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

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 deep-sea 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.