New size data on the enigmatic Spirula spirula (Decabrachia, suborder Spirulina), on a global geographic scale

Abstract

The ram's horn squid Spirula spirula is a unique deep-water marine organism whose life cycle remains enigmatic. Interpretations of its ecology and habitat preferences are currently based solely on dredging, on fishery data, stable isotope data and rare molecular genetic analyses of dead specimens. These methods form the basis to decipher phylogeographic questions of otherwise unobservable deepsea animals such as S. spirula. Here, new morphological data from internal shells (specimens n = 408, analysed n = 260) are presented from 12 different populations over huge distances, from the Atlantic, Indian and the Pacific Oceans. A monospecific status is assumed for Spirula, with its species S. spirula. The dataset shows a highly variable shell morphology including size distribution within distinct populations. Populations from the Indian Ocean are larger than those from the Atlantic and the Pacific. Specimens from the northern Indian Ocean (Maldives, Sri Lanka, Thailand) are larger than specimens from the eastern Indian Ocean (Mauritius, Tanzania) and the south-eastern Indian Ocean (western Australia). Specimens from the eastern Atlantic (Canary Islands) are smaller than those of the western Atlantic (Brazil, Tobago). The Canary Islands yielded by far the smallest specimens, while the largest specimen comes from Thailand. Specimens from the locality at eastern Australia (south-west Pacific) have an intermediate size range. Morphologic and geographic data suggest a geographically induced size differentiation within S. spirula. Preliminary findings on conchs mirror the known (from soft parts) existence of two sexual dimorphs in Spirula. The next step would be to collect more material from other localities. A more detailed morphometric approach based on specimens from which the sexes are known is required to enable a detection of the presence of sexual dimorphism by morphometric analyses on internal shells of Spirula.

Rediscovery of Ram’s horn squid, Spirula spirula (Cephalopoda: Spirulidae), from the Arabian Se

“…The species, Spirula spirula (Linnaeus, 1758), commonly known as Ram's horn squid, is one of the most inscrutable cephalopods with an internal loosely coiled chambered shell (Lukeneder, 2016). This species is considered monospecific under the genus Spirula (Lamarck, 1799) and recent findings from chambered shell structure challenge the monospecific status of the genus but fall short of proving the occurrence of more than one species (Neige and Warnke, 2010;Haring et al, 2012;Lukeneder et al, 2008;Lukeneder, 2016). Spirula inhabits subsurface waters of the tropical and subtropical regions with a disjunctive range of geographic occurrence (Nesis, 1998;Lukeneder, 2016).…”

“…This species is considered monospecific under the genus Spirula (Lamarck, 1799) and recent findings from chambered shell structure challenge the monospecific status of the genus but fall short of proving the occurrence of more than one species (Neige and Warnke, 2010;Haring et al, 2012;Lukeneder et al, 2008;Lukeneder, 2016). Spirula inhabits subsurface waters of the tropical and subtropical regions with a disjunctive range of geographic occurrence (Nesis, 1998;Lukeneder, 2016). The internal, loosely coiled, chambered shell of S. spirula starting from a spherical initial chamber, resembles the external chambered shells of Ammonoidea, the extinct cephalopod subclass that flourished for more than 340 million years from the middle Devonian to the end of the Cretaceous period (Bandel and Bolezky, 1979;Neige and Warnke, 2010).…”

The First In Situ Observation of the Ram’s Horn Squid Spirula spirula Turns “Common Knowledge” Upside Down

“…Although live observations of Spirula are very rare, there have been important advances in our understanding of its shell structure and function [11,12], life history and biogeography [13,14] and phylogeny and systematics [15][16][17]. Still, live observations offer researchers a unique opportunity to directly examine the behavioral responses of organisms about which behavior otherwise has to be merely surmised.…”