Data from: Controlled iris radiance in a diurnal fish looking at prey

Active sensing using light, or active photolocation, is only known from
deep sea and nocturnal fish with chemiluminescent "search"
lights. Bright irides in diurnal fish species have recently been proposed
as a potential analogue. Here, we contribute to this discussion by testing
whether iris radiance is actively modulated. The focus is on behaviourally
controlled iris reflections, called "ocular sparks". The
triplefin Tripterygion delaisi can alternate between red and blue ocular
sparks, allowing us to test the prediction that spark frequency and hue
depend on background hue and prey presence. In a first experiment, we
found that blue ocular sparks were significantly more often "on"
against red backgrounds, and red ocular sparks against blue backgrounds,
particularly when copepods were present. A second experiment tested
whether hungry fish showed more ocular sparks, which was not the case.
Again, background hue resulted in differential use of ocular spark types.
We conclude that iris radiance through ocular sparks in T. delaisi is not
a side effect of eye movement, but adaptively modulated in response to the
context under which prey are detected. We discuss the possible alternative
functions of ocular sparks, including an as yet speculative role in active
photolocation.