ИСТИНА

For a number of the White Sea crustaceans the intriguing details of their life cycle, external morphology and anatomy remain unknown. Mostly these are various parasitic taxa. Probably the most mysterious are the so-called Y-larvae of the subclass Facetotecta, found in marine plankton in all over the world. Adult stages of of Y-larvae are not discovered yet. For the first time nauplii of Facetotecta were described more than a hundred years ago (Hansen, 1899). Afterwards Y-nauplii were found in different regions of Atlantic, Pacific and Indian oceans. The body of the facetotectan nauplius consists of frontal part covered with dorsal head shield and protruding rear part. The dorsal side of nauplii ornamented with cuticular ridges forming polygonal pattern. Only many years later a post-naupliar larval stage called Ycypris was discovered (Bresciani, 1965). Y-cypris consists of cephalon with 5-segmented antennules armed with grasping hook, labrum and modified head appendages; thorax with 6 swimming thoracopods, and 2-4-segmented abdomen with large telson terminated with furcal rami. Univalved dorsal carapace with cuticular ridges covers thoracic somites of the Y-cypris. The position of Y-larvae among Crustacea remained uncertain for a long time. Now they are recognized as a separate subclass Facetotecta within the class Thecostraca. The species Hansenocaris itoi was described from the White Sea, and the larval development was considered to be fully traced for this species, including 5 naupliar and one cypridiform stages (Kolbasov, Høeg, 2003). The facetotectan larvae were collected in plankton in April – July 2018 at the WSBS, MSU. It has been found that Facetotecta, in fact, has at least 6 naupliar stages. Additional laboratory experiments with molting hormone (20-Hydroxyecdysone) showed that induced molting of the Y-cypris leads to a peculiar worm-like juvenile stage – ypsigon. This stage has been described recently (Glenner et al., 2008). We have started the studies of the external morphology and anatomy of the larval stages and ypsigon of the Facetotecta, as well as the analysis of molecular genetics of these crustaceans. These studies were supported by RFBR grant 18-04-00624 A.