Lindsay, Dhugal J., Hunt, James C., McNeil, Mardi, Beaman, Robin J., and Vecchione, Michael (2020) The first in situ observation of the ram's horn squid Spirula spirula turns "common knowledge" upside down. Diversity, 12 (12). 449.

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Abstract

[Excerpt] The ram's horn squid Spirula spirula (Linnaeus, 1758) is the only extant cephalopod with an internal calcareous, chambered shell that is coiled, making it the sole living representative of the once speciose order Spirulida. As also supposed for its Cenozoic and Cretaceous ancestors, the function of the septate, many-chambered shell of Spirula has been considered as primarily for buoyancy. Behavioral observations of this species have been confined to those made in aquaria involving freshly net-caught specimens. Invariably, during those aquaria observations, the posterior end containing the open planispiral shell pointed towards the top of the tank, while the upward-oriented terminal fins moved with a rapid "waving or fluttering motion", presumably attempting to keep the animal submerged. A large photophore is present between the two fins on the posterior end of the body, and this has been observed to emit a "pale, yellowish-green light" that can glow "uninterruptedly for hours". We report here the first in situ observations of S. spirula in its natural habitat, illustrating the importance of such observations for a correct understanding of the ecology of deep-water organisms.

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