The size structure of plankton communities is an important determinant of their functions in marine ecosystems. However, few studies have quantified how organism size varies within species across biogeographical scales. Here, we investigate how planktonic foraminifera, a ubiquitous zooplankton group, vary in size across the tropical and subtropical oceans of the world. Using a recently digitized museum collection, we measured shell area of 3,799 individuals of nine extant species in 53 seafloor sediments. We first analyzed potential size biases in the collection. Then, for each site, we obtained corresponding local values of mean annual sea‐surface temperature (SST), net primary productivity (NPP), and relative abundance of each species. Given former studies, we expected species to reach largest shell sizes under optimal environmental conditions. In contrast, we observe that species differ in how much their size variation is explained by SST, NPP, and/or relative abundance. While some species have predictable size variation given these variables (Trilobatus sacculifer, Globigerinoides conglobatus, Globigerinella siphonifera, Pulleniatina obliquiloculata, Globorotalia truncatulinoides), other species show no relationships between size and the studied covariates (Globigerinoides ruber, Neogloboquadrina dutertrei, Globorotalia menardii, Globoconella inflata). By incorporating intraspecific variation and sampling broader geographical ranges compared to previous studies, we conclude that shell size variation in planktonic foraminifera species cannot be consistently predicted by the environment. Our results caution against the general use of size as a proxy for planktonic foraminifera environmental optima. More generally, our work highlights the utility of natural history collections and the importance of studying intraspecific variation when interpreting macroecological patterns.

Related materials:

Marina C. Rillo, C. Giles Miller, M Kucera and THG Ezard (2020) Intraspecific size variation in planktonic foraminifera cannot be consistently predicted by the environment. Ecology and Evolution (in press) https://doi.org/10.1002/ece3.6792

Marina C. Rillo, M Kucera, THG Ezard and C. Giles Miller, (2019) Surface Sediment Samples From Early Age of Seafloor Exploration Can Provide a Late 19th Century Baseline of the Marine Environment. Frontiers in Marine Science 5:517; DOI: https://doi.org/10.3389/fmars.2018.00517

Marina C. Rillo, J. Whittaker, THG Ezard, A. Purvis, A.S. Henderson, S. Stukins & C.G. Miller, (2016) The unknown planktonic foraminiferal pioneer Henry A. Buckley and his collection at the Natural History Museum, London, Journal of Micropalaeontology, 36, 191-194, DOI: https://doi.org/10.1144/jmpaleo2016-020

Marina Costa Rillo (2016). Dataset: Henry Buckley Collection of Planktonic Foraminifera. Natural History Museum Data Portal (data.nhm.ac.uk). DOI: https://doi.org/10.5519/0035055