Complex reproduction reflects life-history trade-offs in early Cambrian multicellular algae.

The Cambrian explosion diversified metazoan consumers and restructured marine food webs, yet how planktonic primary producers responded to this escalating ecological pressure remains poorly understood. Here we report Polyostiosphaera operculata gen. et sp. nov., an exceptionally preserved multicellular alga from the early Cambrian Kuanchuanpu Biota, South China. High-resolution X-ray microtomography, light microscopy and scanning electron microscopy reveal an outer envelope of small cells enclosing a membrane-bound cluster of larger cells, with regularly spaced apertures, in the best-preserved specimens, sealed by plug-like opercula. The two spatially segregated cell populations are interpreted as somatic and reproductive cells, and the operculate apertures as a specialized controlled-release apparatus. This architecture would have permitted propagules to exit through pre-formed channels without catastrophic rupture of the parental body, making staged or repeated release anatomically plausible, although not directly demonstrated. Comparative analysis of Polyostiosphaera with Cambrian multicellular algae and modern Volvocaceae indicates that it had attained a high level of multicellular and reproductive complexity by the early Cambrian. Its reproductive strategy reflects a life-history trade-off: investment in a complex release system may have protected propagules and distributed release under elevated grazing pressure and environmental instability. Polyostiosphaera therefore suggests that consumer-producer interactions during the Cambrian radiation were more intricate than previously recognized.