Macroecological patterns of life history traits, such as larval size, are fundamental to understanding the biogeography and diversity of marine invertebrates. The long-established Thorson's Rule hypothesizes an increase in maternal energy allocation per offspring with increasing latitude, primarily as an adaptation to offset slower development in cold, food-limited waters. Here, we conduct the largest analysis to date on decapod crustaceans using the size of the first larval instar as a proxy for per offspring investment, to test whether reproductive and developmental patterns are governed by environmental factors on an evolutionary timescale. Our results confirm a strong phylogenetic signal in larval size and reveal that it systematically increases with latitude and greater water depth. This pattern aligns with ambient habitat conditions of lower temperature and higher dissolved oxygen. As expected, we found a strong inverse relationship between larval size and the number of larval stages, confirming that larger, provisioned offspring undergo abbreviated development. In addition, our findings support the hypothesis that increased per offspring investment and larval abbreviation represent a conservative, bet-hedging evolutionary strategy to minimize the vulnerable planktonic duration in cold, high-latitude and deep-sea environments. The deep-sea trend is, therefore, a conceptual extension of the latitudinal gradient into the vertical dimension. This study consolidates the notion that temperature, oxygen and developmental constraint, are the overriding global drivers of offspring investment strategies in decapod crustaceans, shaping their evolutionary pathways and global macroecological patterns.
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