The cardinal fish (Ostorinchus doederleini) is a paternal mouthbrooder: the male incubates fertilized eggs and newly-hatched larvae in its mouth for extended periods of time. During the first few weeks of life, the larvae are unable to swim, and, once released from the mouth of their parent, are at the mercy of the ocean currents, which can cause them to drift up to 20 miles away from their place of birth (the natal, or home, reef). Cardinal fish larvae, can, however, find their way back to the reef where they hatched, unlike the larvae of many other fish species.
Now, a multidisciplinary study, to be published later this week on the website of the Proceedings of the National Academy of Sciences, shows that the cardinal fish larvae are able to discriminate the odour of their home reef from that of other reefs, and may use their sense of olfaction to home in on their birthplace after dispersal by ocean currents.
Gabriele Gerlach and her colleagues, of the Marine Biological Laboratory in Wood’s Hole, Massachusetts, first used hydrodynamic models to examine the prevailing ocean currents around a group of five coral reefs in the Great Barrier Reef. From the models, the extent of fish larvae dispersal, and hence mixing of the fish populations, was predicted. They then performed genetic analyses of fish populations around the group of reefs. These analyses showed that, although some fish species are widely dispersed as predicted by the model, others, such as the cardinal fish, are instead concentrated around their home reef.
Olfactory choice tests were then performed to determine whether or not the fish could use odour to find their way back to the home reef. Tanks containing separate but connected compartments were filled with small volumes of water obtained from the different reefs. Larvae of different fish species were caught and placed in the tanks. It was found that cardinal fish larvae moved to the compartment containing water from their home reef, while other species tested, such as the neon damselfish, did not.
It appears that the water around the home reef of the cardinal fish has a particular odour, which is somehow imprinted in the olfactory system of the larvae. It is this ‘olfactory imprinting’ that enables the larvae to find their way back to the population from which they came, even though it is one of several fish populations inhabiting the same reef, and despite the home reef being in close proximity to other reefs.
According to Gerlach, who is lead author of the forthcoming paper:
This research shows that the spatial distribution of these aquatic organisms is far from being random despite long larval dispersal stages of several weeks. Apparently, these larvae – small as they are – use elaborate sensory mechanisms to orientate and find their way to appropriate habitats or express successful homing behavior to their natal spawning sites. This might play a major role in processes of population separation and, eventually, of speciation. This information should be considered by marine managers as they designate the location and spacing of Marine Protected Areas.
However, it remains unclear whether the cardinal fish larvae are detecting a “social” scent of its own species, or a mixture of compounds which gives the natal reef its own particular smell. Nor is it known how the fish initially learn the odour of their natal reef.