Abstract
Acanthocephalans of the family Polymorphidae Meyer, 1931 are cosmopolitan parasites that infect the intestines of fish-eating birds and mammals. Polymorphid acanthocephalans recovered from the intestines of red-billed gulls (
Chroicocephalus scopulinus
(Forster)) from the Otago coast, New Zealand, although morphologically similar to the genus
Arhythmorhynchus
Lühe, 1911 nevertheless have a unique molecular profile showing considerable genetic differentation, and are here diagnosed and described as
Tenuisoma tarapungi
n. g., n. sp. Characters which distinguish
T. tarapungi
include a very elongate, cylindrical hindtrunk, swollen anterior trunk with a spinose region, a secondary swelling in males only containing the testes, and hypodermal nuclei distributed throughout the length of the trunk. Molecular data (
cox
1, 18S, 28S) confirm that the representative of the new genus is closest to, but nonetheless strongly divergent from species of
Pseudocorynosoma
Aznar, Pérez-Ponce de León & Raga, 2006. Immature specimens are described and illustrated, demonstrating the extreme degree of hindtrunk inversion occurring in immature individuals of this species. We provide a key to the genera of the family Polymorphidae.