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The reproductive strategy of planktonic foraminifera, key pelagic calcifiers, has long remained elusive, hampering efforts to understand and model their population dynamics. This is particularly critical in polar oceans where their success relies on rapid population growth after the polar night. Here, we provide field and laboratory observations constraining the reproductive cycle of the dominant polar species Neogloboquadrina pachyderma in Antarctic and Arctic waters. We observe that asexual reproduction is a rare but persistent element of the reproductive strategy and that it is reflected in a dimorphism in proloculus and in the ratio of shell size versus the number of chambers between sexually and asexually produced individuals. This dimorphism in natural populations reveals that asexual reproduction supplies more than 75% of adult individuals. This indicates a multigenerational reproduction strategy, where the majority of the population releases gametes to facilitate recombination, while a minority persistently reproduces asexually, allowing rapid population growth as a prerequisite for success in the polar oceans. [ABSTRACT FROM AUTHOR]

The relationship between adult and offspring size is an important aspect of reproductive strategy. Although this filial relationship has been extensively examined in plants and animals, we currently lack comparable data for protists, whose strategies may differ due to the distinct ecological and physiological constraints on single-celled organisms. Here, we report measurements of adult and offspring sizes in 3888 species and subspecies of foraminifera, a class of large marine protists. Foraminifera exhibit a wide range of reproductive strategies; species of similar adult size may have offspring whose sizes vary 100-fold. Yet, a robust pattern emerges. The minimum (5th percentile), median, and maximum (95th percentile) offspring sizes exhibit a consistent pattern of increase with adult size independent of environmental change and taxonomic variation over the past 400 million years. The consistency of this pattern may arise from evolutionary optimization of the offspring size-fecundity trade-off and/or from cell-biological constraints that limit the range of reproductive strategies available to single-celled organisms. When compared with plants and animals, foraminifera extend the evidence that offspring size covaries with adult size across an additional five orders of magnitude in organism size. [ABSTRACT FROM AUTHOR]