Van Nynatten, A. | Search bioRxiv

bioRxiv2026

Low Symbiodiniaceae diversity across Acropora hosts on the Central Pacific atoll Kiritimati

The transformative impacts of global bleaching events are now a ubiquitous feature of coral reefs, often resulting in taxonomically simplified coral assemblages that can persist long-term due to ongoing ocean warming, heatwaves, and localized anthropogenic stressors. Understanding to what extent these processes shape symbiont biodiversity continues to be a major knowledge gap. Here, we examined Acropora host and symbiont communities on Kiritimati (Republic of Kiribati), seven and a half years after the 20152016 El Nino-associated marine heatwave, which caused a near total loss of the genus in the atoll's shallow reefs. We quantified Acropora cover and growth-form composition at 37 sites spanning a gradient of local human disturbance and characterised Symbiodiniaceae communities using ITS2 DNA metabarcoding from 109 coral tissue samples at 13 of the sites. Corymbose and tabular Acropora growth forms remained evenly represented across the atoll and cover declined with increasing human disturbance. Symbiodiniaceae communities were dominated by Cladocopium with 93 % of colonies associated with a single ITS2 profile. Communities were homogenous, with low diversity and insignificant differences amongst growth forms, but slight differences across disturbance levels. DNA barcoding of the PaxC intron identified the majority of host colonies as most similar to Acropora granulosa (62 %) and Acropora hyacinthus (24 %). Although dominance by a single symbiont lineage is common among Acropora species across the Pacific, the low Symbiodiniaceae diversity observed at Kiritimati contrasts with the substantial within- and among-lineage diversity documented elsewhere. The depauperate Symbiodiniaceae communities observed in this study may thus represent a lasting legacy of the 2015-2016 disturbance, with the recovering Acropora assemblage representing an early stage of community reassembly drawing from a potentially depleted environmental symbiont pool.

ecology↗

bioRxiv2023

Evolution of rhodopsin in flatfishes (Pleuronectiformes) is associated with depth and migratory behaviour

Visual signals are involved in many fitness-related tasks and are therefore essential for survival in many species. Aquatic organisms are ideal systems to study visual evolution, as the high diversity of spectral properties in aquatic environments generates great potential for adaptation to different light conditions. Flatfishes are an economically important group, with over 800 described species distributed globally, including halibut, flounder, sole and turbot. The diversity of flatfish species and wide array of environments they occupy provides an excellent opportunity to understand how this variation translates to molecular adaptation of vision genes. Using models of molecular evolution, we investigated how the light environments inhabited by different flatfish lineages have shaped evolution in the rhodopsin gene, which is responsible for mediating dim-light visual transduction. We found strong evidence for positive selection in rhodopsin, and this was correlated with both migratory behaviour and several fundamental aspects of habitat, including depth and freshwater/marine evolutionary transitions. We also identified several mutations that likely affect the wavelength of peak absorbance of rhodopsin, and outline how these shifts in absorbance correlate with response to the light spectrum present in different habitats. This is the first study of rhodopsin evolution in flatfishes that considers their extensive diversity, and our results highlight how ecologically-driven molecular adaptation has occurred across this group in response to transitions to novel light environments.

evolutionary biology↗