WINDY WEIRDOS – NEW INSIGHTS TO THE SPIRULA SHELL

The deep-sea cephalopod Spirula possesses a coiled internal shell which is unique for recent members of this taxon. The shell serves as a buoyancy device during life. During death, the buoyant shell detaches from the animal’s remains and begins floating atop the ocean’s surface, where it can persist for longer periods before being transported to shore or the seafloor. While being transported on the surface, the shell serves as a floating hard substrate allowing for attachments of epibionts. Although these thin-walled shells occur in high-energy environments, they are often found well-preserved on the beaches. Morphologically, structurally, and taphonomically, Spirula shells are unique but poorly understood and widely underrepresented in the literature.

A total of 336 Spirula shells were collected from the east-coast of Fuerteventura, Canary Islands, Spain to analyze their morphology, floating behavior, and infestation by epibionts. 3D models were obtained by x-ray micro-computed tomography scans to assess the shell’s floating capability.

Data show that more than 80% of the shells are infested by the goose barnacle Lepas. In most cases, multiple Lepas were found on a given Spirula shell. In some cases, the shell of Spirula is infested by up to 17 Lepas, which can grow over 2 cm in length. Even though the accumulation of Lepas on Spirula shells increases the weight, the shells are capable maintaining their floating ability. Micro-CT analyses indicate that Spirula possesses a light-weight shell which, together with the large gas-filled volume, enhance buoyance ability. In addition, the morphological layout of the siphuncle function as an air-lock mechanism after the death of an individual which further enabling the shell to be a reliable floating device.