Structure and functional implications of photoreceptive systems in non-marine ostracods: a review

Ostracods are small, bivalved crustaceans living in all types of aquatic habitats. All non-marine species belong to the order Podocopida. They usually possess a simple optical system composed of three eyecups called naupliar eye. Phylogenetic data and morphological characteristics support the homology of naupliar eyes between ostracods and other crustacean groups. The photoreceptive system is formed by several specialised cells and can be approximated by a lens-mirror optical apparatus. In some cases, a transparent area of the calcitic carapace can form anadditional lens. The visual stimuli are processed by the protocerebrum, possibly via monoaminergic neurons. The presence or absence of one or more specialised cells influence the function of the eyes, and, most likely, their evolution or loss are associated with the adaptation to different habitats. Podocopid ostracods may express long wavelength-sensitive rhabdomeric opsins and may possess nonvisual opsins. The few ethological experiments so far conducted demonstrate that non-marine ostracods might be capable of associative learning when trained with light or specific light wavelengths. This work will provide an overview of what is known and what remains to be further investigated about vision and how light cues affect the behaviour in non-marine ostracods.