The distribution of seaweed forms and foundational assumptions in seaweed biology.

Seaweeds are the most phylogenetically diverse group of multicellular organisms and rank foremost among marine keystone species. Due to their taxonomic diversity and functional importance, previous studies have classified seaweeds into functional groups based on qualitative or semi-quantitative traits, such as seaweed form, anatomy, and thickness. Despite the widespread use of seaweed functional groups from basic marine ecology to coastal monitoring, it is not known how accurate such morphology-based proposals are in grouping seaweeds by their form. To address this uncertainty at the foundations of seaweed biology, we surveyed and gathered all available data on seaweed forms using PRISMA protocols. We used the surface area to volume ratio (SA:V), a quantitative and universal measure of seaweed form, to assess the distribution and diversity of seaweed morphology across 99 species from three phyla. We show that seaweed surface area to volume ratio values span 3.64 orders of magnitude and follow a continuous and exponential distribution, without any significant gaps or clusters. We also tested current functional group schemes based on morphology and anatomy and showed that only 30% to 38% of their groups showed any significant pairwise differences in morphology. Our results challenge the basis of the current functional group approach in seaweed biology and suggest that a trait-based framework based on quantitative and continuous measures of seaweed form could provide a simpler and more accurate alternative to functionally assess seaweed ecology and physiology, as well as its implications for coastal ecosystem management.