Invertebrate color vision and the tuned-receptor paradigm

'Phototaxis' in the absence of light? Locomotory patterns in unionid mussels

Locomotion in benthic invertebrates can strongly affect habitat selection and ecosystem nutrient cycling. In the case of freshwater mussels, the drivers of locomotion are largely unresolved. Our aim was to assess the influence of light presence and intensity on the locomotory behaviour of freshwater mussels in controlled laboratory experiments. The species investigated in our study were Anodonta anatina and Unio pictorum, two widely distributed mussels in European lentic and lotic inland waters. At low algal concentrations, known to be associated with more frequent locomotory activities, we found that both species moved primarily in the absence of light (72.7% of all movements across experiments). However, the movements of both species were directed towards the light source, resembling a net-positive 'phototactic' response but in the absence of light. The distance to the light source, which was negatively correlated to light intensity, had a positive effect on the distance covered in locomotory activities by A. anatina but not by U. pictorum. Intraspecific variation in shell size had no impact on movement distance, indicating that the energetic costs of movement were not a limiting factor. We suggest that the observed movement towards brighter locations helps to enhance food quantity and quality, whilst movement in darkness mitigates predation risks.

Predation and the persistence of melanic male mosquitofish (Gambusia holbrooki)

The empirical reasons for the persistent rarity of a genotype are typically complex and tedious to identify, particularly in nature. Yet rare morphs occur in a substantial fraction of phenotypic polymorphisms. A colour polymorphism has persisted for decades in the eastern mosquitofish, yet why this is so remains obscure. Here, I report the results of (1) intensive sampling at 45 natural sites to obtain the frequency distribution of the melanic (black) mosquitofish morph in Florida, (2) predation trials, conducted independently in mesocosms, with three different predatory species and (3) two mark-recapture studies, conducted in nature. This work (1) documents the rarity of melanic mosquitofish in nature, (2) demonstrates that melanic males experience a selective advantage over silver males in the presence of predators, (3) indicates no difference in the colour morphs' survival at a natural site essentially devoid of these predators, although suggesting a higher rate of recapture for melanic males at a site rife with predators. Overall, selective predation appears to contribute to the persistence of the melanic morph, despite its rarity in nature.

Photopic sensitivities to ultraviolet and visible wavelengths and the effects of the macular pigments in human aphakic observers

Psychophysical spectral sensitivities were obtained from aphakic human subjects in the photopic range. The foveal spectrum has a primary peak near 550 nm and a secondary peak about 0.7 log units lower around 380 nm. Off fovea, beyond most of the macular pigment, the curve is quite broad. While the peak is in the green, the gradual drop-off to 330 nm varies over only half a log unit. The two curves were compared in order to obtain an estimate of the macular pigments' spectrum. The macular pigments' curve has peaks at 460 and 490 nm as expected. It is broad and extends into the UV. These data suggest that the ocular media, including the macular pigments, have a profound effect on the spectral sensitivity.