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

Small benthic fish strike at prey over distances that fall within theoretical predictions for active sensing using light

Small, benthic, cryptic fishes represent a species-rich guild on marine substrates. Most of them are micropredators that feed on crustaceans that are often smaller than 1 mm. Typical examples are seahorses and pipefishes (Syngnathidae), most gobies (Gobiidae), dragonets (Callionymidae) and triplefins (Tripterygiidae). Previous work on the yellow black-headed triplefin Tripterygion delaisi demonstrated that it actively redirects downwelling sunlight sideways using its iris and can use this to locally illuminate objects of interest. We call this form of active sensing using light "diurnal active photolocation". Visual modelling predicted that light redirection can be sufficient to induce a perceivable change in luminance in the eyes of one of its prey species, a cryptic gammarid crustacean (Cheirocratus sp.), over distances of 1-2 cm. Empirical validation, however, was not possible because measurements of predation distances have not been quantified for free-ranging, small, benthic fishes before. Here, we present interaction distances measured from videos of T. delaisi approaching and striking at prey in the field. Out of 160 recordings, we were able to quantify 78 prey approaching distances and 100 striking distances. Approaching distances ranged from 2.1 to 4.1 cm (interquartile range, IQR) and involved one to five approaching steps before the actual strike occurred. The distance over which the final strike took place varied from 0.7 to 1.6 cm (IQR). Both approaching and striking distances increased with fish body size. We conclude that most approaching sequences started too far away to be explained by prey detection through light redirection. Striking distances, however, fell well with the distances predicted by the model. We conclude that if diurnal active photolocation plays a role in prey detection, it is during the final decision whether to strike or not.

The round goby genome provides insights into mechanisms that may facilitate biological invasions

Background

The invasive benthic round goby (Neogobius melanostomus) is the most successful temperate invasive fish and has spread in aquatic ecosystems on both sides of the Atlantic. Invasive species constitute powerful in situ experimental systems to study fast adaptation and directional selection on short ecological timescales and present promising case studies to understand factors involved the impressive ability of some species to colonize novel environments. We seize the unique opportunity presented by the round goby invasion to study genomic substrates potentially involved in colonization success.

Results

We report a highly contiguous long-read-based genome and analyze gene families that we hypothesize to relate to the ability of these fish to deal with novel environments. The analyses provide novel insights from the large evolutionary scale to the small species-specific scale. We describe expansions in specific cytochrome P450 enzymes, a remarkably diverse innate immune system, an ancient duplication in red light vision accompanied by red skin fluorescence, evolutionary patterns of epigenetic regulators, and the presence of osmoregulatory genes that may have contributed to the round goby’s capacity to invade cold and salty waters. A recurring theme across all analyzed gene families is gene expansions.

Conclusions

The expanded innate immune system of round goby may potentially contribute to its ability to colonize novel areas. Since other gene families also feature copy number expansions in the round goby, and since other Gobiidae also feature fascinating environmental adaptations and are excellent colonizers, further long-read genome approaches across the goby family may reveal whether gene copy number expansions are more generally related to the ability to conquer new habitats in Gobiidae or in fish.

Electronic supplementary material

The online version of this article (10.1186/s12915-019-0731-8) contains supplementary material, which is available to authorized users.

Redirection of ambient light improves predator detection in a diurnal fish

Cases where animals use controlled illumination to improve vision are rare and thus far limited to chemiluminescence, which only functions in darkness. This constraint was recently relaxed by studies on Tripterygion delaisi, a small triplefin that redirects sunlight instead. By reflecting light sideways with its iris, it has been suggested to induce and detect eyeshine in nearby micro-prey. Here, we test whether 'diurnal active photolocation' also improves T. delaisi's ability to detect the cryptobenthic sit-and-wait predator Scorpaena porcus, a scorpionfish with strong daytime retroreflective eyeshine. Three independent experiments revealed that triplefins in which light redirection was artificially suppressed approached scorpionfish significantly closer than two control treatments before moving away to a safer distance. Visual modelling confirmed that ocular light redirection by a triplefin is sufficiently strong to generate a luminance increase in scorpionfish eyeshine that can be perceived by the triplefin over 6-8 cm under average conditions. These distances coincide well with the closest approaches observed. We conclude that light redirection by small, diurnal fish significantly contributes to their ability to visually detect cryptic predators, strongly widening the conditions under which active sensing with light is feasible. We discuss the consequences for fish eye evolution.