Results
Of the 3,315 experiments in the dataset, only 1.7% followed individuals through multiple stages of the life cycle (Figure 2b). Studies were most commonly conducted in the laboratory (88.2%) and focused on a single stage (67.4%; Figure 2b). Studies beginning with metamorphosis or the juvenile stage were rare across all experimental design methods—generally, experiments most often began with measuring F0 adults, embryos and larvae (Figure 3).
Cohort longitudinal studies focused on the F0 adult, embryo and larval stages (Figure 4a-b) whereas individual longitudinal studies focused on juveniles and F1 adult stages (Figure 4c-d). Across all the experimental design approaches, no experiment measured all six stages of the life cycle.
Because species with direct development do not have free-swimming larvae and do not metamorphose, we analysed those data in isolation. There were 260 experiments that used species with direct development—most experiments followed a cohort (46.9%) or focused on a single stage (51.9%) (Figure S2). Broadly, most experiments began with measuring the F0 adult stage (Figure S3), and experiments measuring sequential stages usually ended at the juvenile stage, meaning measurements of F1 adults were rare (Figure S4). However, there was one individual longitudinal study that measured all four stages (Figure S4C).
We also compared the relative frequency of the three development modes in our dataset to their frequency reported in Marshall et al.(2012). Studies of planktotrophic species (i.e. planktonic, feeding larvae) were overrepresented in our literature map—they were used in 61.8% of experiments and 64.1% of articles (some articles had more than one experiment [e.g. multiple single stage experiments] or used one species that exhibits more than one development mode [i.e. “poecilogony”]). Compared to Marshall et al. (2012), lecithotrophic species (i.e. planktonic, non-feeding larvae) were underrepresented in articles by 25.7%, and direct developing species (i.e. crawl-away juveniles) by 45.7%.
The most common phyla studied were Mollusca (34.9%), Echinodermata (21.8%) and Arthropoda (13.1%), accounting for ~70% of the species in the map (Figure 5a). Of the 1,225 resolved species, the ten most common species make up 13.8% of the dataset (n = 459 experiments). Further, five of the ten most common species were echinoderms: four sea urchins (Arbacia punctulata ,Paracentrotus lividus , Strongylocentrotus droebachiensis ,Strongylocentrotus purpuratus ) and one sand dollar (Dendraster excentricus ) (Figure 5b).