Green2009-herbivore monitoring

Herbivorous Coral Reef Fishes
to which territorial pomacentrids rely on an invertebrate component in their diet is also unclear (Choat
1991).
ONTOGENETIC CHANGES IN FEEDING MODES
Most reef fishes go through a planktonic larval stage (Leis and Rennis 1983), and little is known about
their feeding modes while they are in the plankton (Leis 1991). However, most are presumed to feed
on other plankton (Choat 1991).
Once they settle onto the reef, the majority of herbivorous species become herbivores and stay that
way throughout their lives, including most acanthurids, blenniids and siganids (reviewed by Bellwood
1988 and Choat 1991). Similarly, the pomacanthid Centropyge bicolor appears to be an omnivore
throughout its benthic life (Bellwood 1988).
Others change feeding modes at different stages of their life history (ontogeny) after they have settled
onto the reef. Some species of unicornfishes commence benthic life as herbivores then feed on open
water plankton as adults (Naso annulatus, N. brevirostris, N. maculatus, N. mcdadei and N. vlamingii:
Choat 1991, Choat and Clements 1998, Choat et al 2002). For these species, ontogenetic change in
diet tends to occur by at least 20cm standard length (SL).
In contrast, parrotfishes pass through a period of carnivory during their early post-settlement phase
(Bellwood 1988), before progressively changing to herbivores within the first few weeks of benthic life
(by 32mm SL: Bellwood 1988). Similarly, herbivorous pomacentrids undergo ontogenetic changes in
diet from an omnivorous to a predominantly herbivorous diet (reviewed in Bellwood 1988).
These differences may be explained by phylogeny (Choat 1991). Acanthuroid fishes (including
acanthurids and siganids) settle at relatively large sizes with well-developed sensory, locomotor, and
alimentary systems (Leis and Rennis 1983), and can function as herbivores (Choat 1991). Labroids
generally, and scarines in particular, settle at smaller sizes (Leis and Rennis 1983). Herbivory in
scarines is based on the development of specialized mouthparts (the pharyngeal mill), associated
musculature and the alimentary tract (reviewed in Choat 1991). The functioning of the mill may be
size dependent, working efficiently only when individuals reach a certain size or mass. Therefore,
scarines may feed on copepods while they are small (Bellwood 1988), because their particular mode
of feeding has a size threshold. Similarly, Lassuy (1984) suggested that the inclusion of crustaceans
in the diet of small juveniles of the herbivorous pomacentrid Stegastes lividus may be a result of
limited digestive capabilities of these individuals.
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