The great diversity of body shapes among small coral reef fishes suggests differences in life-styles and benthic relationships as well as adaptation to habitats with different physical properties. Damselfishes are among the three most species-rich groups of small coral reef fishes and they exploit numerous habitats ranging from hyperbenthic to cryptobenthic. The latter include many species associated with branching corals to varying degrees. Most eco-morphological studies of damselfishes have focused on the trophic niche and examined the trophic morphology through associated anatomical structures such as the head skeleton. The present study investigates body shape differences with respect to differences in habitat use of four coral-associated damselfishes. Dascyllus aruanus, D. marginatus, Chromis viridis and C. flavaxilla represent four of the most abundant coral-associates among damselfishes in the northern Red Sea. Using a combination of traditional and geometric morphometrics, we discovered great shape differences between the two genera, but also within-genus differences along the same difference axis, which can be interpreted as differences in habitat use. The more cryptic species D. marginatus has a considerably deeper but less wide body than its larger sister species D. aruanus. This body shape pattern and the small maximum size can be explained by the physical constraints provided by the interbranch space of host corals of D. marginatus. Similar differences were found in the genus Chromis, although these species are generally longer and more compressed. This body shape enables Chromis to exploit a wider spatial niche by using more host corals with different architecture than the two species of Dascyllus. The high body shape diversity of coral-associated damselfishes mirrors the structural complexity of reef habitats, and small differences in coral architecture lead to a fine-scale tuning of fishes. Apart from interspecific differences, shape examinations revealed a common allometric axis indicating similar ontogenetic shape changes in all species, although to different extents. This shows that certain growth patterns are typical for small reef fishes and may be associated with a change in diet after settlement.
Key words: Morphology, habitat associations, adaptation, reef fishes, geometric morphometric, allometry
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