Gobies on coral reefs

The family Gobiidae comprises at least 2000 species (Nelson, 2006), with many still undiscovered or undescribed. More than half of all goby species are found on coral reefs and thus they contribute greatly to the astonishing fish diversity of this environment. Most coral reef gobies are closely associated with the substratum, where they seek shelter from predators in the complex physical structure of reef habitat (Munday and Jones, 1998). As a consequence of their small size and cryptic life-style, careful investigations by trained experts are necessary to reveal their full diversity. Although there are few such researchers, new species are still being described (e.g. Coryphopterus (Victor, 2007), Eviota (Gill and Jewett, 2004), Gobiodon (Harold and Winterbottom, 1995; Harold and Winterbottom, 1999; Winterbottom and Harold, 2005), Priolepis (Nogawa and Endo, 2007) and Trimma (e.g. Winterbottom, 2004, 2006; Hagiwara and Winterbottom, 2007; Suzuki and Senou, 2007; Winterbottom and Zur, 2007). Despite the diversity of coral reef gobies-the genus Trimma, for example, may include nearly 100 species (Winterbottom and Southcott, 2008)-most species are easily overlooked and are commonly excluded from sampling or ecological surveys. However, a recent upsurge of interest in the ecological role of small reef fishes (e.g. Munday and Jones, 1998; Depczynski and Bellwood, 2003) and the use of several coral-reef goby genera as model groups in ecology and evolution (e.g. Forrester and Steele, 2000; Munday et al., 2001; Taylor and Hellberg, 2003; Munday, 2004b) has greatly increased our understanding of the biology and ecology of this diverse and interesting group of coral reef fishes. Gobies are not only diverse in terms of species richness, but also with regard to their life-styles and life-histories. Gobies occupy all major habitats around coral reefs, including sand and rubble, seagrass meadows, coral reef matrix, live corals and numerous other living organisms (Greenfield and Johnson, 1999; Nakamura and Sano, 2004; Syms and Jones, 2004). Size diminution has played an important role in the evolution of coral reef gobies and has enabled them to utilize many restricted or specialized microhabitats (Munday and Jones, 1998; Herler, 2007). The most habitatspecialized gobies are associated with invertebrates such as sponges, soft coral and hard coral (Larson, 1985, 1990; Munday et al., 1997; Herler and Hilgers, 2005). Others do not use invertebrates as habitat, but use the structures created by them, such as the burrows of shrimps. Mutualistic relationships between shrimps and gobies are found in about 15 genera and most are obligate associates of burrowing alpheids. Coral reef gobies hold two world records: the smallest size and the shortest lifespan among marine vertebrates (Winterbottom, 1990; Depczynski and Bellwood, 2005; Winterbottom and Southcott, 2008). Most reef gobies are short-lived species. Many are annuals (Kritzer, 2002; Hernaman and Munday, 2005a) and some have generation times of just a few months (Depczynski and Bellwood, 2005; Depczynski et al., 2007; Winterbottom and Southcott, 2008). They are also among the most abundant families of fishes on coral reefs (Ackerman and Bellwood, 2000). High abundance combined with short lifespans means that they account for a very large proportion of the total growth production of reef fishes (Depczynski et al., 2007). Consequently, these tiny reef fish contribute significantly to energy transfer throughout coral reef systems. Minute body size and short lifespan are also associated with a range of morphological adaptations and reductions in the skeleton, fins, squamation and lateral line system in the smallest species (Winterbottom, 1990; Herler, 2007; Winterbottom and Zur, 2007). High predation risk and a short lifespan are associated with early maturation in many small coral reef gobies. Early maturation is advantageous because it results in a higher probability of survival to first reproduction and shorter generation times (Stearns, 1992). A disadvantage is that maturation at a small body size limits batch fecundity and, potentially, spawning opportunities (i.e., in social systems where larger size confers greater success in competition for mates or spawning sites). Perhaps as a result of these limitations, many coral reef gobies invest in somatic growth throughout life including during reproduction (Hernaman and Munday, 2005a), which contrasts with many small, long-lived coral reef fishes where growth ceases soon after sexual maturation (e.g. Choat and Robertson, 2002). Coral reef gobies also exhibit a diversity of reproductive behaviours and patterns of social organisation. Most, if not all, coral reef gobies are substrate spawners with parental egg care, but mating strategies vary from monogamy to polygyny (e.g. Kuwamura et al., 1993; Sunobe and Nakazono, 1999a, 1999b; Takegaki and Nakazono, 1999; Hernaman and Munday, 2005b). Some species form complex social hierarchies where individuals queue for breeding rights, and growth rates are controlled by social interactions (Wong et al., 2007, 2008a, 2008b). Many species also exhibit remarkable sexual flexibility and are able to change sex, in some cases more than once (Nakashima et al., 1996; Munday et al., 1998; Manabe et al., 2007a). In this chapter we examine the ecology, life-histories and reproductive behaviour of coral reef gobies. We also explore morphological adaptations that are associated with the unique life-styles exhibited by many coral reef gobies, and the extreme miniaturization of certain groups of species. Although frequently overlooked in studies of reef fish assemblages due to their small size and cryptic behaviour, we show that gobies play an important role in coral reef ecoystems, and that they have proven to be a useful model group for studying a broad range of topics in ecology and evolution, including: species diversification, ecological and morphological adaptation, the evolution of animal life history traits, mechanisms of community structure and population regulation, and the evolution of animal behaviour