The functional morphology, systematics and behavioural ecology of parrotfishes (family Scaridae)

The functional morphology and behavioural ecology of the parrotfishes (family scaridae) were investigated to assess the degree of correlation between them. Twenty-two species of parrotfishes from the subfamily Scarinae were examined mainly from reefs around Lizard Island, Great Barrier Reef, Australia. Morphological studies of the osteology and myology of the heads of adult parrotfishes revealed two morphological groups within the genus Scarus, viz. the 'sordidus' and 'frenatus' groups. Species in the 'sordidus' group are characterized by: uneven cutting edges on the jaws, an entopterygoid lateral process, a tightly bound interdigitating maxilla and premaxilla, three rows of teeth on the upper pharyngeal bones, well developed adductor mandibulae sections, A2 and A3 and an additional unique muscle, the Awy. Species in the 'frenatus' group are characterized by: even cutting edges on the jaws, a slightly protrusible premaxilla, two rows of teeth on the upper pharyngeal bones, fusion of the abductor muscle section A1 and A2 in some species and a thin strap-like A3 section which inserts only on the articular. The morphology of the heads of species in other genera, namely, Cetoscarus bicolor, Bolbometopon muricatum and Hipposcarus Longiceps was marked by their possession of a quadrato mandibularis internus muscle and the non-articulatory articular-dentary joint in C. bicolor and B. muricatum. Analyses of the two morphological groups indicated a marked difference in the functional abilities of species in the two groups. The morphological characteristics of 'sordidus' group species enable them to bite the substratum with a large powerful cracking bite. Species in this group are therefore described as 'biters', The morphological characteristics of 'frenatus' group species are consistent with the requirements necessary for scraping substratum with small, weak bites. Species in this group are therefore described as 'scrapers'. Of the species examined in this study, the following species are functional ‘biters’: S. bleekert, S. gitbbus and S. sordidus, whereas the following are functional 'scrapers': S. brevifilis,, S. dimidiatus, S. ftavipectoralis, S. frenatus, S. ghobban S. globiceps, S. longipinnis, S. niger, S. oviceps, S. psittacus, S. rivulatus, S. rubroviolaceus, S. schlegeli, S. spinus, S. tricolor and Scarus sp. (cf. lunula). Because they lack some specialized morphological features associated with the biting or scraping strategies of species in the genus Scarus, Cetoscarus bicolor and Bolbometopon muricatum are considered 'proto-biters', whilst Hipposcarus longiceps is considered a 'proto-scraper'. These differences are not apparent in juvenile specimens less than 50 mm standard length. specimens of Scarus smaller than this are extremely similar. Small specimens(< 14 mm S.L.), in particular, differ markedly from the adults in the possession of caniform teeth and a simple, non-sacculated intestine. Functional interpretations of the anatomy of the head and intestine of juvenile scarids suggest that they progress from an initial carnivorous phase to a selective grazing stage before becoming functional 'scrapers' at about 50 mm S.L. 'Biting' species only possess the full complement of biting characteristics above 90 mm S.L. Field observations of adults revealed two feeding guilds within the genus Scarus which correspond with the two functional groups. (1) 'Biting' i.e. 'sordidus' group species are characterized by: i) infrequent, large bites which scar the Substratum, ii) a propensity to feed upon convex substrata, and iii) aggressive interspecific interactions, when displayed, predominantly directed towards other 'sordidus' group species. (2) 'scraping' i.e. 'frenatus' group species are characterized by: i) numerous small bites which rarely scar the substratum, ii) a tendency to feed on a range of substrata, and iii) aggressive interspecific interactions, when displayed, predominantly directed towards other 'frenatus' group species, These feeding strategies strongly influence the roles played by parrotfishes as coral predators and bioeroding agents. The major coral predator is Bolbometopon muricatum. Bioerosion is primarily the result of feeding by 'sordidus' group species, Cetoscarus bicolor and B. muricatum. It is therefore proposed that the distribution of these species within and between reefs may influence the extent and rates of bioerosion, the topography of the substratum and the distribution of various coral morphs. Field observations of juvenile scarids revealed a wide range of behavioural traits, although these did not correspond with the 'sordidus' and 'frenatus' groupings. S. frenatus in particular, differed from other juvenile 'frenatus' group species in its feeding behaviour. An analysis of the gut contents of the juveniles of several species showed marked changes in the diet, from being initially predominantly carnivorous to herbivorous. These changes were strongly correlated with changes in morphology and behaviour. In addition to functional considerations, the morphological analyses in this study were used to examine the systematic relationships between genera in the subfamily Scarinae. The present generic status of Cetoscarus, Bolbometopon and Hipposcarus is supported. The genus Scarus contains two distinct groups which may be recognized at the generic level. A descriptive account of the ontogeny of the colour patterns of the juvenile phase of 22 species is presented to facilitate the study of juvenile scarids in the field. The colour patterns of juvenile scarids are interpreted in terms of concealment colouration, including crypticism, camouflage and mimicry. In the species examined, there is a strong correlation between schooling behaviour, the range of patterns displayed, and the speed with which the patterns may be changed.