Limacina helicina

Limacina helicina is a species of small swimming planktonic sea snail in the family Limacinidae, which belong to the group commonly known as sea butterflies (Thecosomata). Limacina helicina is a keystone species of mesozooplankton in Arctic pelagic ecosystems. The first written record of this species was by Friderich Martens from Spitsbergen in 1675. Limacina helicina was also observed during a 1773 expedition to the Arctic led by Constantine John Phipps on the ships HMS Racehorse and on HMS Carcass and the species was described one year later, in 1774. Limacina helicina is the type species of the genus Limacina. In contrast to the traditional view, it was shown in 2010 that the distribution of this species is not bipolar; Arctic and Antarctic individuals belong to two genetically distinct species: Limacina helicina in the Arctic, and Limacina antarctica in the Antarctic. Limacina helicina has been recognised as a species complex comprising two sub-species and at least five forms. In addition, the taxonomic category “forma” has been applied to designate at least three morphotypes of Limacina helicina helicina (acuta, helicina and pacifica) and two morphotypes of Limacina helicina antarctica (antarctica and rangi). It is also known as Limacina helicina rangii (d'Orbigny, 1835). These forms typically have different geographical ranges, but it remains unclear as to whether forms represent morphological responses to different environmental conditions or are indeed taxonomically distinct, and if the latter, their level of taxonomic separation. However, at the species level the geographical distribution is considered to be bipolar, as it occurs in both the Arctic and Antarctic oceans. Remigio and Hebert (2003) provided initial evidence for the genetic separation of Limacina helicina helicina and Limacina helicina antarctica. Hunt et al. (2010) have quantified genetic distance within these taxa. Hunt 2010 found a 33.56% difference in cytochrome c oxidase subunit I (COI) gene sequences between the 'Limacina helicina' which were collected from the Arctic and the Antarctic oceans. This degree of separation is sufficient for ordinal level taxonomic separation in other organisms, and provides strong evidence for the Arctic and Antarctic populations of Limacina helicina differing at least at the species level. Subspecies Limacina helicina antarctica Woodward, 1854 can be considered as a separate species Limacina antarctica Woodward, 1854. A conservative divergence time estimate of 31 Ma (95% HPD interval 12–53 Ma) for Arctic and Antarctic taxa, indicates that they have undergone rapid independent evolution since the establishment of cold water provinces in the early Oligocene. Also there is different structure of the shell between Limacina helicina and Limacina antarctica. The type locality of Limacina helicina is 'Arctic seas'. Limacina helicina is the only thecosome pteropod in Arctic waters.