IS POROSITY A FUNCTIONAL TRAIT IN PLANKTIC FORAMINIFERA?

Morphology is key for taxonomy, but if linked to a function can also provide insight into a species role in a given ecosystem. Identifying and testing the functionality of morphological traits in deep time is difficult, however. Planktic foraminifera provide an ideal study system to understand functional morphology due to their abundance in modern oceans, where we can measure traits that are lost during the taphonomic process and ascertain what clues are left behind in the fossil record . Porosity in planktic foraminifera is hypothesized to be directly linked to metabolic gas exchange resulting in a correlation between porosity and temperature.

In this study we investigated the roles of genetics and environment on the porosity of planktic foraminifera by using two thermocline dwelling species: Sphaeroidinellopsis paenedehiscens and Sphaeroidinella dehiscens. We hypothesize that if genetics is the dominant driver in test porosity these two foraminifera will have statistically different porosities despite experiencing the same environmental drivers. Micro-CT scans and 3D measuring software Dragonfly were used to image and measure the final chamber pores on 50 individuals from each species. Following imaging the specimens then underwent Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) to obtain Mg/Ca values for the final chamber to allow estimates of depth habitat.

We show statistically significant (p<0.05) different porosities between Sphaeroidinellopsis (6.77%) and Sphaeroidinella (9.75%). Sphaeroidinella have on average 262 more pores which are on average 3.90 x 10^-7 mm³ larger than those found on Sphaeroidinellopsis. Current environmental data implies no difference between Sphaeroidinellopsis and Sphaeroidinella, which suggests that at least in these two species porosity is not a solely environmentally driven trait.

These results show that porosity does vary between species but not systematically with the environment. Whilst this doesn’t mean that porosity is independent of metabolic gas exchange it does indicate that gas exchange is not related solely to temperature, and that these two species have evolved different functionally relevant traits that allow them to co-exist.