Across plants and animals, genome size is often correlated with life history traits: large genomes are correlated with larger seeds, slower development, larger body size, and slower cell division. Among decapod crustaceans, caridean shrimps are among the most variable both in terms of genome size variation and life history characteristics such as larval development mode and egg size, but the extent to which these traits are associated in a phylogenetic context is largely unknown. In this study, we examine correlations among egg size, larval development, and genome size in two different genera of snapping shrimp, Alpheus and Synalpheus, using phylogenetically informed analyses. In both Alpheus and Synalpheus, egg size is strongly linked to larval development mode: species with abbreviated development had significantly larger eggs than species with extended larval development. We produced the first comprehensive dataset of genome size in Alpheus (n = 37 species), and demonstrated that genome size was strongly and positively correlated with egg size in both Alpheus and Synalpheus. Correlated trait evolution analyses showed that in Alpheus, changes in genome size were clearly dependent on egg size. In Synalpheus, evolutionary path analyses suggest that changes in development mode (from extended to abbreviated) drove increases in egg volume; and larger eggs, in turn, resulted in larger genomes. These data suggest that variation in reproductive traits may underpin the high degree of variation in genome size seen in a wide variety of caridean shrimp groups more generally.

Egg size: Egg length and width were measured (and egg volume calculated) as described in the methods. Egg data were log-transformed.

Development mode: Development mode was coded from the literature, and from examination of first-stage larval phases from collections or from the literature.

Body size: Body size was measured as carapace length.

Genome size: Genome size was measured using Feulgen Image Analysis Densitometry or flow cytometry, as detailed in the methods.