Argonauts: astronauts of the sea

by Mark Carnall, Life Collections manager

Cephalopods are a remarkable group of molluscs that includes nautilus, octopuses, cuttlefish and various groups of ‘squid’. The other major groups of molluscs includes more familiar shelled animals such as gastropods (snails and slugs), bivalves, and chitons, as well as some less familiar forms.

In natural history museums, molluscs are normally represented by shell collections because the hard shelly parts are easier to preserve and store than the soft tissue. This creates a bias against the soft-bodied cephalopods, such as squids, octopuses and cuttlefish, because aside from the cuttlebones of cuttlefish and the thin gladius in squids there aren’t many hard parts that can be preserved to represent these animals in dry collections. For octopuses it’s normally only the beak and microscopic radulae, a toothed tongue-like structure, that can be preserved. But there is one notable exception: the eggcases of argonauts.

Model of Argonauts argo. Image: Mark Carnall
Model of Argonauta argo. Image: Mark Carnall

Argonauts, four* species of octopuses in the genus Argonauta, are unusual in that they produce a paper-thin eggcase, sometimes referred to as a shell. Unlike a true shell it’s not attached to the body of the argonaut, but secreted by two specialised webbed arms. The eggcases themselves are sometimes called paper nautiluses because they resemble the spiral shells of nautiluses, but they are structurally and functionally very different.

External morphology of a female paper nautilus (Argonauta argo) with egg case. Poli, Giuseppe Saverio. Testacea utriusque Siciliae. (1791-1827).
External morphology of a female paper nautilus (Argonauta argo) with eggcase. Poli, Giuseppe Saverio. Testacea utriusque Siciliae. (1791-1827).

Argonaut eggcases wash on up shorelines around the world and have been known for centuries. But it’s only comparatively recently that the origin and use of these cases has been described. When eggcases containing live argonauts were first encountered it was supposed that argonauts were reusing empty shells created by another animal, much like hermit crabs repurpose gastropod shells.

Pioneering research by marine biologist Jeaneatte Villepreux-Power in the 19th century led to observations of Octopus and Argonauta, confirming that the eggcases are made and repaired by female argonauts. It wasn’t until 2010 that we understood how argonauts use these cases to float in the ocean. It turns out that they ‘bob’ their shells to gulp a pocket of air. Then, using their second pair of arms, they trap the air in the top of the shell and dive releasing enough air to maintain the required buoyancy.

Only female argonauts make the eggcases, so the free-floating males are tiny in comparison. In addition to providing a home for female argonauts, these structures are used to brood embryos in. One eggcase was described with nearly 50,000 embryos attached to the inside of the shell.

Preparation showing series of argonaut egg cases of varying sizes.

Thanks to their oddity and beauty these eggcases are common in museum collections, but they represent one of the marvels of evolution. Unlike many bottom-dwelling octopuses, female argonauts have evolved this amazing structure to function as an underwater craft to allow them to leave the ocean floor and inhabit the open oceans: the true astronauts of the sea.

To celebrate the pioneering work of Jeaneatte Villepreux-Power, these amazing animals, their eggcases, and a selection of museum specimens are on display in the Museum’s Presenting… case until the 3 July 2018.

Mark writes more about these ‘astronauts of the sea’ on the Guardian’s Lost Worlds Revisited blog.

* Tens of species of living argonauts have been described, however four are currently recognised with a few dubious species.

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