Size range, morphotypes, and test deformations in juvenile megalospheres of the symbiont-bearing foraminifer Peneroplis: Windows on evolutionary processes and past environments

Foraminifera's diverse reproductive strategies contribute greatly to their capacity for evolutionary change and their remarkable success throughout Earth's history. This includes sexual reproduction (where genetic material from two individuals is combined), asexual reproduction (where offspring are produced from a single parent) and multiple fission events, giving them the ability to evolve and thrive over time, and the potential to adapt to new environments, develop new traits and diversify into new species. Previous laboratory studies have consistently shown that foraminifera can each produce a large number of offspring from a single reproductive event, but finding foraminifera during their reproductive phase in nature is a rarity.

 

Here we report on symbiont-bearing foraminifera of the genus Peneroplis in a reproductive state from two natural sites in the Pacific Ocean that are in the process of releasing their juveniles. Findings of adult Peneroplis parent shells with pre-emergent juveniles contained within the test provided a rare opportunity to examine the range of morphologic variability within a single brood of offspring resulting. The adult specimens were complete and contained numerous calcified megalospheric juveniles. We examined the parental test and the megalospheric juveniles by high-resolution Scanning Electron Microscopy to document the quantity, morphology, and size range of the juveniles and to provide novel insight into the reproductive biology and ontogenetic constraints of P. pertusus. The juveniles are non-uniform and highly heterogenous, varying in size, ornamentation, position of the flexostyle, and extent of test deformations. We also report on morphological features in normal and deformed juvenile tests as displayed in the proloculus and the flexostyle. To test whether the juvenile test deformations are expressed in adults, more than 100 specimens of P. pertusus were examined. Previous studies suggested that juvenile test deformities can be carried through into the adult. Our study shows, however, that test abnormalities among juveniles are not expressed in adults of natural population, indicating that they are either transitioning to normal growth forms or are not viable. The small juveniles ultimately become the proloculus and associated juvenile chamber(s) of adults, features that are used in the taxonomy of some larger foraminifera. Morphological features of juveniles and prolocular size across contemporary populations and among populations through time may be used to infer ecological and paleoecological conditions. Results of this study inform such investigations.