Background: At depths below 10 m, reefs are dominated by blue-green light because seawater selectively absorbs the longer, 'red' wavelengths beyond 600 nm from the downwelling sunlight. Consequently, the visual pigments of many reef fish are matched to shorter wavelengths, which are transmitted better by water. Combining the typically poor long-wavelength sensitivity of fish eyes with the presumed lack of ambient red light, red light is currently considered irrelevant for reef fish. However, previous studies ignore the fact that several marine organisms, including deep sea fish, produce their own red luminescence and are capable of seeing it.

Why do some marine fishes exhibit striking patterns of natural red fluorescence? In this study, we contrast two non-exclusive hypotheses: (i) that UV absorption by fluorescent pigments offers significant photoprotection in shallow water, where UV irradiance is strongest; and (ii) that red fluorescence enhances visual contrast at depths below −10 m, where most light in the ‘red’ 600–700 nm range has been absorbed. Whereas the photoprotection hypothesis predicts fluorescence to be stronger near the surface and weaker in deeper water, the visual contrast hypothesis predicts the opposite. We used fluorometry to measure red fluorescence brightness in vivo in individuals belonging to eight common small reef fish species with conspicuously red fluorescent eyes. Fluorescence was significantly brighter in specimens from the −20 m sites than in those from −5 m sites in six out of eight species. No difference was found in the remaining two. Our results support the visual contrast hypothesis. We discuss the possible roles fluorescence may play in fish visual ecology and highlight the possibility that fluorescent light emission from the eyes in particular may be used to detect cryptic prey.

The sessile suspension-feeding worm-snail Dendropoma maxima Sowerby, 1825 (Vermetidae) secretes a mucous web to capture planktonic prey. In dense groups, the feeding webs of neighbouring snails frequently overlap and stick together. This may create direct food competition between neighbours because the earlier retracting snail may get more than its fair share of the prey. While field observations indicate that web overlap may generate retarded growth, we experimentally studied whether web overlap also triggers a phenotypic response in feeding behaviour. In our experiment we consecutively placed focal snails in a place by themselves (solitary) or close to a neighbouring snail such that webs overlapped, starting with either of the 2 conditions in half of the experimental individuals to exclude sequence effects. We found that focals retracted their feeding web significantly earlier when close to a neighbour than when solitary. Our experiments thus confirm a phenotypic response through early web retraction in D. maxima, indicating that direct interference competition affects worm-snail behaviour.

Summary1. Studying the effect of anthropogenic noise on animal behaviour and physiology is a field of growing scientific and management interest. Anthropogenic noise is internationally seen as major environmental concern, but knowledge of the response of animals to noise and its ecological and evolutionary consequences is disparate. Even though fish and invertebrates form the majority of aquatic taxa, the effects of noise on these taxa are largely understudied. Especially the consequences of noise for short-range communication and behavioural interactions are yet unknown. While experimental studies on this subject are thus greatly needed, the expense of the equipment can be a main factor limiting data generation. Speakers that produce low-frequency sounds under water are currently either too large or too expensive to allow for sufficient replication in many research set-ups. 2. Here, we describe a device that can produce a low-frequency sound, which can be used as an experimental source of noise both in aquaria and in the field. The device is completely self-contained and costs around 10 euros per 'noise egg'. 3. The sound created consists of frequency bands (harmonics), which has the advantage that broadband and pulsed sounds, such as sounds produced by vocal fish, are easily detected in a spectrogram. Because the sound from the egg attenuates quickly, it can be used in aquaria and in the field to target certain study species or individuals without affecting a large part of the surrounding area. 4. We have developed the device to study the effects of noise on communication and behaviour in small aquatic animals; however, it could be used for other purposes, such as testing the propagation of certain frequencies in shallow-water habitats. 5. We hope the described method will facilitate the generation of experimental data on the effect of noise on behaviour and communication in aquatic animals in a wide variety of study systems and study areas.

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