ability of ish to learn to discriminate
objects they see on a regular basis.
So how do we test face and object
discrimination in ish? It all started with
our discovery that some damselish have
faces decorated with complex patterns
that uniquely differ between individuals,
much like our own ingerprints. What
makes these patterns even more inter-
esting is the fact that we cannot see them
because their colours lie in the ultravi-
olet range of the electromagnetic spec-
trum.
Earlier work in our lab had already
established that, unlike humans, these
ish can see light in the UV range so we
selected one such species, the ambon
damselish, as our model. One of the
many useful things about these ish is
that they are territorial, and thus aggres-
sive towards intruders. They also adapt
quickly to being housed in ish tanks, which they rapidly adopt
as their new territory.
To human eyes, ambon damselish look very similar in colour
as well as shape to another reef ish species, the lemon damselish,
which also have faces decorated with ultraviolet patterns. We
wanted to know whether territorial ambon damselish males
would be able to tell the difference between an intruder of their
own species and an intruder belonging to the lemon damselish.
Since the main visual difference between the two species lies
in their facial features, we displayed the intruders inside
containers that manipulated the visibility of these features.
One container allowed the ish to see the facial patterns while
another did not as it iltered out ultraviolet light.We found that the ambon damselish could only tell the
species apart when the facial features were visible. As long as the
UV patterns were visible, the ish warded off intruders of their
own species but showed relatively little interest in the other
species. However, as soon as the UV signal was removed they
approached both equally. From this we learned that the ambon
damselish do indeed use the ultraviolet patterns for species
discrimination.
We then changed our methodology as we wanted to control
our stimuli more carefully by controlling for the brightness of
the stimuli or the degree or type of motion of the target ish.
Instead of displaying live ish in containers, we photographed
the ish and created stimuli that only displayed the faces. This
was not an easy feat because we needed to capture the faces in
a spectrum, including ultraviolet light, and to create stimuli
that resembled the original faces.
We found that the best way to do this was to use an ultra-
violet-relecting white substance on an ultraviolet-absorbing
background, and spent hours cutting out the patterns and illing
the stencils with the ultraviolet-relecting material. In the end
we had stimuli that looked plain white to the human eye but
displayed an ultraviolet pattern on a white background to the
ish.
Having created the stimuli, all that remained was to see if the
ish could discriminate the two types of face. Sounds easy, but
how do you do it?
We developed a set of behavioural training methods that
allow us to test visual learning and discrimination abilities in
various ish species. Our basic method relied on the ish learning166 JUNE 2016
Some damselfish have faces decorated with complex patterns that are unique between
individuals. These markings are visible in the ultraviolet range of the electromagnetic
spectrum (left), and are invisible to us.Ambon damselfish are territorial, and
thus aggressive towards intruders.