33 Coral reefs are structurally complex environments that provide a diverse range of habitats for fish. Many small reef- fishes are habitat specialists (Munday and Jones, 1998) and some of these species display unique adaptations to the habitats they occupy (e.g. anemonefishes; Allen, 1972). Among the most habitat-specialized fishes on coral reefs are coral- dwelling gobies from the genus Gobiodon that live among the branches of coral colonies, mostly from the genus Acropora (Patton, 1994; Munday et al., 1997). Species of Gobiodon are arguably exceptionally cowardly fishes. They secrete a poisonous mucus, and are therefore probably inedible to most predators (Schubert et al., 2003). Nevertheless, they spend virtually their whole adult life in narrow spaces formed between the branches of Acropora, a shelter that should make them inaccessible to most predators. Moreover, the need to leave the coral to find a breeding partner is minimized by the ability to change sex in either direction. Thus, if two individuals of the same sex end up in a coral colony, one of them will change its sex unless other breeding partners are nearby (Munday et al., 1998). In addition to shelter, corals also provide a site for reproduction, and for some gobies, a source of food (Patton, 1994; Nakashima et al., 1996). Coral colonies are a limited resource and there are significant consequences to individual fitness to inhabiting different species of coral (Munday et al., 1997; Munday, 2001). Consequently, there is intense competition for colonies of preferred coral species (Munday et al., 2001). Although coral colonies are an essential resource for coral- dwelling fishes, they may also present unique problems that could jeopardize the survival of its inhabitants. Physiological studies have shown that coral tissue becomes hypoxic at night (Jones and Hoegh-Guldberg, 2001). It is therefore possible that water between the coral branches becomes hypoxic on calm nights because of the combined effects of the nocturnal cessation of photosynthesis, continued respiration of the coral (and associated organisms), and lack of convective water movements. Furthermore, the entire coral colony may be exposed to air during spring tides. For example, coral colonies on the reef flat at Lizard Island (Great Barrier Reef) may be completely air exposed for 1–4·h during exceptionally low tides that occur up to 30 times per year. Despite these potential problems, coral gobies rarely leave the shelter of their host coral colonies, even when corals are fully exposed at low tide. Staying there under such conditions could be considered an act of bravery. In this study we investigated hypoxia tolerance in the broad- barred goby, Gobiodon histrio, a common coral-dwelling goby on the Great Barrier Reef (Munday et al., 1999). This goby exhibits a strong preference for colonies of Acropora nasuta (Munday et al., 1997, 2001). First, we estimated the nocturnal water O 2 level likely to be experienced by gobies inhabiting A. nasuta on calm nights. We then used closed-system The Journal of Experimental Biology 207, 33-39 Published by The Company of Biologists 2004 doi:10.1242/jeb.00713 Coral reef fishes are not known for their hypoxia tolerance. The coral-dwelling goby, Gobiodon histrio, rarely leaves the shelter of its host coral colony. However, our measurements indicate that this habitat could become hypoxic on calm nights ([O 2 ] minima=2–30% of air saturation) due to respiration by the coral and associated organisms. Moreover, at very low tides, the whole coral colony can be completely air exposed. Using closed respirometry in water, we found that G. histrio maintains O 2 uptake down to 18% of air saturation, and that it can tolerate at least 2·h at even lower O 2 levels. Furthermore, during air exposure, which was tolerated for more than 3·h, it upheld a rate of O 2 consumption that was 60% of that in water. The hypoxia tolerance and air breathing abilities enables this fish to stay in the safety of its coral home even when exposed to severe hypoxia or air. To our knowledge, this is the first report of hypoxia tolerance in a teleost fish intimately associated with coral reefs. Key words: hypoxia, Gobiidae, air breathing, Great Barrier Reef, Gobiodon histrio. Summary Introduction Coward or braveheart: extreme habitat fidelity through hypoxia tolerance in a coral-dwelling goby Göran E. Nilsson 1, *, Jean-Paul Hobbs 2 , Philip L. Munday 2 and Sara Östlund-Nilsson 1 1 Department of Biology, University of Oslo, PO Box 1051 Oslo, Norway and 2 School of Marine Biology and Aquaculture, James Cook University, Townsville, Australia *Author for correspondence (e-mail: g.e.nilsson@bio.uio.no) Accepted 9 September 2003