Australian Tropical Marine Micromolluscs: An Overwhelming Bias

Characterising tropical oyster reefs: invertebrate-environment associations and a newly documented reef building species

Oyster reefs were once widespread across the global tropics but have often been excluded from global assessments of historical distributions, declines and conservation needs. Consequently, little is known about tropical oyster reefs, including ecological function, which species are reef-building, structural characterisations of remnant reefs, associated biodiversity, and whether conservation actions are needed to ensure their recovery. We compared the characteristics of newly documented tropical oyster reefs across three Australian locations to gain a foundational understanding of their ecology. At each location, structural reef traits were assessed, and associated invertebrate communities were quantified taxonomically and functionally. Location had a strong effect on invertebrate communities, with reefs in Gladstone hosting a greater abundance, and larger, invertebrates, followed by Mapoon, and then Proserpine. Most invertebrates were positively associated with the lower intertidal zone and shelly sediment - we hypothesise that heat stress, interstitial reef spaces, and the proportion of habitat edges could be explanations for these observed patterns, however, further research is needed to confirm this. Reef-building oysters identified using molecular markers at all locations were predominantly Saccostrea lineage B, a species broadly distributed across tropical Australia and the Indo-Pacific. These characterisations provide novel insights into the ecology of tropical oyster reefs and baseline information to inform appropriate conservation strategies.

Molluscan systematics: historical perspectives and the way ahead

Mollusca, the second-most diverse animal phylum, is estimated to have over 100,000 living species with great genetic and phenotypic diversity, a rich fossil record, and a considerable evolutionary significance. Early work on molluscan systematics was grounded in morphological and anatomical studies. With the transition from oligo gene Sanger sequencing to cutting-edge genomic sequencing technologies, molecular data has been increasingly utilised, providing abundant information for reconstructing the molluscan phylogenetic tree. However, relationships among and within most major lineages of Mollusca have long been contentious, often due to limited genetic markers, insufficient taxon sampling and phylogenetic conflict. Fortunately, remarkable progress in molluscan systematics has been made in recent years, which has shed light on how major molluscan groups have evolved. In this review of molluscan systematics, we first synthesise the current understanding of the molluscan Tree of Life at higher taxonomic levels. We then discuss how micromolluscs, which have adult individuals with a body size smaller than 5 mm, offer unique insights into Mollusca's vast diversity and deep phylogeny. Despite recent advancements, our knowledge of molluscan systematics and phylogeny still needs refinement. Further advancements in molluscan systematics will arise from integrating comprehensive data sets, including genome-scale data, exceptional fossils, and digital morphological data (including internal structures). Enhanced access to these data sets, combined with increased collaboration among morphologists, palaeontologists, evolutionary developmental biologists, and molecular phylogeneticists, will significantly advance this field.

Functional and phylogenetic responses of motile cryptofauna to habitat degradation

Biodiversity of terrestrial and marine ecosystems, including coral reefs, is dominated by small, often cryptic, invertebrate taxa that play important roles in ecosystem structure and functioning. While cryptofauna community structure is determined by strong small-scale microhabitat associations, the extent to which ecological and environmental factors shape these communities are largely unknown, as is the relative importance of particular microhabitats in supporting reef trophodynamics from the bottom up. The goal of this study was to address these knowledge gaps, provided coral reefs are increasingly exposed to multiple disturbances and environmental gradients that influence habitat complexity, condition and ecosystem functioning. We compared the density, biomass, size range, phylogenetic diversity and functional roles of motile cryptofauna in Palau, Western Micronesia, among four coral-derived microhabitats representing various states of degradation (live coral [Acropora and Pocillopora], dead coral and coral rubble) from reefs along a gradient of effluent exposure. In total, 122 families across ten phyla were identified, dominated by the Arthropoda (Crustacea) and Mollusca. Cryptofauna biomass was greatest in live Pocillopora, while coral rubble contained the greatest density and diversity. Size ranges were broader in live corals than both dead coral and rubble. From a bottom-up perspective, effluent exposure had mixed effects on cryptic communities including a decline in total biomass in rubble. From a top-down perspective, cryptofauna were generally unaffected by predator biomass. Our data show that, as coral reef ecosystems continue to decline in response to more frequent and severe disturbances, habitats other than live coral may become increasingly important in supporting coral reef biodiversity and food webs.