We take the plunge to find out why Damselfish larvae swim in different directions in different regions.

Populations of nearly all bony fishes that live on different coral reefs maintain connection, not by movement of adults, but by dispersal of tiny larvae through relatively featureless open waters. My research team has shown larvae of Black-axil Puller (Chromis atripectoralis), a damselfish, are super swimmers, but without the ability to navigate, such swimming ability has little effect on dispersal outcomes. Understanding dispersal - or lack of it - is essential for design of marine parks and management of fisheries.

Tiny, 1 cm-long larvae of the Black-axil Puller can swim at more than 1 km per hour, or 28 body lengths per second, which makes human Olympic swimmers, who can only manage a bit over 1 body length per second, look pretty slow. Swimming at half this speed, the larvae can travel an average of 15 km without food or rest. This is equivalent to a human swimming 3000 km!

How a larva uses its swimming prowess determines where it will live for the rest of its life, and so is vitally important to understand. Ten years of ARC-funded research at the Australian Museum's Lizard Island Research Station on the northern Great Barrier Reef, revealed that larvae of Black-axil Puller consistently swim to the south, regardless of which side of the reef, and how far from shore: swim direction is location-independent at local scales. However, we discovered that off Townsville, 620 km to the south, and in the lagoon of New Caledonia, 2400 km to the south-east, the larvae swim to the east. Why?

We put on our SCUBA gear, and studied the swimming direction and speed of Black-axil Puller larvae by releasing them in the ocean, and following behind, measuring their swimming direction with a compass, and their speed with a flow meter. In each region, we discovered that the average larva is swimming into the average, prevailing current (not the real-time current, which is dominated by tidal currents moving back and forth). This tends to minimize the larvae being swept away by currents, keeping them near the reef where they were spawned.

This behaviour explains how populations of reef fishes are maintained on individual reefs and islands, although the odd larva swimming in a different direction can travel many kilometers to a different reef. “Long-distance dispersers” like these are important for maintaining genetic connectivity, and replenishing over-fished reefs, or reefs that have been damaged by cyclones or climate-change-induced coral bleaching.

Scientists used to think that fish larvae drifted passively with the currents, but our research and that of others demonstrates that larvae of coral-reef fishes are very competent swimmers that have a lot of control over where they end up. We need to learn more about how larvae do this, for example, what sensory cues they use to maintain a course, but the discovery of differences in larval swimming directions among regions is an important step in understanding dispersal of fish larvae in the ocean.

Dr Jeff Leis
Senior Fellow, Ichthyology

More information:
Leis, J.M., U.E. Siebeck, A.C. Hay, C.B. Paris, O. Chateau and L. Wantiez. (2015). In situ orientation of fish larvae can vary among regions. Marine Ecology Progress Series. 537: 191-203 + supplement DOI: 10.3354/meps11446