The biphasic life cycle of the non-spinose planktonic foraminifera is characterised by an aberrant coiling signature

The complete lifecycle of the planktonic foraminifera has continued to remain obscure. Benthic foraminifera exhibit an obligate alternation of generations between haploid gamonts and diploid agamonts, yet despite years of observation, the fusion of gametes to form a zygote, gamont or agamont has never been observed in the planktonic groups. Recent evidence from Neogloboquadrina pachyderma culture confirms that they do have a biphasic reproductive cycle and the agamonts may exhibit dimorphism in coiling direction. In this study we examine the morphology of normal and aberrant coiling tests of both N. pachyderma and N. incompta from three Arctic and North Atlantic water masses, to explore whether aberrant coiling is an indicator of a biphasic life cycle in the non-spinose planktonic foraminifera group as a whole. Using a Nano-Computed Tomography scanning approach, we morphologically compared the proloculus and growth trajectories of aberrant and normal coiling tests. Aberrant coiling tests were found to have a slightly larger proloculus and test size, consistent with a left coiling N. pachyderma agamont in the literature. This degree of dimorphism is insufficient to discriminate agamonts in the wild population and coiling direction remains their only distinguishable feature without further morphological investigation. The constant low-level aberrant coiling signature in a range of non-spinose planktonic foraminiferal genera implies that a biphasic life cycle is the principal mode of reproduction in the non-spinose planktonic group.