How baby fish find a home: orientation by reef-fish larvae in the pelagic environment
A project undertaken at the Australian Museum, Sydney and supervised by J M Leis (Australian Museum) and C B Paris (University of Miami, Florida)One of the most important questions in marine biology today is: what are the spatial scales over which animals populations are connected by larval dispersal? Answering this question is essential for theoretical understanding of population dynamics in marine systems, for effective management of fisheries, conservation of marine biodiversity, including design and operation of Marine Protected Areas, and for predicting effects of climate change on marine systems. The vast majority of bottom-associated (demersal), bony fishes have a pelagic larval stage subject to dispersal in open water over a pelagic larval duration of days to weeks. Thus, it is the pelagic larval stage, not the demersal adult stage, that sets the spatial scale for population connectivity and for the geographic size of fish populations. As a result, unlike terrestrial vertebrate populations, marine fish populations are generally considered open, with young potentially derived from distant sources and with management scaled accordingly. However, we now know marine fish populations are often demographically structured at modest spatial scales implying that demographically-relevant dispersal may also operate at such scales. The focus of our research is how the larvae behaviourally interact with the blue-water environment to influence dispersal outcomes. In other words, how do baby fish (ie, larvae) find a reef home after days to weeks in a relatively featureless blue-water environment.
Based on cold water species, until recently, it was assumed that fish larvae were so small and had such limited behavioural capabilities, that they could have no significant influence on dispersal by currents. Recent work has shown, however, that fish larvae can swim at speeds similar to currents for long periods of time. Speed without orientation abilities is unlikely to influence dispersal other than to increase diffusion. So, orientation is important in determining the extent to which fish larvae might be able to influence dispersal. Jeff Leis' previous work on the Great Barrier Reef shows that fish larvae can orientate with some precision in apparently featureless blue-water, pelagic environments: larvae of some species consistently swim to the south, whereas other species swim away from shore during daylight. Understanding how larvae orientate is key to understanding how the biophysical process of larval dispersal actually takes place. Investigation of orientation must take place in the ocean to avoid being misled by lab artifacts. Until now, this has been done by direct observation by divers, leaving open the question of bias that the presence of divers might introduce. Plus, observations by divers are restricted by depth, time and light limitations. Claire Paris, working in Florida has now developed a floating chamber that can be set adrift with larvae inside, and their behaviour recorded remotely at any time or depth. Paris and Leis will directly compare their methods for the study of orientation of fish larvae at Lizard Island Research Station (Great Barrier Reef). The team will also study the influence of visual and auditory cues on orientation of fish larvae in the ocean using Paris' innovative approach. Lizard Island is ideal due to the availability of large numbers of larvae of many species in the summer, relatively calm waters, an ability to work on both sides of the island, and the extensive work already done there by Leis.
For further reading:
Cowen, RK, CB Paris, et al. (2006). Scaling of connectivity in marine populations. Science 311: 522-527
Leis JM (2006). Are larvae of demersal fishes plankton or nekton? Advances in Marine Biology 51:59-141
Figure 1. Claire Paris' underwater larval fish behavioural apparatus, which was developed with funding from the National Science Foundation of the USA. The apparatus is set adrift with a larva in the disc-shaped chamber at the bottom. The larva's behaviour is filmed by the video camera at top. Photo © University of Miami.
Figure 2. 10 mm long larva of Rainford's butterflyfish Chaetodon rainfordi. The orientation abilities of larvae like this one will studied in this project. Photo by David Priddle (©Australian Museum).
Figure 3. Adult reef fishes like these were all once larvae out in the blue water that can be seen in the background. All have successfully solved the problem of finding a reef at the end of their pelagic, larval phase. Photo by Jeff Leis (©Australian Museum).
Figure 4. 14 mm long larva of the Barramundi Cod, Chromileptes altivelis. Compared to the adult, larvae like this one look very different, live in different places and have different ecologies. We know very little about their behaviour, but, increasingly, we are finding they have remarkable swimming, orientation and sensory abilities. Photo by Paul Ovenden (©Australian Museum).