Crustacea on coral reefs: habitat associations and trophic relationships

Crustaceans are one of the most widespread and speciose marine groups, performing key ecological roles to many ecosystems. On coral reefs they are part of one of the most diverse and complex ecosystems on Earth. It would be reasonable to expect that crustaceans are important coral reef organisms, yet the body of work concerning coral reef crustaceans pales in comparison to other popular groups, such as fishes and corals. To rectify this, I investigated the importance of crustaceans as a component of the coral reef faunal assemblage, quantifying their distribution and abundance across a range of reef microhabitats, comparing the tropical assemblage to the better-understood temperate assemblage, and examining the role of reef Crustacea as a dietary resource for fishes. To determine the community structure, abundance, biomass and productivity of benthic Crustacea on a typical coral reef, I investigated 5 major microhabitats: dead coral, coral rubble, sand, epilithic algal matrix (EAM) and fine-branching live coral at Lizard Island, a mid-shelf reef on the Great Barrier Reef, Australia (Chapter 2). Crustacean communities differed significantly among habitats. Dead coral was by far the most important microhabitat type in terms of crustacean abundance (7838 ± 628 ind. 100 cm⁻², mean± SE), biomass (0.75 ± 0.13 g m⁻², wet weight) and estimated productivity (0.92 ± 0.13g100 cm⁻² yr⁻¹ ash-free dry weight). These values were 2 to 3 orders of magnitude greater than those for the least important habitats (EAM and fine-branching live coral). The average crustacean body length was just 0.79 ± 0.32 mm, largely due to the dominance of relatively small harpacticoid copepods. In contrast, decapods exhibited very low abundances, but yielded the greatest biomass and productivity and were particularly abundant in dead coral and coral rubble. The results highlighted the importance of small crustaceans and dead coral microhabitats as valuable contributors to the trophic structure of coral reefs. Although it is well established that fish, coral and algal assemblages vary across large spatial scales, very little is known of the differences in crustacean assemblages across similar scales. To determine whether crustaceans had similar spatial patterns to other reef organisms, I investigated the EAM cryptofaunal community, dominated by Crustacea, at three locations on the Great Barrier Reef: two inner shelf locations - Orpheus Island and the Turtle Island group - and a mid-shelf location, Lizard Island (Chapter 3). Although the EAM appears to be a relatively simple and consistent habitat, significant differences in cryptofaunal assemblages were found between locations. EAM assemblages from Orpheus Is land were markedly different to those from the Turtle Island group and Lizard Is land. This appears to be a function of the sediment profile (grain size >60 μm) at Orpheus Island, as many cryptofaunal taxa displayed a positive relationship with sediment volume. However, sediment volumes did not differ significantly between the three locations, highlighting the possibility of cyclonic activity affecting the sediment profile at Orpheus Island in the months preceding the study, in addition to the nutrient input from major terrigenous sources. The results show that EAM cryptofaunal assemblages are not uniform across the Great Barrier Reef and suggest that dissolved nutrients, sediment loads and distance from river systems may be significant drivers of cryptobenthic faunal compositions. Tropical and temperate marine habitats have long been recognised as fundamentally different systems; yet, comparative studies are rare, particularly for small organisms such as Crustacea. I investigated the ecological attributes (abundance, biomass and productivity) of benthic Crustacea in selected microhabitats from a tropical and a temperate location, revealing marked differences in the structure of crustacean assemblages (Chapter 4). In general, microhabitats from the tropical location (dead coral, the EAM and sand) supported high abundances of small individuals (mean size = 0.53 mm vs. 0.96 mm in temperate microhabitats), whilst temperate microhabitats (the brown seaweed Carpophyllum sp., coralline turf and sand) had substantially greater biomasses of crustaceans and higher estimated productivity rates. In both locations, the most important microhabitats for crustaceans (per unit area) were complex structures: tropical dead coral and temperate Carpophyllum sp. It appears that the differences between microhabitats are largely driven by the size and abundance of key crustacean groups. Temperate microhabitats have a higher proportion of relatively large Peracarida (Amphipoda and Isopoda), whereas tropical microhabitats are dominated by small detrital and microalgal feeding crustaceans (i.e. harpacticoid copepods and ostracods). These differences indicate the vulnerability of tropical systems, especially to the loss of complex benthic structures and the associated crustacean assemblages as a result of habitat degradation. Crustaceans are one of the most influential groups in aquatic trophic networks by providing a major connection between primary production and higher consumers. Although coral reefs support a high diversity and abundance of crustaceans, and crustacean predators, their trophic interrelationships remain unclear. Using predator gut content analyses, I investigated trophic relationships between Crustacea and adult fishes of the family Labridae, which are one of the most abundant and diverse families of marine crustacean predators (Chapter 5). Crustaceans were present within the guts of 93 % of the 30 wrasse genera investigated. I found a distinct division between micro- and macro-crustacean predators: wrasses 90 mm SL displayed a predominantly macro - crustacean diet. Notably, micro-crustacean predators tended to specialise on certain crustacean taxa, whereas macro-crustacean predators consumed mostly brachyurans. My findings highlight complex patterns of feeding diversity within crustacean predators that prompt a more detail-oriented approach to defining the role of crustacean-feeding fishes in coral reef trophodynamics. Having established Crustacea as an important component in coral reef ecosystems, occupying a broad range of coral reef microhabitats, with the greatest biomass in dead coral and coral rubble, I investigated the relationship between foraging in wrasses and the major reef microhabitats occupied by Crustacea (Chapter 6). Although the greatest biomass of crustaceans is in dead coral and coral rubble, crustacean-feeding wrasses displayed strong selection for a broader range of microhabitats. Of the 14 macro crustacean predators, only 6 selectively foraged in dead coral or coral rubble. The 10 micro-crustacean predators likewise displayed a wide range of microhabitat fo raging associations, reflecting specific prey type preferences. The relationships between crustacean predators and their prey appear to be more complicated than previously assumed, and may be mediated by other morphological and behavioural factors. This thesis represents an important contribution to the relatively new, emerging field of coral reef crustacean ecology. By establishing key baseline information about the contribution of crustaceans to the overall coral reef faunal assemblage, I have confirmed the long-held assumptions that crustaceans are highly abundant and potentially very important on coral reefs. However, crustacean assemblages differ considerably across large spatial scales due to local environmental factors. I also present the first tropical-temperate comparison of crustaceans within comparable microhabitats, which has given a new perspective on the trophic functioning of each ecosystem. Importantly, crustaceans are a major component of the diet of coral reef fishes; wrasses are a key example and provide evidence for a trophic division within crustacean-feeding taxa. However, the relationship between crustaceans and their fish predators appears to be complex, mediated by various factors including microhabitat, fish morphology and behaviour. Crustaceans have often been perceived as a group of organisms that are simply 'present' on coral reefs, yet this thesis demonstrates that their importance in reef trophodynamics cannot be underestimated. Crustacea have a pivotal role in coral reef ecology.