A New Kinorhynch Pycnophyes australensis sp. n. (Kinorhyncha: Homalorhagida: Pycnophyidae) from Magnetic Island, Australia

Abstract

Abstract A new homalorhagid kinorhynch, Pycnophyes australensis sp. n., found in sediment probes from the eulitoral zone of a sandy beach of Magnetic Island, Australia is described and illustrated. This is the second species of Pycnophyes to be described and only the third record of a homalorhagid kinorhynch for Australia. P.australensis is characterized by a unique combination of characters. Like 17 other species of Pycnophyes the new species lacks a well-developed middorsal process on the posterior margin of the 3rd segment. From these only three other species, namely P. beaufortensis Higgins, 1964, P. kielensis Zelinka 1928, and P. zelinkaei Southern, 1914 share the possession of prominent anteromesial thickenings (Mittelwulste) on only the 12th segment with P.australensis . Whereas P. kielensis and P. zelinkaei clearly differ from the new species by the prominent scalloped or a strongly denticulated anterior margin of the 3rd tergite, P. beaufortensis closely resembles P. australensis in several characters, namely the similar body dimensions (TL, MSW) and the relative length of the lateral terminal spines (∼17% of trunk length). However, the new species differs from P. beaufortensis in having lateral setae on segments 4—11 and a pair of setae on segment 12, the lack of the lateral armor joints of the 12th segment and a much broader caudal end. Moreover P. australensis is characterized by a conspicuous reticulate sculpture of cuticular folds on the pachycycli of segments 4—12. In addition the six juvenile stages of P. australensis are described. Whereas the first three stages possess a long midterminal spine and represent the so-called ‘Centrophyes’ stage, the following three stages lack the midterminal spine and possess only small immovable anlagen of the lateral terminal spines. These juveniles represent thus the so-called ‘Leptodemus’ stages. This is the first report of such a development via a series of ‘Centrophyes’ and ‘Leptodemus’ stages into the adult for the Kinorhyncha. Other species of Pycnophyes (e.g. P. beaufortensis and P. cryopygus Higgins & Kristensen, 1988) develop either via three ‘Centrophyes’ and three ‘Hyalophyes’ stages, the latter are characterized by larger, movable lateral terminal spines, or at least the first five juveniles represent the ‘Leptodemus’ stage. The latter mode is known for P. kielensis and P. dentatus (Reinhard, 1881) but is otherwise characteristic for species of Kinorhynchus . The phylogenetic implications of this third developmental mode are discussed.