Plasticity in Daphnia eye size is determined by intra-generational environmental conditions

Variation in structure and size of the eye in relation to light
environments is well-documented across animal taxa. However, the long-term
effects of ultraviolet radiation (UVR) on eye size across generations
remain understudied, despite natural fluctuations in UVR being common in
aquatic habitats. Here, we used Daphnia magna to test whether an
evolutionary history of UVR threat modifies plasticity and
transgenerational responses in eye size. We conducted a reciprocal
split-brood experiment across three parthenogenetic generations using two
groups maintained under either UVR or non-UVR conditions for over 150
generations, enabling the potential accumulation of adaptive responses and
the establishment of distinct evolutionary lineages for comparison. We
found strong plasticity in eye size, primarily shaped by current
environmental conditions rather than transgenerational effects. Smaller
eyes developed in response to UVR but rapidly reverted to larger
dimensions in its absence. Furthermore, changes in relative eye size were
associated with fitness-related traits, including swimming behaviour and
reproductive output, although these relationships varied in strength and
direction between UVR-exposed and unexposed individuals. Our study
highlights the role of environmental stressors and evolutionary history in
shaping sensory adaptations, providing conceptual advances on how sensory
traits among aquatic invertebrates may rapidly evolve under changing
environmental conditions.