Vision and learning in non-marine ostracods

Photoreception and vision are probably some of the most important ways in which animals belonging to a multitude of different evolutionary lineages explore their environment, modulate their behavior, and shape their learning processes. Ostracods are small, bivalved crustaceans that inhabit a variety of aquatic environments (SMITH et al., 2015). Available information on the structural and physiological characteristics of the photoreceptive apparatuses of ostracods refers largely to marine species, while knowledge of these topics for non-marine species is still rather limited. The non-marine species, all belonging to the order Podocopida (MEISCH et al., 2019), typically have a simple optical system consisting of three eyecups known as naupliar eye, which exhibit similar features in different groups of crustaceans (ELOFSSON, 1992). This photoreceptive system is composed of several specialized cells and can be approximated by a lens-mirror optical apparatus (ELOFSSON, 2006). Notably, podocopid ostracods in the genus Heterocypris are known to express long-wavelength sensitive rhabdomeric opsins (HENZE & OAKLEY, 2015; PALECANDA et al., 2022). Ostracods use various sensory systems to explore their environment and interact with other organisms, such as chemoreceptors with olfactory function and mechanoreceptors that pick up tactile stimuli and, in some specific cases, hydrodynamic changes in the surrounding water generated by pressure waves and particle movement (MADDOCKS, 2000; SMITH & MATZKE-KARASZ, 2008). In podocopid ostracods, the integration of chemical, photoreceptive, and other sense stimuli to detect predators and mates can be hypothesized. Non-marine ostracods seem to be able to respond to preferred light components, as demonstrated by phototaxis experiments. The role of photoreception and vision in associative learning processes, experimentally confirmed in several invertebrate groups, is probably also important in non-marine ostracods. The first behavioral study of light stimulus perception in non-marine ostracods is probably that of APPLEWHITE & MOROWITZ (1966), in which Cyclocypris forbesi Sharpe, 1897, preferred dark environments and learned to avoid light by solving a light-driven maze. It also showed associative abilities of classical pseudoconditioning by closing valves when illuminated by a light source passing through a blue frosted glass filter associated with successive electric shocks. To date, the only ethological experiments combining color perception and learning in non-marine ostracods are those conducted by ROMANO et al. (2022), in which miniaturized analytical systems were used to demonstrate that Heterocypris incongruens (Ramdohr, 1808) can exhibit associative operant conditioning. It is not currently possible to determine whether these capabilities are present in other species of the superfamily Cypridoidea or other non-marine podocopid superfamilies. Given that the ability to learn through visual stimuli represents a significant adaptive advantage for survival and resource utilization, the paucity of research devoted to investigating the role of vision from an evolutionary and adaptive perspective in non-marine ostracods is surprising.