Oithona minuta (Kriczagin, 1873)
Synonyms: Oithona nana Giesbr., 1892.
Common names: None.
Order (Scientific): CYCLOPOIDA.
Family (Scientific): OITHONIDAE.
Taxonomic descriptions: The cephalothorax has an enlarged middle part of the body. The rostrum is absent in both sexes. The anal segment is shorter than the preceding one. The female's first antennae extends to the end of the third thorax segment. The fifth pair of legs has two bristles. Aciniform egg carrying bags. The length of the female is 0.5-0.7 mm and of the male 0.4-0.6 mm.
Black Sea Regional level:
Subregion level: EN
Habitats type, Critical habitats, Limiting factors: Mainly inhibits the surface layers of the Black Sea. Euryhaline species. The high degree of eutrophication and great rivalry in feeding with larval forms of Mnemiopsis leidyi during the last 10 years appeared to be important limiting factors. Another factor is the adult forms of M. leidyi which feed on O. minuta. The powerful elimination of O. minuta by the ctenophore is mostly due to the fact that its eggs are gathered in bags. Thus, they together with the individual become prey to the ctenophore and are quickly and effectively eliminated.
Biology: Inhabits mainly the upper sea layers. Performs incessant migrations with a large vertical range. Observed during the whole year. Feeds on tiny phytoplankton, bacteria and representatives of the class Infusoria. The female lays its eggs in two bags located by the two sides of the genital segment.
Population trends: Until 1984, O. minuta had been common species - representative of the mesoplankton in Black Sea waters in front of the Bulgarian coast. After the appearance of the ctenophore M. leidyi during the period 1986-1991 the quantity of O. minuta rapidly fell to almost its total extinction in the period 1991- 1994. Not until 1995 did it appear again in the mesoplankton but quite modest numbers of 134 - 152 ind/m3 compared with 7,000 - 8,000 ind/m3 in the period 1970-1984.
Threats: Increases in eutrophication and M. leidyi populations will lead to a reduction in O. minuta numbers.
Conservation measures taken: None.
Conservation measures proposed: A reduction in eutrophication will lead to a fall in M. leidyi numbers.
Compiled by: A.Konsulov.