The shelled pteropod mollusk Limacina helicina is a key organism in polar pelagic ecosystems, where it can can account for 50% of total zooplankton abundance (number of individuals per unit volume). Pteropods are strictly pelagic mollusks that are highly adapted to life in the open ocean. They are commonly referred to as ‘‘sea butterflies’’ due to their wing-like parapodia, which are evolutionary modifications of the gastropod foot. Pteropods produce large mucus webs to filter-feed on phytoplankton as well as small zooplankton (including their own juveniles). Species of the Order Thecosomata, including L. helicina (the only thecosome pteropod in Arctic waters), produce a fragile external calcium carbonate (aragonite) shell. Shelled pteropods play a significant geochemical role in the oceans, as they contribute to the export of calcium carbonate and can represent a major component of the carbon transport to the deep ocean. The likely effects of ocean acidification (resulting from the absorption of increasing quantities of CO2 from the atmosphere) have been studied in a variety of shell-building marine organisms, including L. helicina, which may be especially vulnerable as a result of its aragonite shell and its distribution in polar oceans, which acidify especially quickly due to their particular chemical and physical characteristics. (Comeau et al. 2010 and references therein; Hunt et al. 2010)
Although L. helicina was formerly believed to have a bipolar distribution, further investigation has shown that the Arctic and Antarctic forms are genetically distinct species. Traditionally, northern and southern hemisphere forms were recognized as subspecies and referred to as L. h. helicina and L. h. antarctica, respectively. Three morphotypes, or "forma", of the subspecies L. h. helicina have often been recognized (acuta, helicina, and pacifica) along with two morphotypes of L. h. antarctica (antarctica and rangi). However, genetic studies have revealed that L. h. helicina and L. h. antarctica differ by around 34% in mitochondrial COI sequence, far more divergence than would be seen intraspecifically. Sequence data are not yet available to help clarify the taxonomic status of the various morphotypes, which may or may not actually be genetically distinct. (Hunt et al. 2010)
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