DOI: 10.25338/b89924
title: Morphology and herbivory of Egregia menziesii at sites from California to Washington

description: Herbivores can drastically alter the morphology of macroalgae by directly
 consuming tissue and by inflicting structural wounds.  Wounds, in
 particular, can result in large amounts of tissue breaking away from
 macroalgae, amplifying the damage initially caused by
 herbivores.  Herbivores that commonly wound macroalgae often only
 occur over a portion of a macroalga’s lifespan or geographic
 range.  However, we know little about the influence of these
 periodic or regional occurrences of herbivores on the large-scale seasonal
 and geographical patterns of macroalgal morphology.  We used the
 intertidal kelp Egregia menziesii to investigate how the kelp’s morphology
 and the prevalence of two prominent kelp-wounding herbivores (limpets and
 gammarid amphipods) changed over two seasons (spring and summer) and over
 the northern extent of the kelp’s geographic range (six sites from central
 California to northern Washington).  Wounds from limpets and
 amphipods often result in the kelp’s fronds being pruned (intercalary
 meristem broken away), so kelp morphology was quantified as size (combined
 length of all fronds) and pruning (proportion of broken fronds). 
 In both seasons, limpets were the dominant herbivore at southern sites
 while amphipods were dominant at northern sites.  This pattern
 was likely driven by each herbivore species’ unique response to local wave
 action, temperature, and tidal regimes.  Within each season, most
 kelp had a similar morphology and collective herbivore prevalence (limpets
 and/or amphipods).  Our results suggest that large-scale
 geographic similarities in macroalgal-wounding, despite regional variation
 in the herbivores inflicting the wounds, can maintain similar macroalgal
 morphologies over large geographic areas.
description: We surveyed five sites over the northern distribution of
 <i>Egregia menziesii </i>(from central California to
 Washington) in spring and summer 2016, with an additional site added in
 the summer.  Survey sites, dates, and sample sizes are listed in Table 1. 
 At each site, we haphazardly placed a 20-m transect parallel to the
 shoreline within the <i>E. menziesii </i>zone.  Along the
 transect, we selected every third kelp that was a mature sporophyte
 (<i>sensu </i>Henkel and Murray, 2007) and measured two
 morphological features: (1) size - the combined length of all fronds on a
 kelp, measured to the nearest 1 cm, and (2) pruning - the fraction of
 fronds that were missing their intercalary meristem (i.e., the region of
 the rachis where the fastest growth occurs).  Only fronds that were longer
 than 10 cm were included in these measurements because fronds &lt;10
 cm made up a negligible portion of a kelp’s total size and could not
 always be reliably counted.  By this measurement, kelp can be large by
 having numerous short fronds or a few long fronds (Burnett and Koehl,
 2020).  We also scored whether the kelp showed signs of herbivory from
 limpets and amphipods, which included the presence of the organisms on the
 kelp or their distinctive wounds (i.e., home scars on rachis caused by
 limpets, burrows in rachis caused by amphipods).  We measured as many kelp
 as possible within a single low tide, using multiple transects when
 necessary (samples sizes per site ranged from 25 to 52 kelp).