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The genus Dolops (Audouin, 1837) is endemic among ectoparasites present in fish, with nine species described in Brazil, five of which are from the Amazon region. Among the genus, the species Dolops discoidalis (Bouvier, 1899) stands out due to its high prevalence of parasitism in Amazonian fish. The present study aims to characterize the development phases of D. discoidalis. The parasites were obtained from infected fish species Rhytiodus argenteofuscus from a natural environment, kept in polyethylene experimental boxes (500 liters). The physical-chemical parameters of the water (pH, electrical conductivity, dissolved oxygen and temperature) were monitored daily. A total of 30 egg layings were analyzed. After identification of the oviposition, the eggs were transferred to another container containing water (0.5 liters). The number of eggs was recorded, with observations made in stereomicroscopic and photographic records until hatching occurred. The egg layings exhibited differences in colour throughout their development. On the seventh day, the embryos exhibited visible ocelli and on the 17th day the hatching period began. Hatching occurred in batches, with around 5–10 hatches/day/laying. However, peak hatching was recorded on the 19th day of the cycle. Therefore, the cycle of D. discoidalis comprised a total period of 17–22 days at 28.C, with hatching in batches and pigmentation of the eggs observed close to hatching. Newly hatched parasites did not survive for more than 24 hours in the absence of the host. This study contributes to knowledge of the biology of D. discoidalis, and adds to information regarding possible studies into its control.

This study investigated the acute toxicity (LC50-24 hr ) effects of the essential oil of Cymbopogon citratus for adult Argulus sp. and Dolops discoidalis, before and during oviposition. In vitro acute toxicity (LC50-24 hr ) was tested using 20, 40, 60, 80, 100, 120, 140 and 160 μg/L of C. citratus essential oil, and two control groups (one with cultivation tank water and one with cultivation tank water + alcohol) were used. Specimens of Argulus sp. and D. discoidalis submitted to acute toxicity were evaluated using histological procedures. The major chemical compounds of C. citratus essential oil were geranial (47.5%), neral (35.6%) and myrcene (6.7%). The LC50-24 hr for Argulus sp. adults was 67.97 μg/L, while for D. discoidalis it was 59.55 µg/L. In the oviposition of both species of argulids, maximum mortality began with treatments of 140 μg/L, while the LC50-24 hr for Argulus sp. and D. discoidalis was 83.98 μg/L and 82.48 μg/L, respectively. In both argulid species exposed to C. citratus essential oil, morphological alterations were observed only in the eyes, and they occurred in the ommatidium and rhabdomeres and were dependent on the concentration of C. citratus essential oil and the parasite species.