Larvae of sea spiders (Arthropoda, Pycnogonida) from the White Sea

The postembryonic development of five pycnogonid species from the White Sea is described. Three types of their development are specified. The first type is characterized by the development of six-legged (protonymphon) larvae of medium size (120-220 mum). They develop on ovigerous legs of males for some time, and then parasitize on hydrozoan polyps. The second developmental type is characterized by small-sized (about 50 mum) protonymphon larvae. Six-legged larvae of Phoxichilidium femoratum have some adaptations for pelagic circulation. Afterwards, they are endoparasites. Pycnogonids with postembryonic development of the third type have no typical protonymphon larvae. Large larvae of the third type are of about 300-700 mum long and with a high yolk reserve. They remain on egg cocoons carred by male on its ovigerous legs until the development of abdomen and three or four pairs of walking legs and then leave the male. Some morphological features of larvae are discussed depending on a type of development.