Background reflectance Haliopteris filiciana

Spectrophotometric measurements of Haliopteris filician

SA scores Gammarid as perceived by Td pupil

Solid angle of the gammarid eye as perceived from the pupil of T. delaisi based on the interaction distanc

Downwelling and sidewelling illuminant for analyses

Downwelling and sidewelling light field

Visual modelling supports the potential for prey detection by means of diurnal active photolocation in a small cryptobenthic fish

Active sensing has been well documented in animals that use echolocation and electrolocation. Active photolocation, or active sensing using light, has received much less attention, and only in bioluminescent nocturnal species. However, evidence has suggested the diurnal triplefin Tripterygion delaisi uses controlled iris radiance, termed ocular sparks, for prey detection. While this form of diurnal active photolocation was behaviourally described, a study exploring the physical process would provide compelling support for this mechanism. In this paper, we investigate the conditions under which diurnal active photolocation could assist T. delaisi in detecting potential prey. In the field, we sampled gammarids (genus Cheirocratus) and characterized the spectral properties of their eyes, which possess strong directional reflectors. In the laboratory, we quantified ocular sparks size and their angle-dependent radiance. Combined with environmental light measurements and known properties of the visual system of T. delaisi, we modeled diurnal active photolocation under various scenarios. Our results corroborate that diurnal active photolocation should help T. delaisi detect gammarids at distances relevant to foraging, 4.5 cm under favourable conditions and up to 2.5 cm under average conditions. To determine the prevalence of diurnal active photolocation for micro-prey, we encourage further theoretical and empirical work

Coaxial reflectance values categorical

Reflective properties of Gammarid eyes measured with coaxial light sourc

Eye reflectance conversion values categorical

Conversion factors to produce non-coaxial reflectance values from co-axial reflectance values for gammarid eye

Average gammarid body reflectance

Spectrophotometric data for body reflectance of Cheirocratus gammarid

Ocular spark conversion on continuous scale

Conversion curves for transforming downwelling irradiance into ocular spark radianc

Ocular media values

Transmission properties of the ocular media of Tripterigyon delais

SA scores Os as perceived by Gammarid eye MATRIX

Solid angles of the Ocular spark from T delaisi as perceived by the gammarid eye based on the interaction distanc