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. Recently, 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 complete physical and theoretical process would provide a more compelling case supporting this mechanism. In this paper, we investigate the conditions under which diurnal active photolocation could assist T. delaisi in detecting potential prey items. In the field, we sampled prey gammarids (Genus Cheirocratus) from foraging substrates, and characterized the spectral properties of their body and eyes, which possess strong reflectors between the ommatidia. In the laboratory, we quantified ocular spark sizes and the angular dependence of their radiance. Together with environmental light measurements and the visual properties of T. delaisi, we modeled diurnal active photolocation under various scenarios. Our results corroborate that diurnal active photolocation can 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. Because ocular sparks are widespread across many different fish species, diurnal active photolocation for micro-prey may be a common predation strategy.