Loneliness on the reef

New research suggests the usually social butterflyfish is becoming more of a recluse as the coral reefs it depends on degenerate.

Cassandra Thompson is the lead author of a new study published in Coral Reefs that relates live coral cover to the social behaviour of the Chaetodon genus of fish. She says the percent of live coral on a reef has a particularly significant effect on the relationships of Chaetodon baronessa—the eastern-triangular butterflyfish.

This particular butterflyfish is normally observed swimming in pairs. However, Thompson says only one pair was observed during 2017 surveys at Lizard Island, in the northern sector of the Great Barrier Reef. Otherwise, she says, they swam as solitary individuals. At this time, successive habitat degradation caused coral cover to reach an all-time low, around seven percent.

The loss of live coral was caused by several disturbance events in addition to a mass bleaching event in 2016. This includes the Crown of Thorns Starfish outbreaks (1995-1999 and 2009-2015), and severe tropical cyclones (2014, 2015).

The degradation and loss of reef habitat is one of the leading threats to coral reef fishes. This is especially relevant to highly specialised species, such as the eastern-triangular butterflyfish, which rely on very specific species of live corals.

Social relationships are important in most animal groups. In particular, pairing is a useful behaviour that can benefit both resource protection and consumption. And of course, in heterosexual pairs, it often leads to babies. Social connections, or sociality, also influence a species’ ability to survive and persist throughout large-scale disturbances.

“The eastern-triangular butterflyfish is the species that shows the strongest positive effect of coral cover on sociality,” says Thompson.

“So, they are more often found in pairs when coral cover is high, and solitary when coral cover is low.”

“When conditions are favourable, or normal, they re-pair within a matter of days, sometimes hours after losing a partner, but we are not seeing that at the moment.”

“The few we observed—apart from that one pair—were solitary, possibly due to the large distances between potential mates and their high reliance on the most affected corals.”

While the overall trend across all species sees the number of pairs decreasing, interestingly, three species (Threadfin, Speckled, and Vagabond butterflyfish) that feed on a wider variety of corals than the eastern-triangular butterflyfish, displayed the opposite effects: as coral cover declined, they were observed in pairs more often. This is likely due to solitary individuals diminishing as food resources became scarce.

“This is more related to the lack of solitary individuals, rather than an increase in pairs,” says Thompson. “But it does give some hope for recovery because paired individuals are normally healthier than solitary fish, as they can more easily defend resources and have continual access to a mate when the urge to make babies occurs.”

Previous studies on the abundance of Chaetodon butterflyfish neglect their sociality, though it was suggested as early as 1981 that pairing incidences should be recorded.

Thompson’s study forms part of a long-term data set on coral cover and butterflyfish abundance at Lizard Island that Prof Morgan Pratchett has looked after since 1995.

Thompson C, Matthews S, Hoey A, Pratchett M (2019). ‘Changes in sociality of butterflyfishes linked to population declines and coral loss’. Coral Reefs: https://doi.org/10.1007/s00338-019-01792-x

You can follow Cassy on Twitter @cassdives and her studies with the hashtag: #staysocialfish