Controlled iris radiance in a diurnal fish looking at prey

Nico K. Michiels, Victoria C. Seeburger, Nadine Kalb, Melissa G. Meadows, Nils Anthes, Amalia A. Mailli, Colin B. Jack

Preprint posted on February 19, 2018

Threefin blennies can adjust “ocular sparks” in their irises between blue reflections and red fluorescence to suit their background, and do so in the presence of prey, potentially indicating a role in “photolocation” of transparent plankton.

Selected by James Foster

The Story

Echolocation and electrolocation represent well known examples of so-called “active” senses in animals, which allow them to scan their environments. The authors present evidence that suggests some fishes may adjust the way that light is redirected from their irises, and propose this may be used to “photolocate” their prey, transforming vision from a passive to an active sense. The authors also assess alternative functions of ocular sparks as bright lures for prey or as intraspecific signals.

The Study

Vision, for the most part, is a passive sense, relying on sunlight reflected from objects towards the viewer. It has been suggested that iris reflections in some fishes, known as “ocular sparks”, could be used as a kind of torch, illuminating prey in the fish’s vicinity by reflecting and otherwise re- emitting downwelling sunlight onto them. This study investigated the degree to which threefin blennies can adjust their ocular sparks, which can be either red or blue, in response to both the colour of their background and the availability of prey. The reflectance and radiance of the two types of ocular spark was also measured with reference to ambient illumination. The authors propose that, since the fish produced ocular sparks that contrasted with their background and did so more often when prey were present, these iris reflections may act as a form of “photolocation”, helping them to identify reflected eyeshine from their relatively transparent planktonic prey.


Before visual detection via photolocation, what aspect of the presence of prey causes the fish to adjust their ocular sparks. Could this be mediated by partial detection with ambient light, or perhaps olfactory cues?

Video shows T. delaisi ocular sparks in the lab. Reproduced from the preprint with permission from the authors.


Might a calibrated camera system be used in future to automate the detection and differentiation of ocular sparks (avoiding the adaptive mechanisms in GoPros and other cameras)?

Could ocular sparks have a secondary role in camouflaging the eye by counterbalancing the dim spot produced by the adjacent dark pupil and breaking up its outline (hiding it from prey with poor visual resolution)?

(Have I been using the wrong plural for “iris” all this time?)


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