Abstract:
In marine environments, active sensing using light is rare and has mainly been
described in nocturnal and deep-sea fishes. Previous work showed that triplefins, a
diurnal fish, can redirect downwelling light to improve the detection of scorpionfish.
However, this mechanism is expected to operate mainly in shallow water (3–10 m),
where sufficient downwelling light is available. With increasing depth, or in shaded
microhabitats, the underwater light field becomes dimmer, more scattered, and
increasingly shifted towards blue-green wavelengths. Under these conditions,
redirecting downwelling light is likely to become less effective, whereas red
fluorescence may provide an alternative light source.
In this dissertation, I test whether red iris fluorescence in the yellow black-faced
triplefin, Tripterygion delaisi, can enhance the detection of the black scorpionfish,
Scorpaena porcus, by specifically making their retroreflective pupils light up. This
hypothesis is tested in four linked chapters that combine behavioural experiments
with visual modelling.
Chapter 1 shows that triplefins respond more strongly to live scorpionfish when red
fluorescence can contribute to illumination. This suggests that fluorescence can
improve predator detection, but it does not establish the underlying mechanism.
Chapter 2 tests whether this effect can be explained by interaction with the
retroreflective scorpionfish pupil, using 3D-printed scorpionfish models with either
retroreflective or non-retroreflective eye inserts. The results provide partial support
for the retroreflective-pupil hypothesis, but also show that simplified models do not
fully reproduce the response elicited by live scorpionfish.
Chapter 3 uses visual modelling to test whether red fluorescence can increase
pupil-iris contrast in scorpionfish. The model predicts that fluorescence should
extend the distance over which the pupil remains distinguishable from the
surrounding iris.
Chapter 4 then tests this prediction directly by manipulating whether the
scorpionfish eye is visible. Triplefins detect scorpionfish at greater distances when
fluorescence is available, but only when the eye remains visible. When the eye is
hidden, the fluorescence effect disappears.
Taken together, these findings support the conclusion that red iris fluorescence can
function as a short-range sensory mechanism for detecting scorpionfish. Rather
than generally enhancing vision, fluorescence appears to exploit a specific optical
vulnerability in predator camouflage, namely the retroreflective pupil.
This dissertation provides experimental evidence that fluorescence can serve an
active sensory function. More broadly, it identifies a previously unrecognized
mechanism by which prey may exploit the optical properties of predator eyes, and it
suggests that red fluorescence in small micropredatory fishes may have evolved to
enhance survival by improving the detection of cryptic predators.
Fluorescence