An egg-laying rhythm in Zeuxapta seriolae (Monogenea: Heteraxinidae), a gill parasite of yellowtail kingfish (Seriola lalandi)

Egg-laying in Zeuxapta seriolae from the gills of yellowtail kingfish (Seriola lalandi) in South Australia was investigated in situ under a controlled lighting schedule. Eggs were collected every 3 h from mature Z. seriolae infecting 4 S. lalandi kept individually for 7 consecutive days exposed to alternating 12 h periods of illumination and darkness (light on 06:00 h; light off 18:00 h) at 18±0.5 °C and 40 ppt salinity. A well-defined egg-laying rhythm was demonstrated for Z. seriolae. A total of 650,153 eggs was collected during all periods of illumination and darkness. Of these eggs, 71.6% (57.4-81.8%) were collected during the first 3 h period following 'dusk' at 18:00 h. At the conclusion of the experiment, 179 mature Z. seriolae specimens were recovered with a mean clamp number of 47 (37-53) along the major clamp row and 36 (26-44) along the minor clamp row. Of the 179 specimens examined, 177 were found with eggs in utero: mean 321, range 8-1015. Specimens of Z. seriolae produced on average 21.6 (20.8-23.8) eggs/worm/h over the 7 day experiment. It is suggested that Z. seriolae accumulates eggs in utero over a 24 h period. Eggs extracted from the uteri of worms were joined together by filamentous strands and formed a single continuous 'egg string'. The well-defined egg-laying rhythm in Z. seriolae may be related to a specific host behaviour associated with 'dusk' or darkness.