Oncomiracidial development, survival and swimming behaviour of the monogenean Sparicotyle chrysophrii (Van Beneden and Hesse, 1863)

Oncomiracidial development of Sparicotyle chrysophrii, a monogenean parasite of Sparus aurata, was analysed using 450 eggs. Parasite morphological changes in time, data on hatching success, as well as oncomiracidial survival and swimming behaviour were recorded. Eggs were maintained at 20°C and exposed to LD 12:12. They were observed under the stereomicroscope every 8h until they hatched. Thereafter, 155 oncomiracidia were isolated in separate wells and observed every 2h until their death. Most of the hatchings occurred in a short period of time (approximately 24h). Hatching success was 87% and the hatching period ranged from 5 to 10days after deposition, with most of the hatchings occurring during darkness (>75%). Oncomiracidial survival time ranged from 2 to 52h, although only 10% of the oncomiracidia lived more than 24h. No significant Spearman's correlation was found between incubation and oncomiracidial survival times. Survival times of the oncomiracidia emerging during darkness period were significantly higher than survival times of those emerging during light. Seven oncomiracidial behaviour patterns were recorded: vertical swimming (fast/slow), horizontal swimming close to the bottom (fast/slow), continuous whirling, alive without ciliary movement and dead. A highly significant correlation was found between survival and swimming times (r=0.911, p-value=0.000). The mean swimming speed of oncomiracidia was 3.97mm/s (n=10). The complete life cycle of S. chrysophrii (from egg to egg) at 20°C lasts approximately 50days: 5–10days of incubation, 2–3days of oncomiracidial survival time and 36days of worm maturation. Treatment involving two applications, first for adults and juveniles and later (after 12 to 15days) for larvae, would allow long periods without monogenean infection.