Infestation dynamics of gnathiid isopod juveniles parasitic on the coral-reef fish Hemigymnus melapterus (Labridae) |
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Authors: | A S Grutter |
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Institution: | (1) Department of Microbiology and Parasitology, The University of Queensland, Brisbane, Queensland 4072, Australia Fax: 0061 (0)7 3365-1588 e-mail: a.grutter@mailbox.uq.edu.au, AU |
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Abstract: | The dynamics of infestation by parasitic juveniles of gnathiid isopods were investigated at Lizard Island in the summer of
1997/1998 to determine when, and at what rate, they infest fishes. Variation in gnathiid abundance on wild-caught fish (Hemigymnus melapterus) between dawn and sunset was examined, and unparasitized H. melapterus in cages were exposed to gnathiids in the field for 4 h (8 h for fish sampled at 06:00 h) at five different times of the
day and night (10:00, 14:00, 18:00, 22:00, 06:00 hrs) on three reefs. To control for any potential effect of the cleaner-fish
Labroides dimidiatus, which consumes large numbers of gnathiids each day, sampling was carried out on reefs from which all cleaner-fish had been
removed. Gnathiid abundance decreased during the day. Standardized abundance per wild-caught fish was 1.9 times higher at
dawn than at sunset. Gnathiids successfully infested fish in cages. Sixty-one percent of the fish in cages were infested with
≥1 gnathiids, with 51% of the fish having between 1 and 4 gnathiids after 4 h exposure. Gnathiids infested fish both day and
night. The log10 (abundance +1) of gnathiids per caged fish varied significantly between time periods, with higher gnathiid numbers on fish
sampled at 18:00, 22:00, and 06:00 hrs than on fish sampled at 10:00 and 14:00 hrs. In contrast to gnathiid abundance on wild-caught
fish, the number of gnathiids on caged fish sampled at sunset was as high as that at dawn. The estimated mean (SE) cumulative
number of gnathiids per caged fish mean size = 13.7 cm (±0.25)] per day was 7.8 (1.1); this is similar to the estimated mean
number of 7.3 gnathiids on similar-sized wild-caught fish at 6:00 hrs. The high infestation rate of gnathiids on caged fish
in the late afternoon contrasted with the low numbers on wild-caught fish at this time, suggesting that factors other than
infestation behaviour may be responsible for the low numbers of gnathiids on wild-caught fish in the afternoon.
Received: 19 August 1998 / Accepted: 1 June 1999 |
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