The evolution of internal fertilization has occurred repeatedly and independently across the tree of life.  As it has evolved, internal fertilization has reshaped sexual selection and the covariances among sexual traits such as testes size and gamete traits. But it is unclear whether fertilization mode also shows evolutionary associations with traits other than primary sex traits. Theory predicts that fertilization mode and body size should covary, but formal tests with phylogenetic control are lacking. We used a phylogenetically-controlled approach to test the covariance between fertilization mode and adult body size (while accounting for latitude, offspring size, and offspring developmental mode) among 1,232 species of marine invertebrates from 3 phyla. Within all phyla, external fertilizers are consistently larger than internal fertilizers: the consequences of fertilization mode extend to traits that are only indirectly related to reproduction. We suspect that other traits may also coevolve with fertilization mode in ways that remain unexplored.  

We compiled data for adult size, fertilization mode, developmental mode, and latitude for 1,232 species of marine annelids, echinoderms, and molluscs, and for a subset of the species for which it was available, we compiled additional data for offspring size. Most of the species in our dataset came from previously published meta-analyses on various marine invertebrate life-history traits (Marshall et al. 2012; Monro and Marshall 2015), supplemented with additional data for the adult size from the literature. Data were compiled in Microsoft Excel and analyzed in RStudio v. 4.2.2 (RStudio Team 2022).

Literature cited

• Marshall, D. J., P. J. Krug, E. K. Kupriyanova, M. Byrne, and R. B. Emlet. 2012. The biogeography of marine invertebrate life histories. Annual Review of Ecology, Evolution, and Systematics 43:97–114.
• Monro, K., and D. J. Marshall. 2015. The biogeography of fertilization mode in the sea. Global Ecology and Biogeography 24:1499–1509.
• RStudio Team. 2022. RStudio: Integrated Development Environment for R. RStudio, PBC, Boston, MA URL http://www.rstudio.com/.

Programs/software required to open/analyze the data files:

• Microsoft Excel (view raw data and metadata, .csv files for analyses)
• Adobe Reader (view PDF's of phylogenetic trees)
• RStudio (open files, manipulate and analyze data)