Trophi morphology and its usefulness for identification of formalin-preserved species of Synchaeta Ehrenberg, 1832 (Rotifera: Monogononta: Synchaetidae)

A study of temporal and spatial distribution of zooplankton frequently requires identification to species level. In such studies, samples are usually fixed, but according to different authors rotifer genera such as Synchaeta (Ehrenberg, 1832) can only be identified in live samples. A procedure for the identification of preserved specimens of Synchaeta is presented, permitting ecological studies without the examination of live material. Trophus morphology, investigated by light and scanning electron microscopy, was related to morphological characteristics of preserved specimens. In this way, body length was used to group formalin-preserved specimens into Synchaeta gr. tremula-oblonga sensu Ruttner-Kolisko (1974) (⩽170 μm) or Synchaeta gr. stylata-pectinata sensu Ruttner-Kolisko (1974) (⩾190 μm). Furthermore, body length, colour and shape were used to identify Synchaeta grandis (Zacharias, 1893), Synchaeta pectinata (Ehrenberg, 1832), Synchaeta kitina (Rousselet, 1902) and Synchaeta lakowitziana (Lucks, 1930). It was, however, impossible to distinguish Synchaeta tremula (Müller, 1786) from Synchaeta oblonga (Ehrenberg, 1832) by their trophi because of contradictions in the diagnostic keys and monographs regarding trophus morphology.