Carbon Dioxide Exposure Influences Habitat Choice of Coral-Associated Fish

by Hannah Calich, RJD Intern

Elevated carbon dioxide (CO₂) levels have been shown to not only impact the health of coral reefs, but the health of coral reef associated fish as well. Coral-dwelling gobies are among the most habitat-specialized fish on coral reefs. Since these fish are rarely found outside of their preferred habitat they are a great species to monitor when trying to determine how CO₂ levels influence the habitat choice of reef-fish.

A recent study by Devine and Munday (2013) aimed to determine how short-term exposure to high CO₂ levels influences the habitat preference of coral-dwelling gobies. This study used two goby species, yellow-green gobies, Paragobiodon xanthosomus, who preferentially inhabit only one coral species and broad-barred gobies, Gobiodon histrio, who preferentially inhabit a small number of coral species. The gobies were caught in the wild (Lizard Island, Australia) and moved to aquaria with continuous seawater pumps. The pumps either diffused ambient seawater (as a control, 440 µatm CO₂), or high levels of CO₂ (880 µatm) into the water. The fish remained in the aquaria for 4 days before the experiments began.

Laboratory Olfactory Cue Experiment

A laboratory experiment was used to determine if yellow-green gobies that have been exposed to high levels of CO₂ can distinguish between two olfactory (scent) cues. To do this, a goby that had been exposed to either ambient seawater or high levels of CO₂ was put into a container with two different coral species. The goby could not see the corals, but was able to smell them. The coral species were Seriatopora hystrix (the preferred coral species of yellow-green gobies) and Pocillopora damicornis (a non-preferred coral). The researchers noted which coral species the goby associated with during a 2-minute observation period. If the goby did not approach either coral it was recorded as making “no choice”. This procedure was repeated for 20 control gobies and 19 CO₂ exposed gobies.

Field Habitat Choice Experiment

A field experiment was used to determine if broad-barred gobies could locate an alternative habitat if their host coral died. To begin, a goby that had been exposed to either ambient seawater or high levels of CO₂ was released over a dead colony of Acropora nasuta (the preferred coral species of broad-barred gobies). Surrounding the dead coral was live colonies of A. nasuta and Acropora tenuis (a non-preferred coral species). Each goby had the option of either staying on the dead coral, moving to a live preferred coral, or moving to a live non-preferred coral. After 24 hours the researchers came back to the reef and documented what habitat the goby chose. This procedure was repeated for 20 control gobies and 20 CO₂ exposed gobies.

Laboratory Olfactory Cue Experiment

When yellow-green gobies were exposed to ambient seawater (as a control) they chose to swim towards their host corals 70% of the time. In contrast, the gobies exposed to elevated CO₂ displayed no preference for their host coral. In fact, only 16% of the gobies swam towards the host coral, 42% swam towards the non-host coral, and 42% did not choose a coral.

The number of control and CO2 exposed yellow-green gobies that associated with each coral, or displayed no preference. The black bars represent gobies exposed to ambient seawater and the white bars represent gobies exposed to elevated CO2. From left to right the gobies chose to move towards the preferred coral species (S. hystrix), the non-preferred coral species (P. damicornis), or neither. From Devine and Mumby 2013

Field Habitat Choice Experiment

Within 24 hours of being released over a dead host coral, 100% of the broad-barred gobies that were exposed to ambient seawater had relocated to a live preferred coral. In comparison, only 45% of gobies exposed to elevated CO₂ moved to a live preferred coral, 10% moved to a live non-preferred coral, and 45% remained on the dead coral.

The number of control and elevated CO2 exposed broad-barred gobies associated with each habitat type 24 hours after they were released over dead A. nasuta. The black bars represent gobies exposed to ambient seawater and white bars represent gobies exposed to elevated CO2. From left to right the coral choices were a live preferred host (A. nasuta), a live non-preferred host (A. tenuis), and a dead preferred host (where the gobies were initially released, A. nasuta). From Devine and Mumby 2013

Healthy gobies (those raised in ambient seawater) were more attracted to the scent of their preferred coral species than the scent a non-preferred coral species. Additionally, healthy gobies would move from a dead coral to a live preferred coral. However, gobies that had been exposed to high levels of CO₂ were rarely attracted to their preferred coral species. Additionally, when the CO₂ exposed gobies were released on a dead coral they may not leave it.

Results from this study suggest that permanently elevated CO₂ levels may significantly impact the ability of highly specialized fish to locate suitable habitat types following disturbances such as coral bleaching. If a goby is unable to locate a suitable habitat type it will be vulnerable to predation, malnutrition and eventually, death.

Occasionally, a goby may decide to inhabit a non-preferred coral when its preferred coral host dies. This is not an advantageous strategy for the goby as it is adapted to living within its preferred coral. Additionally, since gobies are naturally competitive, if they move onto a new coral they may displace other reef fish that were previously living on that coral. This displacement could theoretically cause a cascading effect that could influence the entire reef.

In conclusion, as CO₂ levels are expected to rise beyond the threshold of many coral communities within the next century, it is not only the corals that are at risk but the coral-associated species as well.

REFERENCE

Devine, B. M., & Munday, P. L. (2013). Habitat preferences of coral-associated fishes are altered by short-term exposure to elevated CO₂. Marine Biology, 160, 1955-1962. doi:10.1007/s00227-012-2051-1