Mechanical properties of the wave-swept kelp, Egregia menziesii, change with season, growth rate, and herbivore wounds

Abstract

The resistance of macroalgae to damage by hydrodynamic forces depends on the mechanical properties of their tissues. Although factors such as water-flow environment, algal growth rate and damage by herbivores have been shown to influence various material properties of macroalgal tissues, the interplay of these factors as they change seasonally and affect algal mechanical performance has not been worked out. We used the perennial kelp Egregia menziesii to study how the material properties of the rachis supporting a frond changed seasonally over a 2 year period, and how those changes correlated with seasonal patterns of the environment, growth rate and herbivore load. Rachis tissue became stiffer, stronger and less extensible with age (distance from the meristem). Thus, slowly growing rachises were stiffer, stronger and tougher than rapidly growing ones. Growth rates were highest in spring and summer when upwelling and long periods of daylight occurred. Therefore, rachis tissue was most resistant to damage in the winter, when waves were large as a result of seasonal storms. Herbivory was greatest during summer, when rachis growth rates were high. Unlike other macroalgae, E. menziesii did not respond to herbivore damage by increasing rachis tissue strength, but rather by growing in width so that the cross-sectional area of the wounded rachis was increased. The relative timing of environmental factors that affect growth rates (e.g. upwelling supply of nutrients, daylight duration) and of those that can damage macroalgae (e.g. winter storms, summer herbivore outbreaks) can influence the material properties and thus the mechanical performance of macroalgae.

Rapid Adaptation to Changing Mechanical Load by Ordered Recruitment of Identified Motor Neurons

“…In their natural environment, Aplysia feed on a variety of seaweeds with different biomechanical properties, including variation in toughness, texture and size (Howells, 1942;Susswein et al, 1984;Carefoot, 1987). Seaweeds' biomechanical properties vary significantly to resist breakage (Koehl, 1986) and change biomechanically in response to herbivory (Burnett and Koehl, 2019).…”

The brain as a dynamically active organ

“…The animal's vision is poorly developed and the smell or the superficial texture of the seaweed are unreliable predictors of its biomechanical properties, such as toughness or size. The different kinds of seaweeds it feeds on not only vary dramatically in these biomechanical properties before the animals arrive, but they can also change in response to herbivory, or once the animal has started to ingest [126][127][128][129]. Therefore, Aplysia has no other choice than trying out how to best ingest the seaweed it is encountering.…”

Age affects the strain‐rate dependence of mechanical properties of kelp tissues

“…ndbc.noaa.gov). In a concurrent study, material properties of E. menziesii were compared between sites and found to be similar (Burnett and Koehl, 2019). At each site, fronds were collected by walking along a transect parallel to the shoreline and haphazardly removing a frond from every third E. menziesii encountered.…”

“…dependence of tissue mechanical properties of macroalgae is poorly understood in the context of the range of strain rates encountered in their natural habitats. The mechanical properties of biological tissues can change with age (e.g., Meinzer et al, 2011;Derby and Akhtar, 2015;Morgan et al, 2018), and a few studies have documented agerelated differences in the material properties of macroalgal tissues (Armstrong, 1988;Johnson and Koehl, 1994;Krumhansl et al, 2015;Burnett and Koehl, 2019). Macroalgae and plants are composed of tissues of different ages that function together to produce the mechanical behavior of the whole organism (e.g., Meinzer et al, 2011;Krumhansl et al, 2015).…”