Ultrastructural observations on the elusive subtegumental cells of the viviparous gill monogenean, Macrogyrodactylus clarii.

The ultrastructure of the tegument, including putative subtegumental cells, of the monogenean gill parasite Macrogyrodactylus clarii is described for the first time. The lateral cells are secretory in nature and can be classified into four types: T1 producing elongated, electron-dense, bodies (t1); T2 manufacturing electron-lucent vesicles (t2); T3 producing thin, dump-bell shaped, electron-dense bodies (t3); and T4 containing lyzosome-like inclusions. Morphologically, the T2 cells resemble those described as presumptive vitelline cells in other gyrodactylids whereas the T1, T3 and T4 cells are reported for the first time in gyrodactylids. The close similarity between the secretory inclusions of the outer syncytial layer of the tegument and those found in the T1, T2 and T3 cells suggests that these cells represent the enigmatic tegumental cell bodies rather than vitelline cells. The possible functions of the distinctive features of the tegument of M. clarii are discussed.