Complex biological traits often originate by integrating previously
separate parts, but the organismal functions of these precursors are
challenging to infer. If we can understand the ancestral functions of
these precursors, it could help explain how they persisted and how they
facilitated the origins of complex traits. Animal eyes are some of the
best studied complex traits, and they include many parts, such as
opsin-based photoreceptor cells, pigment cells, and lens cells. Eye
evolution is understood through conceptual models that argue these parts
gradually came together to support increasingly sophisticated visual
functions. Despite the well accepted logic of these conceptual models,
explicit comparative studies to identify organismal functions of
eye-precursors are lacking. Here, we investigate how precursors
functioned before they became part of eyes in Cnidaria, a group formed
by sea anemones, corals and jellyfish. Specifically, we test whether
ancestral photoreceptor cells regulated the discharge of cnidocytes, the
expensive single-use cells with various uses including prey capture,
locomotion, and protection. Similar to a previous study of Hydra, we
show an additional four distantly related cnidarian groups discharge
significantly more cnidocytes when exposed to dim blue light compared to
bright blue light. Our comparative analyses support the hypothesis that
the cnidarian ancestor was capable of modulating cnidocyte discharge
with light. Although eye-precursors might have had other functions like
regulating timing of spawning, our findings are consistent with the
hypothesis that photoreceptor cells which mediate cnidocyte discharge
predated eyes, perhaps facilitating the prolific origination of eyes in
Cnidaria.