recognising variability in the shells of argonauts (Cephalopoda: Argonautidae): the key to resolving the taxonomy of the family

Abstract

Finn, J.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: 63-104. Argonauts (Cephalopoda: Argonautidae) are a family of pelagic octopuses that are most commonly recognised by the beautiful white shells of females (known as paper nautiluses), prized by beachcombers the world over. Taxonomic delineation of the group has historically relied exclusively on features of the shells of females and has resulted in more than 50 species names being coined worldwide. This approach has created considerable confusion in the taxonomy of the family because argonaut shells are not true molluscan shells and display considerable variation in form. This study closely examined a large number of argonaut shells from museum collections throughout the world. Two types of shell formation that had been previously attributed to separate argonaut species were recognised within individual shells. It is proposed here that the different shell forms reflect the effects of ecological or biological factors or events, often manifesting as dramatic changes in shell growth and shape within the development of an individual shell. The resulting combinations of shell formation types clearly explain both the extreme variation observed across large numbers of argonaut shells and the high number of nominal species names coined in the past. Researchers coining new fossil argonaut species based solely on shell characters are advised to proceed with caution. This study supports parallel morphological and molecular research recognising the existence of only four extant argonaut species worldwide: Argonauta argo, A. hians, A. nodosus and A. nouryi.

A Nature’s Curiosity: The Argonaut “Shell” and Its Organic Content

“…Argonauts belong to a family (Argonautidae) of pelagic octopuses that inhabit tropical and temperate oceans [14,15]. This family comprises only one living genus, Argonauta and four described species, world widely known for their beautiful thin white shell, also named the paper nautilus shell.…”

How Rare Are Argonautoidea Octopuses in the Mediterranean? New Data from Stranding Events, Stomach Contents and Genetics

“…The collection of numerous A. argo oothecae during the study period allowed us to examine a wide dimensional range of these structures (ShL ranged from 11.6 to 157 mm), although the maximum “shell” length for the species is estimated to be at least 300 mm [ 13 ]. Due to the large number of specimens, it was possible to examine the size relationships between different morphometric parameters of the A. argo ootheca for the first time, contributing to the current knowledge on this issue, which until now has been limited to Finn’s studies on A. nodosus , A. hians and A. nouryi [ 25 , 35 ]. Our data showed a good relationship between the morphometric parameters ApL-ShL, ShB-ShL and ApW-ShL, respectively, but the values of the coefficient of determination in the EW-ShL were lower.…”

“…A. argo ootheca is not a true molluscan shell. According to Power [ 34 ] and Finn [ 35 ], unlike the shells of other molluscs (e.g., gastropods), argonaut oothecae are not produced by the derivatives of the shell field, but it is a secondary calcium carbonate structure secreted from webs on the distal ends of the female argonaut’s first (dorsal) arm pair. In this paper, we use the term “ootheca” to indicate the brood case and the term “oothecae” for the plural.…”

“…In this paper, we use the term “ootheca” to indicate the brood case and the term “oothecae” for the plural. However, with the aim of providing comparable results, only for the morphometric features of the ootheca, we followed the terminology of Finn [ 13 , 25 , 35 ], which uses the word “shell” to indicate this structure in measurements.…”

“…A. argo oothecae (including both those belonging to stranded individuals and stranded empty oothecae) were measured to the nearest 0.1 mm by a caliper, recording the “shell” length (ShL), aperture length (ApL), “shell” breadth (ShB), aperture width (ApW) and ear width (EW) [ 25 , 35 ]. The relationship between the ShL and the other dimensions was estimated according to linear regression.…”