Cnidocyte discharge analysis

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, which
we speculate uses an opsin-based phototransduction pathway homologous to
that previously described in Hydra. 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.