Characterizing morphology of Egregia menziesii (Laminariales) in California over 2 centuries using historical and contemporary herbarium specimens

TL;DR

This study used historical and modern kelp specimens to show how Egregia menziesii's shape changes with environmental factors like temperature and waves over 200 years.

Contribution

The study introduces digital morphometrics applied to herbarium specimens to analyze long-term morphological trends in kelp.

Findings

Egregia menziesii morphology varies along a latitudinal gradient and is influenced by seawater temperature and wave height.

Recent decades show region-specific morphological changes, suggesting environmental impacts over time.

Monthly sporophyll presence in southern specimens offers insights into reproductive phenology.

Abstract

The canopy‐forming feather boa kelp Egregia menziesii exhibits remarkable morphological variability across its geographic range. Regional morphotypes of Egregia were once considered separate species, but they were not determined to be genetically distinct; instead, their morphology was thought to reflect local physical or environmental conditions. Although morphological variation in Egregia has long been observed and was previously characterized through field surveys in the early 2000s, we revisited this topic using digital morphometrics (i.e., image analysis) of 1624 macroalgal herbarium specimens from California dating back to the 19th century. We observed that the morphology of Egregia (rachis texture, lateral blade shape, and blade or pneumatocyst density) varied along a latitudinal gradient and could be predicted by seawater temperature and wave height. We also identified some region‐specific morphological changes in recent decades. Further, the monthly presence or absence of sporophylls in southern‐region specimens provided preliminary evidence into the reproductive phenology of Egregia. Herbarium collections are invaluable for studying patterns in morphology because they showcase inter‐ and intraspecific variability and establish a baseline for comparison through time. Integrating natural historical and contemporary data will be critical for understanding and predicting future trends in the context of ocean warming.

Figures