Which way home from the big blue?

Nearly every species of bony reef fish has a larval stage that lives in open water for a couple of weeks to a couple of months. During that time, a larva must not only develop and grow, but also navigate its way around the ocean, and eventually find a reef upon which to settle and metamorphose, all without any help at all from its parents. How larvae manage this is important not only for the larvae, but also for managing coral reefs, however it has been a mystery until recently.

In collaboration with scientists from Queensland, France and the USA, we discovered that larvae of the Blackaxil Puller, Chromis atripectoralis (shown in the image above), a species of damselfish, have very consistent orientation out in the ocean up to 1 km from the reefs off the Australian Museum’s Lizard Island Research Station. The larvae had the same - essentially southerly - swimming direction, regardless of where or how they were observed over a research program spanning 10 years.

We also discovered that the larvae swam more toward the southeast in the morning and toward the southwest late in the afternoon, implying that they were using a sun compass to maintain their orientation. Swimming orientation was less precise under cloudy skies than when it was sunny, supporting this implication.

In contrast, larvae of a second damselfish species had inconsistent, and less orientated, swimming behaviour, but still swam more westerly in the late afternoon than in the morning. This finding is important as it shows that larvae of different species do different things in the ocean.

In this research, we also used a new method to record orientation of fish larvae in the field – the DISC (Drifting In Situ Chamber). Our normal method of observing fish larvae by SCUBA divers gave the same results as did the DISC, but the DISC gives us the ability to use filters and shades to manipulate the light to which the larvae are exposed. We will be experimenting with the DISC to learn more about orientation of fish larvae in the ocean, and the cues that they use to achieve that orientation.

Understanding the behaviour and sensory capabilities of fish larvae is an interesting and challenging subject, but beyond that it is necessary knowledge for managing our living marine resources, particularly those of the World Heritage Great Barrier Reef. We need to know how far from the natal reef the larvae disperse so reef-fish populations can be managed on appropriate spatial scales, and so that design and operation of marine protected areas can be based on sound science, rather than best guesses. Not only the fish and their reef homes, but also fishers, divers and greenies will benefit from a sound understanding of dispersal of fish larvae.

Dr Jeff Leis
Senior Fellow, Ichthyology

More information:
This research was funded by the Hermon Slade Foundation and a Discovery Grant from the Australian Research Council.

Leis JM, Paris CB, Irisson J-O, Yerman MN, and Siebeck UE (2014) Orientation of fish larvae in situ is consistent among locations, years and methods, but varies with time of day. Marine Ecology Progress Series 505:193-208