ИСТИНА

For most of the pycnogonid species the life cycle, development and reproductive biology are poorly studied. Nymphon brevirostre is very common in the upper subtidal zone of the White Sea. Ovigerous males are found from September to April, larvae (instar 1) and postlarvae (instars 2-5) develop from April to July. All postembryonic stages of N. brevirostre dwell on the hydrozoan polyps Obelia. The offspring of N. brevirostre kept in aquarium, were observed and photographed regularly to estimate number of molts and duration of intermolt periods. Instar 1 is a protonymphon larva (six-legged larva), and in the course of anamorphic development the pairs of walking legs appear in a successive pattern. Instar 1 leaves the male’s ovigerous legs and starts feeding on Obelia polyps. Larvae intrude into the hydrothecae and use proboscis to suck prey while only chelae fingers and terminal claws of larval legs touch the hydrant tissue directly to minimize contact with nematocysts. Up to three protonymphon specimens may occupy the same hydrotheca. Up to instar 3 larvae show the same style of feeding and the whole larval body fit in a hydrotheca. As larvae grow, their walking legs and the rear part of the body protrude from the hydrothecae, and instar 5 can only put its cephalic region into a hydrotheca. To walk over the Obelia colonies the larvae use the spinning apparatus located on the chelifores. Silk is produced by the silk gland of two giant cells and extruded from the tip of the spinning spine. The larvae wind the silk threads around the hydrocaulus of Obelia to make a secure net on the surface of the perisarc and clamber on it using chelae and claws of the larval legs. Molting usually takes place outside the hydrothecae. Exuviae are shed in three parts. First, chelifores molt, their exuviae stay fastened to perisarc with silk threads, then larva attaches again and perform characteristic movements to let the rest of the body and legs exuviate and cast the remains of the cuticle in a single piece. The spinning apparatus is functional in instars 1 to 5, and later disappear contemporaneously with the regression of the provisory larval ambulatory legs before they are replaced by the definitive palps and ovigerous legs. These regressive events correlate with transition to the adult manner of locomotion and feeding and mark the beginning of the juvenile phase in the life cycle.