The Whitney Laboratory for Marine Bioscience
Congratulations to Dr. Allison Edgar who published a paper with Dr. Mark Martindale and coauthors in the Proceedings in the National Academy of Sciences (PNAS) - "Ctenophores are direct developers that reproduce continuously beginning very early after hatching."
Ctenophore cydippid larvae are not larvae at all and begin adult reproduction at an early age (∼14 vs. ∼60 d) and small size (∼1 vs. ∼100 mm) relative to attainment of what has been considered the adult stage. This overturns the previous understanding of the ctenophore life cycle, which was believed to be a unique form of biphasic life cycle with two separate sexually reproductive periods. Practically, these results clarify ecological controls regulating ctenophore reproduction and will aid management of this invasive species. Additionally, the 2-wk egg-to-egg generation time will open new avenues of research in this understudied but informative taxon.
A substantial body of literature reports that ctenophores exhibit an apparently unique life history characterized by biphasic sexual reproduction, the first phase of which is called larval reproduction or dissogeny. Whether this strategy is plastically deployed or a typical part of these species’ life history was unknown. In contrast to previous reports, we show that the ctenophore Mnemiopsis leidyi does not have separate phases of early and adult reproduction, regardless of the morphological transition to what has been considered the adult form. Rather, these ctenophores begin to reproduce at a small body size and spawn continuously from this point onward under adequate environmental conditions. They do not display a gap in productivity for metamorphosis or other physiological transition at a certain body size. Furthermore, nutritional and environmental constraints on fecundity are similar in both small and large animals.
Our results provide critical parameters for understanding resource partitioning between growth and reproduction in this taxon, with implications for management of this species in its invaded range. Finally, we report an observation of similarly small-size spawning in a beroid ctenophore, which is morphologically, ecologically, and phylogenetically distinct from other ctenophores reported to spawn at small sizes. We conclude that spawning at small body size should be considered as the default, on-time developmental trajectory rather than as precocious, stress-induced, or otherwise unusual for ctenophores. The ancestral ctenophore was likely a direct developer, consistent with the hypothesis that multiphasic life cycles were introduced after the divergence of the ctenophore lineage.