Irisson, J. O., Paris, C. B., Leis, J. M., & Yerman, M. N. (2015). With a little help from my friends: Group orientation by larvae of a coral reef fish. PLoS ONE, 10(12), 1–14. doi:10.1371/journal.pone.0144060
To shoal or not to shoal?
The ocean is a challenging environment for most creatures, and larval fish are no exception. Many species of fishes reproduce through external fertilization, their eggs floating along in the ocean currents before developing into larvae that have a fighting chance of swimming against the flows and finding suitable areas in which to settle. And make no mistake—larval fish should never be underestimated in how far or fast they can swim. In fact, late-stage larvae actively searching for a new home reef are able to swim up to ~6 inches per second for several days, meaning they can cover immense distances in relation to their body size.
So these fish are well equipped to physically get wherever they need to go, but how do they know where they’re headed? Many scientists have speculated that larval fish orient themselves to solar cues; less is understood about how they continue orienting themselves at night once the sun has set, although it’s been thought sound and odor may play a role. Individual fish have been shown to successfully locate and settle on home reefs, but what if two (or more) heads are better than one? Would there be an advantage? To investigate this, Jean-Oliver Irisson and his team launched an observational study around Lizard Island in the Great Barrier Reef.Methods
Irisson’s team set light traps out overnight, offshore the island’s eastern and western regions, to collect settling larvae. Light traps act to attract fish and lure them into secured chambers. Once inside, the larvae cannot find the way back out, and scientists can identify and sort them the following morning. In this study, the predominant species found in the traps was the black-axil chromis—a relatively common damselfish in the local Australian waters. Adults are known to live in small groups (Fig. 1), so it was expected the larvae would show some affinity for forming shoals while in open water.
Once larvae were collected, they were observed individually or in groups (~12 larvae/group) using two different methods: direct observation by a SCUBA diver, or constrained in floating, rotating chamber (known in the study as a DISC: a drifting in situ chamber). For SCUBA observations, single or groups of larvae were released and followed; their orientation and swimming speed were recorded every 30 seconds for 10 minutes. If groups split, the shoal with the greatest number of larvae was the one followed. Cameras were used to monitor larval position within the DISC during these observations; photos were taken at 10 second intervals.
Irisson and his team compiled two primary types of data: a larva’s ability to maintain its directionality (that is, its ability to swim in a straight line) and its orientation (based on compiling directionality over multiple tests). Across a total of 140 trials, the team determined that individuals would show directionality, with less of a consensus on a unified orientation. However, when placed in groups, larvae were significantly more likely to maintain their orientation over longer periods of time. (Fig. 2)
Overall, shoaling larvae were able to keep a straighter bearing by 15%, and swimming in a group also increased their swimming speed by 7%—likely due to the decreased resistance of the water as individuals arranged themselves in an arched ‘V’ shape (similar to migrating geese). It would seem that shoaling does indeed pay off, at least under these experimental conditions.
The authors were quick to admit that the limited information on the black-axil chromis could call some of their conclusions into question, however, this study does mark one of the first wild observations of shoaling in larvae. The ecological impacts of schooling during the larval phase can be far reaching. For example, shoaling allows fish to reach their settlement sites faster (thereby lowering their chances of getting eaten out in the open water), at the expense of longer swimming endurance. With less endurance, settling areas that are chosen would be closer in distance, which could impact the way populations are structured as the larval generation grows up. Overall, the study serves to open up a new can of questions surrounding how naïve fish manage to find their collective way to their new homes. So, what question would you like to ask of the fish next?