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).