The positions of most fishes’ eyes mean they cannot see their own body. So, how do they recognise others of their own species?
Andrew MacColl
School of Life Sciences, University of Nottingham, UK
Leaving aside the issue of exactly how much of their body they can see (some fish, like eels, can probably see quite a bit of it), the question of how fish recognise their own species is an important one that is surprisingly poorly understood.
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From what we know about other species, fish may “imprint” on a parent, enabling them to recognise adults of their own species later in life. However, most fish never see their parents. This leaves us with some form of inherited, instinctual recognition as the most likely possibility. The exact way in which that might work is unclear and may vary between habitats and species.
Some use of the senses must be involved, and given that most fish don’t vocalise and have limited contact while mating, it seems most likely that sight and/or smell are involved. Sight is likely to be important for many species and could involve the recognition of shape, movement or colour patterns to define species.
The many closely related cichlids in East African lakes have a particular problem when it comes to species recognition
For the many species of fish that live in dark or murky environments, sight must be of limited use and this leaves smell. The use of smell allows the possibility that species recognition could be “self-referential” – that is, dependent on an individual’s perception of itself.
A final mystery is how species recognition keeps pace with divergence during speciation. The many closely related cichlids in East African lakes, for example, have a particular problem when it comes to species recognition and it is likely that any one mechanism is error-prone, but that the overall error rate is reduced by spatial and temporal context.
Mike Webster
School of Biology, University of St Andrews, UK
Recognising members of their own species is important to fishes, allowing them to shoal with others to gain protection from predators and ensuring that they direct courtship behaviour to members of the same species so that they can reproduce successfully.
Some fish, including zebrafish, learn to recognise members of their own species through a process called imprinting. This is a type of long-lasting learning that occurs early in life in a sensitive period, during which they learn recognition cues of other nearby animals, which will tend to be members of their own species. This information provides them with a template for recognising others later in life, and it isn’t necessary that they perceive their own appearance in order to do so.
For zebrafish, both visual and chemical cues are important for imprinting. For many species of fishes, multisensory integration combining information detected using different senses, including visual, but also chemical, tactile, acoustic and electrical cues, allows them to build an accurate representation of their surroundings, including the other animals they interact with.
The extent to which vision is important to fishes varies greatly between species and strongly depends on the habitat that they are adapted to. Fish inhabit a wide variety of environments. In shallow, coastal waters and clear rivers and lakes, sunlight can penetrate easily, and vision may be important to many of the fishes that live there.
In silty estuaries, algae-rich eutrophic lakes and in the deep ocean, however, light is quickly scattered by particles of suspended sediment or phytoplankton and doesn’t penetrate deeper water. Fishes in these environments might still use vision to detect shadows or flashes of bioluminescence from predators or prey, but in the absence of sunlight, they tend to rely more heavily on other senses to detect and recognise others.
Finally, regardless of the sensory systems involved, species recognition in fishes isn’t always accurate. Many fishes live in mixed-species shoals, and at least some of the time this might be down to misidentification of species. When it comes to reproduction, hybridisation – breeding that occurs between different species – is widespread in some fish families and more common in fishes than any other vertebrate.
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