Reconstructing reef fish communities using fish otoliths in coral reef sediments.

Little is known about long-term changes in coral reef fish communities. Here we present a new technique that leverages fish otoliths in reef sediments to reconstruct coral reef fish communities. We found over 5,400 otoliths in 169 modern and mid-Holocene bulk samples from Caribbean Panama and Dominican Republic mid-Holocene and modern reefs, demonstrating otoliths are abundant in reef sediments. With a specially-built reference collection, we were able to assign over 4,400 otoliths to one of 56 taxa (35 families) though mostly at genus and family level. Many otoliths were from juvenile fishes for which identification is challenging. Richness (by rarefaction) of otolith assemblages was slightly higher in modern than mid-Holocene reefs, but further analyses are required to elucidate the underlying causes. We compared the living fish communities, sampled using icthyocide, with the sediment otolith assemblages on four reefs finding the otolith assemblages faithfully capture the general composition of the living fish communities. Radiocarbon dating performed directly on the otoliths suggests that relatively little mixing of sediment layers particularly on actively accreting branching coral reefs. All otolith assemblages were strongly dominated by small, fast-turnover fish taxa and juvenile individuals, and our exploration on taxonomy, functional ecology and taphonomy lead us to the conclusion that intense predation is likely the most important process for otolith accumulation in reef sediments. We conclude that otolith assemblages in modern and fossil reef sediments can provide a powerful tool to explore ecological changes in reef fish communities over time and space.

Young species of cupuladriid bryozoans occupied new Caribbean habitats faster than old species.

The breadth of habitat occupied by a species, and the rate at which a species can expand into new habitats has important ecological and evolutionary consequences. Here we explore when extant species of free-living cupuladriid bryozoans expanded into new benthic Caribbean habitats that emerged during the final stages of formation of the Isthmus of Panama. Habitat breadth was estimated using the abundances of over 90,000 colonies in ten cupuladriid species, along with the ecological and sedimentary characteristics of the samples in which they occurred. Data reveal that all species expanded their habitat breadths during the last 6 Myr, but did so at a different tempo. 'Young' species - those that originated after 5 Ma - expanded relatively quickly, whereas 'old' species - those that originated before 9 Ma - took a further 2 Myr to achieve a comparable level of expansion. We propose that, like invasive species, young species are less restrained when expanding their habitat breadths compared to older well-established species. Understanding the mechanism causing this restraint requires further research.