The Anisakis simplex complex off the South Shetland Islands (Antarctica): endemic populations versus introduction through migratory hosts

Third-stage larvae (L3) of 2 Anisakis nema- tode species, A. simplex C and A. pegreffii (both A. sim- plex sibling species complex), were isolated from migrat- ing myctophids around the South Shetland Islands. Gymnoscopelus nicholsi were parasitized by both nema- tode species, at a prevalence of 22.7% for A. simplex C and 4.0% for A. pegreffii, while Electrona carlsbergi har- bored A. simplex C at a prevalence of 9.1%. The ITS-1, 5.8S, and ITS-2 regions of the nematodes were identical to specimens from Pacific Canada or California (A. sim- plex C) and the coast of China (A. pegreffii), confirming an extensive range of distribution for both species. The occurrence in migrating myctophids coupled with rare findings from other teleosts leads to the conclusion that both species were introduced from outside the Antarctic. Consequently, they are at their most southern range in the Southern Ocean, and an earlier molecular record of A. simplex C from the elephant seal Mirounga leonina may have detected an accidental case of infection. Del- phinids, which frequently occur along South America but not in the Southern Ocean, are suggested as being the typical final hosts in the life cycles of A. simplex C and A. pegreffii. The myctophids had only few prey items within their stomachs, dominated by euphausiids, including Euphausia superba and planktonic copepods. These are common intermediate hosts for anisakid nematodes and are an important component of the Ant- arctic food web. Regular introduction events through migrating whales and myctophids are considered responsible for the occurrence of Anisakis infection in the high Antarctic.