ИСТИНА

There are five modes (pathways) of postembryonic development of pycnogonids known to date. Classification of the developmental pathways is based on the nutrition source which is used during the larval period i.e. yolk or invertebrate host, offspring size, and some characters of metamorphosis e.g. order of the development of the walking legs. In Ammotheidae four out of five pathways are observed, and only one in such families as Phoxichilidiidae or Callipallenidae. The most common, widespread, and apparently primitive mode is ectoparasitic development via protonymphon stage. Protonymphon larva is a three-segmented larval form unique for pycnogonids, and most of protonymphon larvae show very uniform morphology. Among Pycnogonidae and Ammotheidae, however, there are genera with unusual morphology of the protonymphon larvae. In ammotheids and phoxichilidiids a tendency to the endoparasitic development appears independently. Among Nymphonidae, Callipallenidae, and Ammotheidae there are species with lecithotrophic development. In case of Nymphonidae and Callipallenidae, this fact may be put down to their possible close phylogenetic relation. Among nymphonids prolonged lecithotrophic development starts with a large protonymphon larva and may occur along with ectoparasitic development in different species of the same genus. Lecithotrophic developmental mode of callipallenids is unique in that the earliest stage after hatching is a “walking-leg bearing larva”, not a protonymphon, and the postembryonic period is reduced. Development of Callipallenidae species seems to be the most derived of postembryonic development modes of Pycnogonida. The existing versions of nomenclature for the pycnogonid developmental pathways should be revised. This study has been supported by Russian Foundation for Basic Research (grants 05-04-48697-a; 09-04-01132-a).