Ultrastructure of the digestive tract of Diacyclops thomasi (Cyclopoida, Copepoda) during different stages of encystment during a summer diapause

Baud, A., Cuoc, C. and Alekseev, V. 222_. Ultrastructure of the digestive tract of Diacyclops thomasi (Cyclopoida, Copepoda) during different stages of encystment during a summer diapause. — Acta Zoologica (Stockholm) 85 : 181–189 Diacyclops thomasi ( Forbes, 1882 ) has a life cycle comprising a summer diapause with whole‐body encystment at the copepodid IV stage. Formed progressively, the cyst can indicate the extent of entry into diapause. In this study the ultrastructure of the midgut epithelium during encystment of CIV was compared with that of active CIV and females. In active individuals two well‐differentiated epithelial cell types were observed: vacuolar cells (B‐cells), and cells without vacuoles but with more densely packed microvilli and abundant rough endoplasmic reticulum and mitochondria (F/R cells). In encysted specimens a striking transformation of the midgut epithelial layer was noted. Only inactive nuclei with highly concentrated euchromatin and encircled by a thin rim of hyaloplasm were observed. Compartments of both urosome and especially cephalothorax displayed large lipid‐rich lacunae. At the beginning of encystment, the midgut epithelium showed an intermediate state. Changes in ultrastructure observed in the midgut epithelium of D. thomasi clearly reflect different stages of diapause, and represent clear evidence of profound reorganization, which is progressively induced by diapause in the organism.