Biomass-to-volume ratio as a central continuous functional trait for marine zooplankton

Gelatinous zooplankton are an important component of many ecosystems and important for ecosystem structure and carbon cycling. However, this group is generally not considered in biogeochemical models. Here we investigate the biomass-to-volume ratio as an underappreciated “master trait” that allows for the incorporation of a large diversity of zooplankton groups into modeling exercises. By considering the biomass-to-volume ratio as a continuum, we investigate the potential trade-offs between body composition and physiological (e.g., clearance, respiration, carbon mass-specific growth, assimilation) as well as ecological (e.g., predator–prey size ratio, feeding modes) traits. We find that a low carbon composition has a positive effect on the organism's fitness, as more prey could be captured for the same active mass. Thus, taking the biomass-to-volume ratio into account could improve the estimation of physiological rates. Additionally, we show that gelatinous feeding-current feeders (e.g., tunicata, Mnemiopsis spp., Rhizostoma spp.) have an ability to catch smaller prey over a wider size range than non-gelatinous feeding-current feeding organisms (gelatinous feeding-current feeders min–max: 10²–10⁶ μm_predator μm_prey⁻¹; non-gelatinous feeding-current feeders min–max: 5 × 10⁰–8 × 10¹ μm_predator μm_prey⁻¹). However, results are only valid for the respective feeding mode, highlighting new trade-offs. This allows us to re-evaluate the functional role of certain organisms, such as larvaceans (appendicularians), which were previously considered to be super-filters, or pteropods, which remain understudied. This study contributes to a wider representation of the complexity of the zooplankton community in size-structured models. We highlight that the biomass-to-volume ratio, along with size, is the most important parameter required to represent the full diversity of zooplankton.