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Cleaning Up
Cleaning is one of the most common behaviours on coral reefs. The blue and white banded cleaner wrasse is a coral reef fish that spends most of the daytime hours cleaning parasites from other fish. This is assumed to benefit both the cleaner and the client fish, yet we know very little about what drives this behaviour. The cleaner wrasse establishes a cleaning station then does a bobbing and gyrating dance to show that it is ready to begin cleaning. Fish living in the area make frequent visits to the cleaning station to be groomed. Client fish turn off their predatory instincts while being cleaned and allow the cleaner fish to remove parasites from all over their bodies, including inside the mouth and around the gill chambers. The benefit to the cleaner wrasse is clear: it gets a meal of parasites. However, the significance of cleaning behaviour to the client fish is not clear, although it has been suggested that the health of client fish will deteriorate in the absence of cleaners.
Dr Lexa Grutter, a former Lizard Island Doctoral Fellow and now at the University of Queensland, is conducting a series of field and aquarium experiments at Lizard Island to test whether client fish benefit from the attentions of cleaner wrasse. Lexa has demonstrated that particular parasites (gnathiid isopods), are the main food source of cleaner fish and that they eat them in large numbers: about 1,200 gnathiids per cleaner per day. She also found that the abundance of gnathiids on fish decreases during the day, when cleaner fish are active. To test whether this decline is due to predation by cleaner fish, she placed caged fish on reefs with and without cleaner fish at sunset and sampled the fish the following dawn and sunset. At dawn, she found that gnathiid abundance did not differ between reefs with and without cleaner fish. However, by sunset, fish on reefs without cleaner fish had four times as many gnathiids as fish on reefs with cleaner fish. This suggests that cleaner fish predation is so high, it causes the daily decline in parasites observed on fish. Little is known about the effects of different parasite loads on fish so Lexa is now conducting an aquarium-based experiment at the Station to investigate whether client fish suffer decreased "health" when infected with different densities of gnathiid parasites.
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