Repeated cyclone events reveal potential causes of sociality in coral-dwelling Gobiodon fishes

Abstract

Social organization is a key factor influencing a species’ foraging and reproduction, which may ultimately affect their survival and ability to recover from catastrophic disturbance. Severe weather events such as cyclones can have devastating impacts to the physical structure of coral reefs and on the abundance and distribution of its faunal communities. Despite the importance of social organization to a species’ survival, relatively little is known about how major disturbances such as tropical cyclones may affect social structures or how different social strategies affect a species’ ability to cope with disturbance. We sampled group sizes and coral sizes of group-forming and pair-forming species of the Gobiid genus Gobiodon at Lizard Island, Great Barrier Reef, Australia, before and after two successive category 4 tropical cyclones. Group sizes of group-forming species decreased after each cyclone, but showed signs of recovery four months after the first cyclone. A similar increase in group sizes was not evident in group-forming species after the second cyclone. There was no change in mean pair-forming group size after either cyclone. Coral sizes inhabited by both group- and pair-forming species decreased throughout the study, meaning that group-forming species were forced to occupy smaller corals on average than before cyclone activity. This may reduce their capacity to maintain larger group sizes through multiple processes. We discuss these patterns in light of two non-exclusive hypotheses regarding the drivers of sociality in Gobiodon, suggesting that benefits of philopatry with regards to habitat quality may underpin the formation of social groups in this genus.

Size-specific recolonization success by coral-dwelling damselfishes moderates resilience to habitat loss

“…Moreover, very severe and large-scale disturbances caused by anthropogenic climate change, will increase the extent of coral loss 22 and have disproportionate impacts on branching corals 83 . Highly specialized fishes with specific reliance on live corals for food or habitat are therefore, extremely vulnerable to sustained and ongoing changes in the condition and structure of coral reef ecosystems, as has been suggested previously 11,12,40,82,84,85 .…”

“…Even so, we did record at least some displaced fishes (46.7%) recolonizing alternative coral hosts within the experimental plots. The fact these movements did not buffer against overall coral loss is attributable to limited persistence of elevated densities of coral-dwelling fishes on these surviving coral colonies, possibly reflecting inherent limits on the number of fishes that can be sustained within individual corals 10,74,82 . Given our inability to distinguish individual fishes of the same size we do not know whether fishes that moved ultimately succumbed, or whether these individuals effectively displaced other fishes of equivalent size 45 .…”

Habitat size, health and saturation do not alter movement decisions or the preference for familiarity in a social coral-reef fish

“…Both habitat size (small or large) and habitat saturation (0, 1, or 2 residents) were included as fixed factors along with the following covariables: distance to home coral, number of gobies in home coral, proportion of uninhabited corals within 10-m radius, and average group size of conspecifics in inhabited corals within 10-m radius. Recruits (distinguished from other life stages by distinct colour and markings, Hing et al 2018) in the home coral were not included in analysis, because recruits often move between corals before settlement (Froehlich pers. obs.…”

Uneven declines between corals and cryptobenthic fish symbionts from multiple disturbances

“…However, goby group sizes were 28% smaller (p < 0.001), with gobies mostly occurring in pairs, and less so in groups (Fig 2e). Smaller groups were likely due to their coral hosts being smaller than before the cyclones as there is an indirect link between group size and coral size (32).…”

“…Declines from a single disturbance have the potential for a resilience, but multiple events will require long-term recovery (31,32) as most corals are uninhabited after consecutive disturbances (Fig 1b). Although the disturbances in this study were compounded, heatwaves may have had an even stronger effect on gobies since goby communities differed the most after the heatwaves, whereas coral communities remained similarly diverse after each disturbance.…”