Ontogenetic variation in the diet of the cleaner fish Labroides dimidiatusand its ecological consequences

Ontogenetic changes in the diet of the cleaner wrasse Labroides dimidiatus (Labridae) were examined at Lizard Island, Great Barrier Reef. Cleaner fish of all sizes mainly ate gnathiid isopod juveniles; the proportion of these (77 to 85%) did not differ among size classes. However, the proportion of the remaining items (scales, parasitic copepods, and non-parasitic copepods ) differed significantly among size classes largely due to small juveniles which ate more non-parasitic copepods. The number of gnathiid isopods and s cales in the diet of L, dimidiatus increased with the size of fish, with ad ult cleaners having 7 times as many gnathiids and 4 times more scales compa red to small juveniles. The size-frequency distribution of gnathiids differ ed among size classes of cleaners; small juvenile cleaners had more small g nathiids in their diet than medium-sized juveniles and adult cleaners. The 'throat width' increased with the size of cleaner; on small juvenile cleane rs the throat width was equal to or less than the width of the larger gnath iids sampled on a common client fish Hemigymnus melapterus (Labridae). Mout h size constraints may, therefore, explain the low number of large gnathiid s found in the diet of these small cleaners. The lower number and smaller s ize of gnathiids in the diet of juvenile cleaners compared to adult cleaner s suggests that any potential impact of small cleaner fish on gnathiids abu ndance will be less than that of larger cleaners and will be largely on sma ll gnathiids. In addition, although clients may benefit more from cleaning by the larger cleaners, because they remove more parasites, the costs in th e form of loss of scales, which are also removed in larger numbers, will al so be higher.