Aim: Our aim was to evaluate the role of ecological and life-history factors in shaping global variation in offspring size in a marine clade with a diverse range of parental care behaviours.

Location: Global.

Time period: Data sourced from literature published from 1953 until 2019.

Major taxa studied: Marine teleost fishes.

Methods: We compiled a species-level dataset of egg and hatchling size for 1,639 species of marine fish across 45 orders. We used Bayesian phylogenetic mixed models to evaluate the relationship between offspring size and environmental factors (i.e., mean temperature, chlorophyll-a and dissolved oxygen content together with their annual variation), as well as latitude, reproductive strategy, parental body size and fecundity. We also tested long-standing hypotheses about the co-evolution of offspring size and the presence of parental care in BayesTraits.

Results: After controlling for parental body size and phylogenetic history, we find that increased egg size is associated with colder and oxygen-rich waters, while hatch size further depends on food supply and the reproductive strategy exhibited by the species. Irrespective of the initial investment in egg size, species with parental care or demersal egg development yield larger hatchlings compared to pelagic spawners. We also demonstrate that hatch size has co-evolved with advanced forms of care in association with parental body but fail to find a relationship with other types of care.

Main conclusions: Our study shows that parental care behaviours, together with environmental context, influence the evolution of classic life-history trade-offs on a global scale. While the initial investment in eggs is driven primarily by temperature and oxygen content, hatchling size also reflects the impact of care an offspring has received throughout development. In support of the ‘offspring-first’ hypothesis, we find that an increase in hatch size drives the evolution of advanced care provision. 

Dataset includes two proxies for offspring size – egg size and larval size at hatching – as well as taxonomic information, parental body size, reproductive strategy, caregiver identity, fecundity, latitude of species range midpoint and a set of environmental variables characterising each species' range (please see the main manuscript and Supporting Information Methods S1 for further details). Main sources of information included global and regional datasets, larval guides, online databases and family-level reviews; please see the Reference List to Supplementary Data for further details. For consistency, we attempted where possible to score both offspring proxies from the same source. The information was collected primarily from secondary sources (see above), but we also consulted the primary research articles referenced in the secondary literature in cases where only one proxy for offspring size was available. These instances have been marked up in the References columns (e.g., ‘Ref Egg Size’) in the Supplementary Dataset using the following format: [reference to primary source] in [reference to secondary source]. As an additional step, we performed an automated search in Web of Science using search terms ‘repro*’/ ‘egg*’/ ‘larva*’/ ‘care*’/ ‘early life’/ ‘life history’/ ‘spawn*’ in combination with a genus name to identify sources of information in cases where the reproductive strategy for some species within the genus with known offspring size was uncertain or unobserved; asterisks (*) denote multiple possible suffixes.