Visual ecology of talitrid amphipods from Mediterranean and Atlantic coasts

Eye morphology and responses to various visual stimuli were comparatively investigated in several populations of three species of talitrid amphipods: Talitrus saltator, Orchestia gammarellus and Talorchestia spinifera from Mediterranean and Atlantic coasts. The responses to a directional artificial light (phototaxis) and to a black boundary (scototaxis) appeared tuned to orient talitrids downshore or, alternatively, to shelters according to their activity phase. In addition, Mediterranean T. saltator showed a peak of spectral sensitivity to blue light, the most abundant wavelength in the sky over the sea (450 nm), and this appears to be an effective means to orient seaward. In T. spinifera the arrangement of a particular vesicular structure in the lenses of the ommatidia appears suited to amplify the perception of horizontal light reflection on the sea surface, thus achieving a similar orienting device on an anatomical rather than physiological basis as exhibited by T. saltator. In summary, the compound eyes of talitrid amphipods have a common basic structure, but show a variety of morphological and/or physiological features, each suited to optimize vision under different optical conditions and to adapt the animals to the environments they inhabit and behaviours they express.