DOI: 10.5061/dryad.9zw3r22rn
title: Alteration of cleaner wrasse cognition and brain morphology under marine heatwaves

description: Heatwaves, exacerbated by global warming, are progressively affecting
 various ecosystems, with coral reefs among the most susceptible. Within
 these ecosystems, cleaner wrasses (Labroides dimidiatus), engage in
 cooperative interactions with client fish by removing ectoparasites, and
 play an essential role in sustaining client abundance and
 diversity. In 2016, the northern section of the Great Barrier
 Reef experienced widespread and intense bleaching due to unparalleled
 ocean temperatures associated with a marine heatwave. While prior studies
 have connected changes in fish densities following this heatwave to
 modifications in cleaner fish cognitive performance, the immediate impact
 of heatwave exposure on cleaner fish cognition and brain structure has yet
 to be investigated. Here, we exposed cleaner wrasses to a
 laboratory-simulated Category 1 marine heatwave for 55 days, mirroring the
 2016 Great Barrier Reef event. Cleaners’ cognitive performance was
 evaluated through a visual discrimination task during the heatwave and
 after a 14-day recovery phase. This was followed by analysis of brain
 development 30 days after the cessation of the marine heatwave. Our
 results demonstrate that although heatwave exposure temporarily hindered
 cognitive performance, these deficits were recoverable. Interestingly,
 cleaner fish brain morphology, measured after recovery, underwent
 significant changes. Specifically, despite cleaners exposed to heatwaves
 having notably larger brains, their telencephalon was substantially
 smaller, while their brainstem was enlarged. These findings indicate that
 while some cognitive effects may be reversible, marine heatwave exposure
 leads to lasting alterations in brain morphology, particularly in regions
 associated with higher cognitive functions and social behaviour. This
 raises questions about the potential impact on more complex tasks that
 rely on these brain regions. We argue that the significant disruptions in
 cleaners' cognitive performance observed months after the 2016 due to
 neurological impairments linked to brain morphological changes. If so, a
 mere recovery of fish densities may not necessarily lead to a restoration
 of cognitive performance, as experiencing marine heatwaves might induce
 life-long morphological alterations in fish. Our results underscore marine
 heatwaves' intricate and enduring impact on cleaner fish, emphasising
 the need for comprehensive strategies to safeguard these vital components
 of coral reef ecosystems.
description: The dataset was collected from a laboratory experiment involving
 Bluestreak cleaner wrasses (<em>Labroides dimidiatus;
 </em>n=20), which were wild-caught in the Maldives and housed in
 flow-through aquaria at Laboratório Marítimo da Guia, Cascais, Portugal.
 Cleaners were exposed to two experimental treatments: control (28.8°C) and
 a simulated Category I marine heatwave (29.8°C). After a 5-day acclimation
 period, temperature was increased over 6 days to reach the target
 temperature for the heatwave treatment, and exposure lasted for 55 days.
 Afterward, a 30-day recovery period followed before brain
 dissection. Behavioral trials, using an associative
 learning task (FAP test), were conducted at the end of the heatwave
 exposure and after a 14-day recovery period. Cleaners were tested
 individually on their ability to learn colour cues to locate a food
 reward, with up to 100 trials per fish. Brain
 development was analyzed 30-days post exposure, with brains dissected into
 five regions and weighed for further analysis. Statistical analyses were
 performed using R, including survival analysis for learning tasks and
 weighted linear models for brain development. The
 dataset includes variables on experimental treatments, task performance,
 body weight, brain weight, and brain region measurements.
description: # Alteration of cleaner wrasse cognition and brain morphology under marine
 heatwaves
 [https://doi.org/10.5061/dryad.9zw3r22rn](https://doi.org/10.5061/dryad.9zw3r22rn) ## Description of the data and file structure In this study, we investigate whether the response to environmental stress in a laboratory setting would parallel the observations of diminished strategic sophistication in cleaners, as reported by Triki et al. (2018) in the field. To explore this, individuals were subjected to a simulated Category 1 marine heatwave lasting 55 days, replicating the conditions of the 2016 El Niño event. Cleaners’ cognitive performance was evaluated through a visual discrimination task during the heatwave and after a 14-day recovery phase. This was followed by analysis of brain development 30 days after the cessation of the marine heatwave. The potential alignment of our laboratory findings with the field observations would imply that the impairment of cleaners’ cognitive abilities could be linked to the direct consequences of a temperature peak affecting their physiological mechanisms. ### Files and variables #### File 1: brain\_data.csv **Description:** Brain development data ##### Variables * ID: fish ID * Treatment: experimental treatment (Control or Marine Heatwave I) * body: body weight (g) * sl: standard length (cm) * brain: total brain weight (g) * bs: brainstem weight (g) * cer: cerebelum weight (g) * die: diencephalum weight (g) * ot: optic tetum weight (g) * tel: telencephalum weight (g) * bbo: brain/body ration * bsb: brain-stem/body ration * cb: cerebelum/body ration * db: diencephalum/body ration * otb: optic tetum/body ration * tb: telencephalum/body ration #### **File 2: data\_cognition.csv** **Description:** cognitive performance data (associative learning task results) ##### Variables * ID:  fish ID * Treatment: experimental treatment (Control or Marine Heatwave I) * trials_1: trials at the end of exposure period * solved_1: task solved (yes/no) * colour_1: color of the correct plate for trial 1 * colour_2:  color of the correct plate for trial 2 * trials_2:  trials after 14-day recovery period * solved_2: task solved (yes/no) #### File 3: Water\_parameters.xlsx **Description**: This file contains seawater parameter data collected during an experiment assessing the effects of marine heatwaves.  **Structure**: * Page 1: Control treatment - Page 2: Marine Heatwave I treatment **Experimental periods:** Acclimation, ramp period, exposure to MHW, recovery period, and brain sampling **Date**: Represents the experimental days corresponding to each period **Seawater parameters:** Temperature (ºC), Oxygen (mg), pH, Salinity **Replication & Labeling:**  Each parameter was measured across four replicates, labeled as follows: * Temperature: T(ºC)_1, T(ºC)_2, T(ºC)_3, T(ºC)_4 * Oxygen: O2(mg)_1, O2(mg)_2, O2(mg)_3, O2(mg)_4 * pH: pH_1, pH_2, pH_3, pH_4 * Salinity: salinity_1, salinity_2, salinity_3, salinity_4 **Averaged Values:** Represents the daily mean of each parameter across the four replicates. #### File 4: Statistical\_Analysis.pdf Description: This file provides details on the statistical methods used to analyze data_cognition.csv and brain_data.csv * The file includes information on data processing, statistical tests, and significance levels. * To analyze the data, the free and open-source software **R** (Version 3.4.3, RStudio Team, 2022) was used. * **R packages used:**  * **survival (for survival analysis and Cox proportional hazards models)**: Used to analyze time-to-event data from the learning task. * **survminer (for survival curve visualization)**: Utilized for plotting Kaplan-Meier survival curves. * **car (for analysis of variance)**: Applied to test the homogeneity of variances in experimental models. * **performance (for model diagnostics)**: Used to assess normality and homogeneity of residuals in linear models. * **dplyr (for data manipulation)**: Applied to filter, group, and summarize data efficiently during preprocessing. * **stats (for statistical modeling)**: Used to perform weighted general linear models and hypothesis testing. * **ggthemes (for plot aesthetics)**: Provided additional themes and color palettes to enhance ggplot2 visualizations. * **ggplot2 (for data visualization)**: Used to generate plots and graphs for exploratory data analysis and presentation. * **HighstatLibV10 (for data exploration and validation)**: Applied to detect collinearity, outliers, and heteroscedasticity in data.