Adaptations and mutualism between corals and associated fish

Coral-associated fishes of the families of gobies (Gobiidae) and damselfish (Pomacentridae) are among the most habitat-specialized fishes of coral reefs. Although habitat associations and different degrees of specialization are well known, little information is available about the underlying biological factors that govern the relationship between fish and host coral. One of the key questions remains unanswered (in particular for the species-rich Gobiidae): Is this association merely a commensalism or is it mutualistic with benefits for both partners? The proposed project will examine coral-associated fishes in the northern Red Sea and western Pacific. The main issue is whether selected fitness factors, such as growth and predation, are influenced in a way that increases fitness for both partners through their association. This will include examining fish morphology with respect to host coral structure. Morphological investigations of body forms and growth patterns will be performed on coral-associated gobies and damselfish using geometric morphometrics based on landmark data of digitally scanned fishes. Additionally, selected body proportions will be measured to also cover the 3rd body dimension. Multivariate statistical analyses of fish shapes and correlation with host coral interbranch space will then be performed. To gain more information about potential coral predator repellent effects and performance of fish within their host corals, we will collect etho-ecological data on aggressive behavior, predation, competition, feeding and locomotion by using long-term underwater video monitoring. Small finger cameras will be used to observe fishes during day and night (IR-illumination). These observations are to be accompanied by extensive manipulative in situ (field) and ex situ (aquarium) experiments on growth and predation of corals and fishes. Nine short-term (1 to 2 months: Red Sea and Great Barrier Reef) experiments will be performed in the field and laboratory and several will be accompanied by video observations through a second set of finger cameras. Three long-term (up to 2 years) experiments are planned to reveal growth rates of corals with and without fishes (exclusion experiments) and to help determine how coral interbranch space and coral health influences fish growth patterns, body form and occupation rates. The project is designed to study phenotypic adaptation of fishes to their coral hosts and to study two-ways benefit to examine whether the coral/fish-relationship is of mutualistic nature. The project will be carried out by one Post- Doc (the applicant Dr. Jürgen Herler), two PhD-students (Simone Niedermueller and one Egyptian PhD) and will be assisted by technicians and collaborators. It is firmly anchored in national and international (JCU, Australia and Egypt) co-operations and will have a high impact for marine research at the University of Vienna and in Egypt.

A central question in ecology is the significance of biological interactions between key functional groups and their symbionts within ecosystems. In coral reefs, the corals are inhabited by a high diversity of invertebrates and fishes. Branching corals, such as the genus Acropora, are particularly important habitats because of their structural complexity. These corals are frequently occupied by fishes of the family damselfishes and gobies. The relationship between corals and several groups of fishes is mutualistic, where corals provide a home and breeding site for fishes, while receiving oxygen through fish movements, nutrient input through faeces or defence against coral predators. This research project investigated the role of strictly coral-associated fishes (mainly gobies of the genus Gobiodon) for the biology of their host corals (genus Acropora) and evolutionary adaptations between these fishes and their host corals. The branching patterns and growth type of corals create variation in the geometry of coral hosts and the architecture of the coral colony is assumed to provide a physical constraint for the fishes. So the two main questions of the research project were whether 1.) the Acropora-Gobiodon association is mutualistic (mainly based on the assumption that the fishes defend their host coral against corallivorous fishes), and, 2.) these obligatorily coral-dwelling fishes show physical adaptations to the structure of specific host corals, explaining diversification through coral-specific selection for different maximum size and shape of fish species.To answer these questions, we developed an underwater video surveillance apparatus to observe fishes in the field and during experiments, and a buoyant weighing apparatus for measuring coral weight increments in the sea. As experiments revealed, the defensive behaviour and toxic skin mucus of Gobiodon species leads to a deterrence of corallivorous butterflyfishes. Coral growth measurements in cages have shown that corals under predation of butterflyfishes grow slower. This shows that gobies potentially promote coral growth and behave mutualistically. Morphological investigations of gobies and damselfishes revealed a great variety of body forms because fishes are phenotypically well adapted to their particular host corals. In particular specialists that dwell very few (sometimes only one) coral species have evolved extreme and highly adapted body shapes. A slender but high body enables fishes to grow larger in the narrow interbrach space of corals. Larger fishes are harder to catch by predators and are more successful in defending their corals against congeneric species. Thus natural selection seems to act strongly on body shape and only species with a great range of host corals with different architecture have retained a generalized shape. This also helps explaining the high species richness (~ 30 species) of Gobiodon, which all seem to have a very similar general biology (in terms of habitat use and diet). The demand for a sufficient number and size of large host colonies puts specialized fishes at risk because increased coral bleaching will reduce coral growth rates and average colony size. The loss of mutualistic inhabitants will provide negative feedback for coral growth. Therefore it is important to include information about the status of mutualistic coral-associates into coral reef evaluation and conservation efforts.