Habitat selectivity of megalopae and juvenile mud crabs (Scylla serrata): implications for recruitment mechanism

Megalopae of several crab species exhibit active habitat selection when settling. These megalopae usually select structurally complex habitats which can provide refuge and food. The portunid mud crab, Scylla serrata, is commonly found within the muddy estuaries of the Indo-West Pacific after attaining a carapace width > 40 mm. Despite substantial efforts, the recruitment mechanism of juvenile mud crabs to estuaries is not understood because their megalopae and early stage crablets (carapace width < 30 mm) are rarely found. We used laboratory experiments to determine whether megalopae and early stage crablets are selective among three estuarine habitats which commonly occur in Queensland, Australia. These animals were placed in arenas where they had a choice of habitats: seagrass, mud or sand, and arenas where they had no choice. Contrary to the associations exhibited by other portunid crab megalopae, S. serrata megalopae were not selective among these estuarine habitats, suggesting that they tend not to encounter these habitats, or, gain no advantage by selecting one over the others. The crablets, however, strongly selected seagrass, suggesting that residing within seagrass is beneficial to the crablets and likely increases survival. This supports the model that for S. serrata, crablets and not megalopae tend to colonise estuaries, since a selective behaviour has evolved within crablets but not megalopae. Introduction Organisms are rarely randomly distributed throughout the environment (Condit et al. 2000; Bertness et al. 2001). For animals, this non-random distribution is generated by mechanisms that may or may not involve habitat choice. Mortality is one mechanism; animals which recruit randomly and subsequently die in inhospitable places but survive elsewhere will be non-randomly distributed and associated with the hospitable habitats (Crowe and Underwood 1998). This mechanism does not require the exercise of choice or selection yet generates habitat associations. Alternatively, animals may make choices about where they live. For pelagic larvae, choices among settlement habitats may initially establish a non-random distribution (Orth and van Montfrans 1987; Moksnes 2002). Postsettlement movements of individuals towards particular chosen habitat types can redistribute the population (Moksnes 2002; Lecchini et al. 2007). These latter mechanisms involve the selection of particular habitats to create non-random distributions of animals. Organisms are constantly faced with choices, for example choices about feeding, habitat, and breeding. It is a tenet of evolutionary theory that, collectively, these choices or selections serve to enhance the fitness of future generations (Gould and Lewontin 1979; Krebs and Davies 1997). The postlarvae of several decapod species actively choose to settle to habitats that provide refuge or abundant food, or move into these beneficial habitats shortly after settling (Dionne et al. 2003; Moksnes et al. 2003; van Montfrans et Mar Biol (2009) 166:891-899 Habitat selectivity by mud crabs 2 al. 2003). Understanding which habitats are chosen by settling animals enables us to rank the habitats according to the likelihood that they enhance the survival of that species. Places that increase the probability of juveniles surviving and subsequently contributing to future generations can be considered nursery habitats (Beck et al. 2001). Nursery habitats might not, however, be the dominant contributor to future adult populations. Their area may be small compared to other habitats and therefore, despite being relatively more productive, they could contribute a smaller proportion of the future adults compared to larger habitats (Dahlgren et al. 2006). However, highly productive places such as nursery habitats may provide the population a degree of reproductive resilience, which is important during periods of disturbance (Apostolaki et al. 2002). Nursery habitats, therefore, may contribute subtly to the persistence of populations over an evolutionary timescale by enhancing the success of juvenile recruitment in times of more variable disturbance. Seagrass beds are often cited as nursery habitats for juvenile marine animals because they provide refuge from predators and a greater abundance of food (Jackson et al. 2001). For example, the juveniles of two portunid species, the North American blue crab (Callinectes sapidus) and Indo-West Pacific blue swimmer crab (Portunus pelagicus) are associated with seagrass (Orth and van Montfrans 1987; Kenyon et al. 1999). Refuge may be provided by the complex structure of seagrass beds restricting the movement of larger animals potentially preying on the small juveniles. Paradoxically however, the greater abundance of small animals seeking refuge within these habitats may attract more of their predators, preying on those seeking shelter (Connolly 1994; Franco et al. 2006). The postlarvae of many decapod species are strongly selective among settlement habitats, tending to select habitats with complex structures. For example, the number of Carcinus maenas postlarvae settling to structurally complex habitats such as mussel, algae, and eelgrass patches was more than ten times greater than that settling to sand (Moksnes 2002). Where juvenile marine animals strongly select for particular habitats, the behaviour has likely evolved over time because of the benefits gained, such as refuge from predation and enhanced growth (Perkins-Visser et al. 1996; Moksnes et al. 1998). The mud crab (Scylla serrata) is an economically and recreationally important portunid crab distributed throughout the coasts of the Indo-West Pacific. Adult mud crabs are generally found in muddy, mangrove-lined estuaries, and the ovigerous females move offshore to spawn (Hill 1994). Crabs which have a dispersive coastal larval stage and occur within estuaries as adults, usually colonise coastal habitats as megalopae or postlarvae. For example, the velvet swimmer crab megalopae (Necora puber) and blue crab megalopae colonise estuaries and tend to settle to complex estuarine habitats (Tankersley et al. 2002; Lee et al. 2006). Despite considerable searching effort however, mud crab megalopae (total length ca. 5 mm) are rarely found within estuaries (Arriola 1940; Heasman 1980; Forbes and Hay 1988; Knuckey 1999; Moser and Macintosh 2001; Walton et al. 2006). Explanations for rarely finding mud crab megalopae include episodic recruitment, cryptic behaviour, or simply that they tend not to colonise estuaries as