StriatechJournal Club

Echolocation with Light: A New Form of Active Sensing in Fish?

Prof. Nico K. Michiels - University of Tübingen, Germany

Live date was Jul 22nd, 2021

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The brightest part of many fish species is their iris. This effect can be caused by specular reflection, light focusing, and fluorescence. Why send out light from the iris? Radiating light close to and parallel to the own gaze is the ideal configuration to generate and detect eyeshine in the pupils of other organisms (e.g., cat’s eyes). The research goal of Prof. Michiels’ team is to demonstrate that even weak light reflections in the pupils of predator and prey species can be strong enough to be perceptible by the initial light sender/observer. We call this process “active photolocation”.

To test its presence and functionality, we use visual modelling. For this, we collect data on ocular properties (size, retinal map), contrast sensitivity and spatial acuity (Striatech’s OptoDrum), spectral sensitivities of the observer (microspectrometry), spectrometric properties of all the relevant structures, and the natural light field. We also control the ability to redirect light by attaching small shading hats (or transparent controls). We assume that this form of active detection is mainly functional over very short distances and is used to detect otherwise highly cryptic organisms. Our model system is a small fish species (< 5 cm) triplefin. Tripterygion delaisi is a small, active, bottom-dwelling species. It feeds on mm-sized prey, and is prey itself to cryptic, motionless, bottom-dwelling predators, such as the scorpionfish Scorpaena porcus. This work mainly takes place at the marine research station STARESO in Calvi, Corsica. Our latest results indicate that triplefins can detect gammarid crustaceans (prey) as well as scorpionfish (predator) over meaningful distances using active photolocation.

Key Topics

  • Fish can redirect downwelling sunlight sideways by actively controlled eye movement.
  • Over short distances, this is sufficient to induce perceptible eyeshine in other species.
  • Our hypothesis is that it facilitates detection of cryptic prey and predators.
  • First results suggest that the mechanism is functional in triplefins.
  • It is likely that it is present and functional in many more species as well.

Learning Objectives

  • Eyes are not just passive receptors: Eyes (i.e. mainly the irides) are also actively used as light reflectors, turning them into a local light source.
  • Eyes are also a weak point in species that try to be invisible by camouflage: Good eyes can be revealed by shining light at them.

Background Reading

A context analysis of bobbing and fin-flicking in a small marine benthic fish. Santon M, Deiss F, Bitton P-P, Michiels NK.
Ecology and Evolution 11(3), 1254–1263 (2021).
doi: 10.1002/ece3.7116.

Redirection of ambient light improves predator detection in a diurnal fish. Santon M, Bitton P-P, Dehm J, Fritsch R, Harant UK, Anthes N, Michiels NK.
Proc. R. Soc. B 287, 20192292 (2020).

The contrast sensitivity function of a small cryptobenthic marine fish. Santon M, Münch TA, Michiels NK.
J Vis. Feb 1;19(2):1 (2019)
doi: 10.1167/19.2.1.

Visual modelling supports the potential for prey detection by means of diurnal active photolocation in a small cryptobenthic fish. Bitton P-P, Yun Christmann SA, Santon M, Harant UK, Michiels NK.
Scientific Reports 9(1), 8089 (2019).

Daytime eyeshine contributes to pupil camouflage in a cryptobenthic marine fish. Santon M, Bitton P-P, Harant UK, Michiels NK.
Scientific Reports 8(1), 7368 (2018).

About the speaker

Prof. Nico K. Michiels

Prof. Nico K. Michiels

Professor in Animal Evolutionary Ecology and Director of the Institute for Evolution and Ecology
University of Tübingen, Germany


Prof. Nico Michiels was born and educated in Belgium. He did a PhD on the reproductive ecology of dragonflies, with a focus on copulation, genital morphology, and sperm competition, and continued as a postdoc in Hassel (Belgium) and at Brown University (USA). He then became interested in the sexual mechanisms of hermaphrodites using planarians at the University of Sheffield, followed by a group leader position at the MPI for Behavioural Physiology, Seewiesen (Germany). After moving to a professorship in zoology at the University of Münster, he widened his scope and included earthworms, nematodes, marine free-living flatworms, and sea slugs to the hermaphrodite repertoire. In 2004 he moved to Tübingen. Since then, his research focus shifted to the visual ecology of marine fish, induced by the accidental discovery of strong fluorescence in fish in 2007. He dives 2-3 months per year in the Mediterranean Sea, the Red Sea and in Sulawesi, Indonesia. He is an experienced diver and underwater photographer.

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