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

Abstract

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.