Authors: Lindsay, D.J., Hunt, J.C., McNeil, M., Beaman, R.J., Vecchione, M.
Publication: The first in situ observation of the ram’s horn squid Spirula spirula turns “common knowledge” upside down. Diversity 12(12), 449. doi: 10.3390/d12120449
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.
A single individual of S. spirula was observed between 837–860 m depth on 27 October 2020 during dive 402 of the remotely-operated vehicle (ROV) SuBastian (R/V Falkor Voyage FK200930, Leg 2, Schmidt Ocean Institute). The squid was first observed at 01:34:01 UTC (11:34:01 local time; 12.12960371° S, 143.97562933° E, 837 m), at a height of ~1100 m above the seafloor with a depth of 1941 m. The observed location lies generally within the entrance to Wreck Bay on the continental slope of the far northern Great Barrier Reef, Queensland Australia. Environmental parameters were recorded using a Sea-Bird SBE 49 FastCAT conductivity-temperature-depth meter (CTD) attached to the ROV and were as follows: Temperature 5.33 °C, salinity 34.44 and dissolved oxygen 4.1 ml/L (184.8 µM).
The squid appears on video for a total of 4 min 57 s and, when first encountered (i.e., as far away as can be seen in the video recording) and throughout most of the subsequent observations, it was oriented nearly vertically with its head upward.