Article: Rossi T, Nagelkerken I, Simpson SD, Pistevos JCA, Watson S-A, Merillet L, Fraser P, Munday PL, Connell SD. 2015 Ocean acidification boosts larval fish development but reduces the window of opportunity for successful settlement. Proc. R. Soc. B 282: 20151954. DOI: 10.1098/rspb.2015.1954
Sound is of huge importance in the marine environment (as you might have noticed from this week’s OceanBites posts)! Sound pressure waves carry information long distances due to the low attenuation of acoustic energy in water. This information can be in the form of communication (animals “talking” to one another), or proximity detection (helping “show” where and how far objects are). Acoustic cues have been shown to be an extremely important aid in the settlement behavior of fishes and some invertebrates. In other words, larval fishes are responding to the sounds of the ocean in order to find a safe and happy home to grow old in. Unfortunately, ocean acidification is proving to be a difficult barrier for young fish, disrupting their acoustic navigational skills and making it harder for them to find a suitable place to settle.
Nagelkerken et al. set out to examine the effects ocean acidification has on important behavioral traits needed in different stages of a fish’s life, such as sound-driven activities.
How the acoustic cues were tested in fish
The researchers chose to focus on the barramundi fish, a highly valued commercial and recreational fish whose habitat extends from the eastern Indian Ocean to the western Central Pacific. The barramundi is distinctive in that it is a catadromous species (migrates from fresh water to the salty ocean water in order to spawn).
Over the course of fifteen days barramundi (both barramundi raised in normal water conditions and barramundi raised in highly acidified waters) were observed to see if they were attracted to acoustic cues in the environment. These cues were a combination of video and sound recordings of a typical reef environment. For example, the snapping of shrimp would indicate that mangroves are nearby (an ideal settlement spot for the barramundi), and thus the barramundi would likely follow the snapping sound. Over the five minutes in which the sound cues were played, the behavior of the fish was noted: did the fish swim toward the sounds, away from them, or did they even seem to notice the sounds at all?
What they found and why it matters
Under acidified conditions (higher levels of CO2), larval barramundi grow faster and thus enter their metamorphosis phase earlier. This expedites their journey to find salt water and a safe home to settle. If that isn’t stressful enough, the effects of CO2 also affected the barramundi’s sound-driven orientation, swimming behavior, and sheltering behavior (Fig. 1)! When exposed to high CO2 levels, these poor fish became disoriented and lost. When the ocean acidity increased, the neurological pathways of the fish were disrupted and instead of heading towards a potentially safe home in the mangroves (indicated by the snapping of shrimp) the fish became confused and actually tended to swim AWAY from the sound cues (as seen in this video).
Fish can use a suite of senses to locate suitable homes; sound, olfaction (smell), and vision. Sound travels the best in water and is therefore the most important cue for fish, but with the increasing CO2 levels and ocean acidification, sound cues are proving unreliable and even hindering the fish’s ability to find a proper settlement space. While olfaction and vision could potentially compensate for the ineffective processing of the auditory sensory, they, too, are likely to be impaired. Further studies are needed on this, but the results will likely be similar and show just how damaging ocean acidification can be. More and more fish will become lost, making themselves vulnerable to predation and making it difficult for them to successfully reproduce.