Cryptic diversity in southern African kelp

The southern coast of Africa is one of the few places in the world where water temperatures are predicted to cool in the future. This endemism-rich coastline is home to two sister species of kelps of the genus Ecklonia maxima and Ecklonia radiata, each associated with specific thermal niches, and occuring primarily on opposite sides of the southern tip of Africa. Historical distribution records indicate that E. maxima has recently shifted its distribution ~ 70 km eastward, to sites where only E. radiata was previously reported. The contact of sister species with contrasting thermal affinities and the occurrence of mixed morphologies raised the hypothesis that hybridization might be occurring in this contact zone. Here we describe the genetic structure of the genus Ecklonia along the southern coast of Africa and investigate potential hybridization and cryptic diversity using a combination of nuclear microsatellites and mitochondrial markers. We found that both species have geographically discrete genetic clusters, consistent with expected phylogeographic breaks along this coastline. In addition, depth-isolated populations were found to harbor unique genetic diversity, including a third Ecklonia lineage. Mito-nuclear discordance and high genetic divergence in the contact zones suggest multiple hybridization events between Ecklonia species. Discordance between morphological and molecular identification suggests the potential influence of abiotic factors leading to convergent phenotypes in the contact zones. Our results highlight an example of cryptic diversity and hybridization driven by contact between two closely related keystone species with contrasting thermal affinities.

Kelps on demand: Closed-system protocols for culturing large bull kelp sporophytes for research and restoration

Culturing kelps for commercial, conservation, and scientific purposes is becoming increasingly widespread. However, kelp aquaculture methods are typically designed for ocean-based farms, and these methods may not be applicable for smaller scale cultivation efforts common in research and restoration. Growing kelps in closed, recirculating culture systems may address many of these constraints, yet closed system approaches have remained largely undescribed. Extensive declines of the bull kelp (Nereocystis luetkeana), an ecologically important canopy species in the Northeast Pacific, have received widespread attention and prompted numerous research and conservation initiatives. Here, we detail two approaches for cultivating N. luetkeana sporophytes in closed recirculating systems. Nereocystis luetkeana were reared as attached thalli in custom seaweed growth flumes and also free-floating in tumble culture tanks. Careful control of stocking density, water motion, aeration, and nutrient levels allowed for rapid growth and normal morphogenesis of laboratory-grown kelp. Culture systems reached up to 3 kg · m-3 , and individual thalli attained lengths of up to 6 m before the trials were terminated. Our results demonstrate the potential of recirculating, closed culture systems to overcome limitations associated with traditional culture methods. Recirculating systems enable the precise control of culture conditions, improving biosecurity and facilitating cultivar development and other research. Kelps can be grown away from the ocean or outside their native ranges, and seasonal or annual species can be produced year-round without seasonal constraints.

A Life Outside

How do the morphologies of organisms affect their physical interactions with the environment and other organisms? My research in marine systems couples field studies of the physical habitats, life history strategies, and ecological interactions of organisms with laboratory analyses of their biomechanics. Here, I review how we pursued answers to three questions about marine organisms: (a) how benthic organisms withstand and utilize the water moving around them, (b) how the interaction between swimming and turbulent ambient water flow affects where small organisms go, and (c) how hairy appendages catch food and odors. I also discuss the importance of different types of mentors, the roadblocks for women in science when I started my career, the challenges and delights of interdisciplinary research, and my quest to understand how I see the world as a dyslexic.

Diversity and Carbon Sequestration of Seaweed in the Ma’an Archipelago, China

Seaweed communities perform a variety of ecological services, including primary productivity supply, biological habitat construction, water purification, and acting as marine carbon sinks. The abundance of seaweed is the basis for the assessment of ecological services in communities. The Ma’an Archipelago, adjacent to the Yangtze River estuary in China, is an important and typical island group. In this study, the abundance of seaweed in the typical coastal islands of the Ma’an Archipelago, Zhejiang Province, was evaluated by means of sonar detection and scuba diving sampling methods. The organic carbon content of six dominant seaweed species was measured to estimate the carbon sequestration capacity of the dominant species in the Ma’an Archipelago. The results show that 27 species of Rhodophyta, 10 species of Ochrophyta, and two species of Chlorophyta were found in the Ma’an Archipelago. Seaweed was distributed in the coastal areas of the islands, with a distribution width of 2–60 m. Gouqi Island had the longest shoreline, and there, the distribution depth of the seaweed reached 15 m and the area of the seaweed community was the largest. The slope of the rocks in the Sanheng survey area was large and the width of the seaweed community was small. The distribution area of seaweed in the Ma’an Archipelago was 6.51–13.43 km2 and the organic carbon content of the seaweed was 33.16 ± 3.26%. The biomass of Ochrophyta in the Ma’an Archipelago was the largest, followed by Chlorophyta and Rhodophyta. Among the six dominant species, the carbon sequestration of Sargassum thunbergii was the largest, at 277.91–848.74 t per year, and that of Undaria pinnatifida was the smallest. This study provides scientific guidance for the assessment of the primary productivity supply, carbon sink, and conservation capacity of seaweeds in China.

Ecological biomechanics of damage to macroalgae

Macroalgae provide food and habitat to a diversity of organisms in marine systems, so structural damage and breakage of thallus tissue can have important ecological consequences for the composition and dynamics of marine communities. Common sources of macroalgal damage include breakage by hydrodynamic forces imposed by ambient water currents and waves, tissue consumption by herbivores, and injuries due to epibionts. Many macroalgal species have biomechanical designs that minimize damage by these sources, such as flexibly reconfiguring into streamlined shapes in flow, having either strong or extensible tissues that are tough, and having chemical and morphological defenses against herbivores and epibionts. If damage occurs, some macroalgae have tissue properties that prevent cracks from propagating or that facilitate tissue breakage in certain places, allowing the remainder of the thallus to survive. In contrast to these mechanisms of damage control, some macroalgae use breakage to aid dispersal, while others simply complete their reproduction prior to seasonally-predictable periods of damage (e.g., storm seasons). Once damage occurs, macroalgae have a variety of biomechanical responses, including increasing tissue strength, thickening support structures, or altering thallus shape. Thus, macroalgae have myriad biomechanical strategies for preventing, controlling, and responding to structural damage that can occur throughout their lives.

Marine Autotroph-Herbivore Synergies: Unravelling the Roles of Macroalgae in Marine Ecosystem Dynamics

Simple Summary

Invasive species are a leading hazard to marine ecosystems worldwide, coupled with climate change. Tackling the emerging biodiversity threat to maintain the ecological balance of the largest biome in the world has now become a pivotal part of the Sustainable Development Goals (SDGs). Marine herbivores are generally regarded as biological agents that restrict invasive species, and their efficiency depends on their dietary habits, especially the autotrophs they eat. Many researchers have found contradicting findings on the effects of nutritional attributes and novelty of autotrophs on herbivore eating behaviour. In light of the scattered literature on the mechanistic basis of autotroph-herbivore interactions, we provide a comprehensive review to fill knowledge gaps about synergies based on macroalgae, an important group of photosynthetic organisms in the marine biome that interact strongly with generalist herbivores. We also analyse macroalgal defence measures against herbivores, underlining unique features and potential roles in maintaining marine ecosystems. The nutritional qualities, shape, and novelty of autotrophs can alter herbivore feeding behaviour. Future research should explore aspects that can alter marine autotroph-herbivore interactions to resolve inconsistent results of specific features and the uniqueness of the organisms involved.

Abstract

Species invasion is a leading threat to marine ecosystems worldwide, being deemed as one of the ultimate jeopardies for biodiversity along with climate change. Tackling the emerging biodiversity threat to maintain the ecological balance of the largest biome in the world has now become a pivotal part of the Sustainable Development Goals (SDGs). Marine herbivores are often considered as biological agents that control the spread of invasive species, and their effectiveness depends largely on factors that influence their feeding preferences, including the specific attributes of their food–the autotrophs. While the marine autotroph-herbivore interactions have been substantially discussed globally, many studies have reported contradictory findings on the effects of nutritional attributes and novelty of autotrophs on herbivore feeding behaviour. In view of the scattered literature on the mechanistic basis of autotroph-herbivore interactions, we generate a comprehensive review to furnish insights into critical knowledge gaps about the synergies based largely on the characteristics of macroalgae; an important group of photosynthetic organisms in the marine biome that interact strongly with generalist herbivores. We also discuss the key defence strategies of these macroalgae against the herbivores, highlighting their unique attributes and plausible roles in keeping the marine ecosystems intact. Overall, the feeding behaviour of herbivores can be affected by the nutritional attributes, morphology, and novelty of the autotrophs. We recommend that future research should carefully consider different factors that can potentially affect the dynamics of the marine autotroph-herbivore interactions to resolve the inconsistent results of specific attributes and novelty of the organisms involved.

Less Than the Sum of Its Parts: Blade Clustering Reduces Drag in the Bull Kelp, Nereocystis luetkeana (Phaeophyceae)

Nereocystis luetkeana is a large, canopy-forming kelp that is commonly found in nearshore waters between Alaska and California. Despite regularly reaching lengths in excess of 30 m, this alga demonstrates a remarkable ability to endure hydrodynamically stressful environments without being dislodged by waves or currents. While morphological aspects of this kelp, including its long flexible stipe, have been shown to reduce drag, blade clustering has never been directly tested. In this study, we examined how the distinctive multi-bladed morphology of Nereocystis thalli limits drag in flow. We measured drag on whole kelps in a large recirculating flume and quantified how drag was affected by serial removal of blades. We then compared measured drag with predictions of "additive drag", which we defined as the sum of the drag that each blade experiences in isolation. We found that, on average, intact thalli experience only 37% of the predicted additive drag when subjected to a flow rate of 0.40-0.50 m · s-1 . Our results indicate that the subdivision of the photosynthetic area into multiple blades has a drag-reducing effect on Nereocystis thalli. We found drag increases less than proportionally with the cumulative area, meaning the contribution of individual blades to overall drag decreases with increasing blade number. That is, as thalli develop, each additional blade incurs a reduced hydrodynamic cost, perhaps lending insight into how thalli can grow so large.

Leaf morphogenesis: The multifaceted roles of mechanics

Plants produce a rich diversity of biological forms, and the diversity of leaves is especially notable. Mechanisms of leaf morphogenesis have been studied in the past two decades, with a growing focus on the interactive roles of mechanics in recent years. Growth of plant organs involves feedback by mechanical stress: growth induces stress, and stress affects growth and morphogenesis. Although much attention has been given to potential stress-sensing mechanisms and cellular responses, the mechanical principles guiding morphogenesis have not been well understood. Here we synthesize the overarching roles of mechanics and mechanical stress in multilevel and multiple stages of leaf morphogenesis, encompassing leaf primordium initiation, phyllotaxis and venation patterning, and the establishment of complex mature leaf shapes. Moreover, the roles of mechanics at multiscale levels, from subcellular cytoskeletal molecules to single cells to tissues at the organ scale, are articulated. By highlighting the role of mechanical buckling in the formation of three-dimensional leaf shapes, this review integrates the perspectives of mechanics and biology to provide broader insights into the mechanobiology of leaf morphogenesis.

Ripples at Edges of Blooming Lilies and Torn Plastic Sheets

Ripples arise at edges of petals of blooming Lilium casablanca flowers and at edges of torn plastic sheets. In both systems, ripples are a consequence of excess length along the edge of a sheet. Through the use of time-lapse videos of blooming lilies and published images of torn plastic sheets, we find that ripples in both systems are well-described by the scaling relationship a∝w(L-w), where a is amplitude, w is wavelength, and L is arc length. A phenomenological relationship previously reported for self-similar ripple patterns, namely 〈a〉∝〈w〉, can be recovered by assuming that buckling stress is constant. Excess length along petal edges can also influence their overall Gaussian curvature, such that petals invert from a cup shape to a saddle shape upon blooming. Previous simulations of these shape changes have assumed that petal thickness decreases at least quadratically. Here, we evaluate tomograms of several varieties of lily buds and find that this assumption is valid along the short axis of the buds, but not the long axis. A challenge of employing traditional tomography methods to measure petal thickness is that the sample is destroyed; a single bud cannot be followed through the entire blooming process. To address this challenge, we provide proof of principle that the non-destructive, label-free method of X-ray tomography produces high-contrast 3-dimensional scans on time scales short enough to follow lily blooming.

Ecological biomechanics of marine macrophytes

Macroalgae and seagrasses in coastal habitats are exposed to turbulent water currents and waves that deform them and can rip them off the substratum, but that also transport essential water-borne substances to them and disperse their propagules and wastes. Field studies of the physical environment, ecological interactions, and life history strategies of marine macrophytes reveal which aspects of their biomechanical performance are important to their success in different types of natural habitats and enable us to design ecologically relevant laboratory experiments to study biomechanical function. Morphology and tissue mechanical properties determine the hydrodynamic forces on macrophytes and their fate when exposed to those forces, but different mechanical designs can perform well in the same biophysical habitat. There is a trade-off between maximizing photosynthesis and minimizing breakage, and some macrophytes change their morphology in response to environmental cues. Water flow in marine habitats varies on a wide range of temporal and spatial scales, so diverse flow microhabitats can occur at the same site. Likewise, the size, shape, and tissue material properties of macrophytes change as they grow and age, so it is important to understand the different physical challenges met by macrophytes throughout their lives.

Flow, form, and force: methods and frameworks for field studies of macroalgal biomechanics

Macroalgae are ecologically important organisms that often inhabit locations with physically challenging water motion. The biomechanical traits that permit their survival in these conditions have been of interest to biologists and engineers alike, but logistical and technical challenges of conducting investigations in macroalgal habitats have often prevented optimal study of these traits. Here, we review field methods for quantifying three major components of macroalgal biomechanics in moving water: fluid flow, macroalgal form, and hydrodynamic force. The implementation of some methodologies is limited due to the current state and accessibility of technology, but many of these limitations can be remedied by custom-built devices, borrowing techniques from other systems, or shifting lab-based approaches to the field. We also describe several frameworks for integrating flow, form, and force data that can facilitate comparisons of macroalgal biomechanics in field settings with predictions from theory and lab-based experiments, or comparisons between flow conditions, habitats, and species. These methods and frameworks, when used on scales that are relevant to the examined processes, can reveal mechanistic information about the functional traits that permit macroalgae to withstand physically challenging water motion in their habitats, using the actual fluid flows, macroalgal forms, and physical forces that occur in nature.

Kelp Morphology and Herbivory Are Maintained Across Latitude Despite Geographic Shift in Kelp-Wounding Herbivores

AbstractHerbivores can drastically alter the morphology of macroalgae by directly consuming tissue and by inflicting structural wounds. Wounds can result in large amounts of tissue breaking away from macroalgae, amplifying the damage initially caused by herbivores. Herbivores that commonly wound macroalgae often occur over only 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 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 we quantified kelp size (combined length of all fronds) and pruning (proportion of broken fronds). We found similar results in each season: herbivores were most likely to occur on large, pruned kelp regardless of site; and limpets were the dominant herbivore at southern sites, while amphipods were dominant at northern sites. Despite the geographic shift in the dominant herbivore, kelp had similar levels of total herbivore prevalence (limpets and/or amphipods) and similar morphologies across sites. Our results suggest that large-scale geographic similarities in macroalgal wounding, despite regional variation in the herbivore community, can maintain similar macroalgal morphologies over large geographic areas.

Reproductive phenology and morphology of Macrocystis pyrifera (Laminariales, Ochrophyta) from southern New Zealand in relation to wave exposure1

Macrocystis pyrifera is a major habitat forming kelp in coastal ecosystems of temperate regions of the northern and southern hemispheres. We investigated the seasonal occurrence of adult sporophytes, morphological characteristics, and reproductive phenology at two sites within a wave-protected harbour and two wave-exposed sites in southern New Zealand every 3-4 months between 2012 and 2013. Seasonality in reproduction was assessed via the number of sporophylls, the occurrence of sori on sporophylls, and non-sporophyllous laminae (fertile pneumatocyst-bearing blades and fertile apical scimitars), meiospore release, and germination. We found that M. pyrifera was present and reproductive year-round in three of the four sites, and patterns were similar for the wave-exposure conditions. Sori were found on pneumatocyst-bearing blades and apical scimitars in addition to the sporophylls, and viable meiospores were released from all three types of laminae. Morphological variations between sites with different wave exposure indicate that sporophytes from wave-protected sites have bigger blades and holdfasts and are longer than those from wave-exposed sites. We discuss the implications of these biological variables for the ecology of M. pyrifera inhabiting different wave exposure environments in southern New Zealand.

The stress and strain of life - how differences in the mechanical properties and cellular composition enable the kelp Laminaria digitata to thrive in different hydrodynamic environments

Sessile organisms such as macroalgae located in the intertidal and shallow subtidal zones are subject to a hydrodynamically diverse environment, controlling the variation of intraspecific morphology and distribution. Kelp forests experience both waves and/or currents, yet, how kelp blade material mechanically differs between these various hydrodynamic environments and what drives the variation in strength and extensibility are not fully understood. Here, the mechanical properties, cellular composition and blade tissue thickness of the meristematic region and distal tips of the kelp Laminaria digitata blades were quantified and compared between seasons and among three hydrodynamic environments: wave dominated, current dominated and a benign hydrodynamic environment. Kelps associated with energetic environments, generally tended to be stronger yet more extensible than those growing in the benign hydrodynamic environment. Higher extensibility was located at the meristematic region whereas tissue was stronger in the distal tip of the blade. Linking both cellular composition and mechanical properties, results suggest enhancement of medulla cells in the meristematic region increases extensibility, potentially protecting the thallus during increased storm activity while growing in a wave/current exposed habitat. Investment in cortex cells towards the tip of the blade suggests an increase in strength of the region, which is susceptible to breakage. However, the lack of variation in the proportion of medulla and cortex cellular layers between distinct hydrodynamic environments revealed that the potential overall strategy for avoiding breakage in energetic hydrodynamic environments is that of investing energy into the increased thickness of blade tissue.

How kelp in drag lose their ruffles: environmental cues, growth kinematics, and mechanical constraints govern curvature

We reveal how patterns of growth in response to environmental cues can produce curvature in biological structures by setting up mechanical stresses that cause elastic buckling. Nereocystis luetkeana are nearshore kelp with wide ruffled blades that minimize self-shading in slow flow, but narrow flat blades that reduce hydrodynamic drag in rapid flow. Previously we showed that blade ruffling is a plastic trait associated with a transverse gradient in longitudinal growth. Here we consider expansion and displacement of tissue elements due to growth in blades, and find that growth patterns are altered by tensile stress due to hydrodynamic drag, but not by shading or nutrients. When longitudinal stress in a blade is low in slow flow, blade edges grow faster than the midline in young tissue near the blade base. Tissue elements are displaced distally by expansion of younger proximal tissue. Strain energy caused by the transverse gradient in longitudinal growth is released by elastic buckling once the blade grows wide enough, producing ruffles distal to the region where the growth inhomogeneity started. If a blade experiences higher stress in rapid flow, the edges and midline grow at the same rate, so the blade becomes flat as these new tissue elements are displaced distally.

Oxygen metabolism shapes microbial settlement on photosynthetic kelp blades compared to artificial kelp substrates

We examined factors shaping community assembly of the bull kelp (Nereocystis luetkeana) microbiome by comparing microbial biofilm formation on photosynthetic kelp blade tissues and artificial kelp substrates ('agar substrates') deployed into a kelp forest. New kelp blade tissues were colonized by markedly distinct microbial taxa relative to agar substrates during the same time interval, even when agar substrates were infused with N. luetkeana blades, suggesting that microbial settlement onto kelp surfaces is more than just attraction to a polysaccharide-rich surface. Further, common seawater taxa such as Colwellia sp. and Psychromonas sp. became abundant on agar substrates but avoided new kelp blade tissues, indicating that host-specific factors may deter certain surface-associated marine microbial taxa. Over two-thirds of the bacterial taxa in the kelp microbiome were associated with strictly aerobic metabolisms; thus, photosynthetic production of O2 may favour aerobic microbial metabolisms. While living kelp blades primarily recruited aerobic microbes, including the obligate aerobe Granulosicoccus sp., microbes that colonized agar substrates were predominantly facultative anaerobes. We also found that infusion of kelp tissues into agar substrates altered microbial community composition and lowered taxonomic diversity relative to control agar substrates, suggesting that non-living components of the kelp blade also impact microbial community assembly.

Elevated Temperature Affects Phenotypic Plasticity in the Bull Kelp (Nereocystis luetkeana, Phaeophyceae)

The sensitivity of kelps to elevated temperatures has been linked to recent declines in some kelp populations, with cascading impacts on marine communities. However, it remains unclear how thermal stress affects the ability of kelps to respond to other environmental factors, which could influence their vulnerability to climate change. We investigated the effect of thermal stress on the ability of the bull kelp Nereocystis luetkeana to acclimate to its surrounding hydrodynamic environment through tension-regulated plasticity in blade morphology. We first determined optimal and stressful temperatures for N. luetkeana by measuring growth over nine temperatures from 5°C to 22°C. We then exposed N. luetkeana blades to a factorial combination of temperature (13°C and 20°C) and tension (0.5 N and 2.0 N) simulating different flow conditions, and measured changes in blade length and width after 7 days. The temperature at which N. luetkeana exhibited maximum growth was estimated to be ~11.9°C, though growth was high over a relatively wide temperature range. When thermally stressed, N. luetkeana maintained morphological responses to simulated high flow, but were inhibited from acclimating to low flow, indicated by an inability of blades to widen. Our results suggest that N. luetkeana in sheltered habitats may be particularly vulnerable to climate warming, where an inability to adjust blade morphology to local hydrodynamic conditions could drive declines at sublethal levels of warming. As ecologically important foundation species, declines in sheltered kelp populations could result in major biodiversity loss and disrupt ecosystem function.

Morphological plasticity in the kelp Nereocystis luetkeana (Phaeophyceae) is sensitive to the magnitude, direction, and location of mechanical loading

Nereocystis luetkeana is a canopy-forming kelp that exhibits morphological plasticity across hydrodynamic gradients, producing broad, undulate blades in slow flow and narrow, flattened blades in fast flow, enabling thalli to reduce drag while optimizing photosynthesis. While the functional significance of this phenomenon has been well studied, the developmental and physiological mechanisms that facilitate the plasticity remain poorly understood. In this study, we conducted three experiments to characterize how the (1) magnitude, (2) direction, and (3) location of plasticity-inducing mechanical stimuli affect the morphology of Nereocystis blades. We found that applying a gradient of tensile force caused blades to grow progressively longer, narrower, less ruffled, and heavier in a linear fashion, suggesting that Nereocystis is equally well adapted for all conditions within its hydrodynamic niche. We also found that applying tension transversely across blades caused the growth response to rotate 90°, indicating that there is no substantial separation between the sites of stimulus perception and response and suggesting that a long-distance signaling mechanism, such as a hormone, is unlikely to mediate this phenomenon. Meristoderm cells showed morphological changes that paralleled those of their respective blades in this experiment, implying that tissue-level morphology is influenced by cell growth. Finally, we found that plasticity was only induced when tension was applied directly to the growing tissue, reinforcing that long-distance signaling is probably not involved and possibly indicating that the mechanism on display generally requires an intercalary meristem to facilitate mechanoperception.

By hook or by crook: how and why do compound leaves stay curved during development?

This article comments on:

Rivière M, Corre Y, Peaucelle A, Derr J, Douady S. 2020. The hook shape of growing leaves results from an active regulatory process. Journal of Experimental Botany 71, 6408–6417.

Comparing the effectiveness of twine- and binder-seeding in the Laminariales species Alaria esculenta and Saccharina latissima

The continuing expansion of seaweed cultivation could assist in ensuring future global food security. The Laminariales species Alaria esculenta and Saccharina latissima are each cultivated for food across their European ranges. The predominant method for cultivating European kelps involves growing juveniles on twine within a hatchery which is then deployed at a farm site. The associated hatchery and deployment cost of this approach are relatively high. A new and innovative methodology-called binder-seeding-can reduce these costs, but, has yet to be validated. We compare the biomass yield and morphology of A. esculenta and S. latissima cultured using either the traditional twine-longline method or binder-seeding onto AlgaeRope and AlgaeRibbon, specially designed textiles. In a controlled growth experiment, A. esculenta had a similar biomass yield on all materials, but fronds were shorter (23 ± 7%) and thinner on the AlgaeRibbon (42 ± 4%) due to a 3-4-fold higher density of developing sporophytes compared to the twine-longline. In contrast, S. latissima gave a 4-fold higher biomass yield on the AlgaeRibbon in June (4.0 kg m^-1), but frond morphology was not different between materials, despite a 4-fold higher sporophyte density on the AlgaeRibbon. The stipe length of both species also increased at the higher sporophyte density on the AlgaeRibbon. The AlgaeRope gave an intermediate response or was similar to the twine-longline. These results show that binder-seeding onto the AlgaeRibbon significantly increases the achieved biomass yield in S. latissima. These results can assist cultivators to select the most appropriate method of kelp cultivation depending on morphological/yield requirements of the end use. Further study is needed on the optimisation of the binder-seeding density and its impact on thallus morphology.

Water Affects Morphogenesis of Growing Aquatic Plant Leaves

Lotus leaves floating on water usually experience short-wavelength edge wrinkling that decays toward the center, while the leaves growing above water normally morph into a global bending cone shape with long rippled waves near the edge. Observations suggest that the underlying water (liquid substrate) significantly affects the morphogenesis of leaves. To understand the biophysical mechanism under such phenomena, we develop mathematical models that can effectively account for inhomogeneous differential growth of floating and freestanding leaves to quantitatively predict formation and evolution of their morphology. We find, both theoretically and experimentally, that the short-wavelength buckled configuration is energetically favorable for growing membranes lying on liquid, while the global buckling shape is more preferable for suspended ones. Other influencing factors such as the stem or vein, heterogeneity, and dimension are also investigated. Our results provide a fundamental insight into a variety of plant morphogenesis affected by water foundation and suggest that such surface instabilities can be harnessed for morphology control of biomimetic deployable structures using substrate or edge actuation.

Mechanics of a plant in fluid flow

Plants live in constantly moving fluid, whether air or water. In response to the loads associated with fluid motion, plants bend and twist, often with great amplitude. These large deformations are not found in traditional engineering application and thus necessitate new specialized scientific developments. Studying fluid-structure interaction (FSI) in botany, forestry, and agricultural science is crucial to the optimization of biomass production for food, energy, and construction materials. FSIs are also central in the study of the ecological adaptation of plants to their environment. This review paper surveys the mechanics of FSI on individual plants. I present a short refresher on fluid mechanics then dive into the statics and dynamics of plant-fluid interactions. For every phenomenon considered, I examine the appropriate dimensionless numbers to characterize the problem, discuss the implications of these phenomena on biological processes, and propose future research avenues. I cover the concept of reconfiguration while considering poroelasticity, torsion, chirality, buoyancy, and skin friction. I also assess the dynamical phenomena of wave action, flutter, and vortex-induced vibrations.

Individuality in seaweeds and why we need to care

Documenting the causes and consequences of intraspecific variation forms the foundation of much of evolutionary ecology. In this Perspectives piece, we review the importance of individual variation in ecology and evolution, argue that contemporary phycology often overlooks this foundational biological unit, and highlight how this lack of attention has potentially constrained our understanding of seaweeds. We then provide some suggestions of promising but underrepresented approaches, for instance: conducting more studies and analyses at the level of the individual; designing studies to evaluate heritability and genetic regulation of traits; and measuring associations between individual variation in functional traits and ecological outcomes. We close by highlighting areas of phycological research (e.g., population biology, ecology, aquaculture, climate change management) that could benefit immediately from including a focus on individual variation. Algae, for their part, provide us with a powerful and diverse set of ecological and evolutionary traits to explore these topics. There is much to be discovered.

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

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.

Under pressure: biomechanical limitations of developing pneumatocysts in the bull kelp (Nereocystis luetkeana, Phaeophyceae)

Maintaining buoyancy with gas-filled floats (pneumatocysts) is essential for some subtidal kelps to achieve an upright stature and compete for light . However, as these kelps grow up through the water column, pneumatocysts are exposed to substantial changes in hydrostatic pressure, which could cause complications as internal gases may expand or contract, potentially causing them to rupture, flood, and lose buoyancy. In this study, we investigate how pneumatocysts of Nereocystis luetkeana resist biomechanical stress and maintain buoyancy as they develop across a hydrostatic gradient. We measured internal pressure, material properties, and pneumatocyst geometry across a range of thallus sizes and collection depths to identify strategies used to resist pressure-induced mechanical failure. Contrary to expectations, all pneumatocysts had internal pressures less than atmospheric pressure, ensuring that thalli are always exposed to a positive pressure gradient and compressional loads, indicating that they are more likely to buckle than rupture at all depths. Small pneumatocysts collected from depths between 1 and 9 m (inner radius = 0.4-1.0 cm) were demonstrated to have elevated wall stresses under high compressive loads and are at greatest risk of buckling. Although small kelps do not adjust pneumatocyst material properties or geometry to reduce wall stress as they grow, they are ~3.4 times stronger than they need to be to resist hydrostatic buckling. When tested, pneumatocysts buckled around 35 m depth, which agrees with previous measures of lower limits due to light attenuation, suggesting that hydrostatic pressure may also define the lower limit of Nereocystis in the field.

Seawater nitrogen concentration and light independently alter performance, growth, and resource allocation in the bloom-forming seaweeds Ulva lactuca and Ulvaria obscura (Chlorophyta)

Ulva lactuca and Ulvaria obscura are seaweeds that form green tides on Salish Sea shores. They have similar macroscopic morphologies but differ in their biochemistries and physiological responses. To understand how they are affected by changes in environmental conditions, a factorial experiment was conducted in which algae were grown in artificial seawater with either low (10 μM) or high (160 μM) nitrate (NO₃^-) concentrations at high (29 mol photons·m^-2.day^-1) and low (4 mol photons·m^-2.day^-1) light levels. Light and NO₃^- affected algal responses, but always independently. After two weeks, U. lactuca grown in high light were larger, had lower maximum quantum yields (MQYs), and lower nitrogen (N), carbon (C), pigment, and dimethylsulfoniopropionate (DMSP) concentrations, respectively, relative to algae in low light. In contrast, U. obscura growth was unaffected by light. Like U. lactuca, U. obscura grown in high light had lower MQYs, and N, pigment, and DMSP concentrations. In high light, U. obscura also had 89% higher dopamine concentrations and a tendency to fragment. Both U. lactuca and U. obscura grown in 160 μM NO₃^- were larger, had higher MQYs, and higher N, pigment, and DMSP concentrations, respectively, than algae in 10 μM NO₃^-. Also, when U. obscura was grown in the 160 μM NO₃^- medium, it significantly increased its surface area/mass ratio. Although both species grew faster in high NO₃^-, high light only promoted the growth of Ulva, which may explain the dominance of Ulva in summer months. High light was physiologically stressful to both species and caused increases in photoprotective mechanisms, such as the production of dopamine, a melanin precursor, in Ulvaria, and DMSP lysis in Ulva to generate antioxidants. Growing in 10 μM NO₃- produced responses that were consistent with nitrogen limitation and had greater impacts on Ulvaria than Ulva, suggesting that Ulvaria responds more strongly to eutrophication.

Shell buckling: from morphogenesis of soft matter to prospective applications

Being one of the commonest deformation modes for soft matter, shell buckling is the primary reason for the growth and nastic movement of many plants, as well as the formation of complex natural morphology. On-demand regulation of buckling-induced deformation associated with wrinkling, ruffling, folding, creasing and delaminating has profound implications for diverse scopes, which can be seen in its broad applications in microfabrication, 4D printing, actuator and drug delivery. This paper reviews the recent remarkable developments in the shell buckling of soft matter to explain the most representative natural morphogenesis from the perspectives of theoretical analysis in continuum mechanics, finite element analysis, and experimental validations. Imitation of buckling-induced shape transformation and its applications are also discussed for the innovations of sophisticated materials and devices in future.

From geometric optics to plants: the eikonal equation for buckling

Optimal buckling of a tissue, e.g. a plant leaf, growing by means of exponential division of its peripheral cells, is considered in the framework of a conformal approach. It is shown that the boundary profile of a tissue is described by the 2D eikonal equation, which provides the geometric optic approximation for the wavefront propagating in a medium with an inhomogeneous refraction coefficient. A variety of optimal surfaces embedded in 3D is controlled by spatial dependence of the refraction coefficient which, in turn, is dictated by the local growth protocol.

Biological responses to environmental heterogeneity under future ocean conditions

Organisms are projected to face unprecedented rates of change in future ocean conditions due to anthropogenic climate-change. At present, marine life encounters a wide range of environmental heterogeneity from natural fluctuations to mean climate change. Manipulation studies suggest that biota from more variable marine environments have more phenotypic plasticity to tolerate environmental heterogeneity. Here, we consider current strategies employed by a range of representative organisms across various habitats - from short-lived phytoplankton to long-lived corals - in response to environmental heterogeneity. We then discuss how, if and when organismal responses (acclimate/migrate/adapt) may be altered by shifts in the magnitude of the mean climate-change signal relative to that for natural fluctuations projected for coming decades. The findings from both novel climate-change modelling simulations and prior biological manipulation studies, in which natural fluctuations are superimposed on those of mean change, provide valuable insights into organismal responses to environmental heterogeneity. Manipulations reveal that different experimental outcomes are evident between climate-change treatments which include natural fluctuations vs. those which do not. Modelling simulations project that the magnitude of climate variability, along with mean climate change, will increase in coming decades, and hence environmental heterogeneity will increase, illustrating the need for more realistic biological manipulation experiments that include natural fluctuations. However, simulations also strongly suggest that the timescales over which the mean climate-change signature will become dominant, relative to natural fluctuations, will vary for individual properties, being most rapid for CO2 (~10 years from present day) to 4 decades for nutrients. We conclude that the strategies used by biota to respond to shifts in environmental heterogeneity may be complex, as they will have to physiologically straddle wide-ranging timescales in the alteration of ocean conditions, including the need to adapt to rapidly rising CO2 and also acclimate to environmental heterogeneity in more slowly changing properties such as warming.

Morphogeometric Approaches to Non-vascular Plants

Morphometric analysis of organisms has undergone a dramatic renaissance in recent years, embracing a range of novel computational and imaging techniques to provide new approaches to phenotypic characterization. These innovations have often developed piece-meal, and may reflect the taxonomic specializations and biases of their creators. In this review, we aim to provide a brief introduction to applications and applicability of modern morphometrics to non-vascular land plants, an often overlooked but evolutionarily and ecologically important group. The scale and physiology of bryophytes (mosses, liverworts, and hornworts) differ in important and informative ways from more "traditional" model plants, and their inclusion has the potential to powerfully broaden perspectives in plant morphology. In particular we highlight three areas where the "bryophytic perspective" shows considerable inter-disciplinary potential: (i) bryophytes as models for intra-specific and inter-specific phenotypic variation, (ii) bryophyte growth-forms as areas for innovation in architectural modularity, and (iii) bryophytes as models of ecophysiological integration between organs, individuals, and stands. We suggest that advances should come from two-way dialog: the translation and adoption of techniques recently developed for vascular plants (and other organisms) to bryophytes and the use of bryophytes as model systems for the innovation of new techniques and paradigms in morphogeometric approaches.

Evidence of an evolutionary-developmental trade-off between drag avoidance and tolerance strategies in wave-swept intertidal kelps (Laminariales, Phaeophyceae)

Kelps are a clade of morphologically diverse, ecologically important habitat-forming species. Many kelps live in wave-swept environments and are exposed to chronic flow-induced stress. In order to grow and survive in these harsh environments, kelps can streamline (reducing drag coefficient) to avoid drag or to increase attachment and breakage force to tolerate it. We aimed to quantify the drag tolerance and streamlining strategies of kelps from wave-swept intertidal habitats. We measured drag coefficient and tenacity of populations from eight kelp species over a wide range of sizes to determine whether kelps avoid dislodgement by reducing drag coefficient or by increasing tenacity as they grow, and whether these traits are traded off. We employed phylogenetic comparative methods to rule out potentially confounding effects of species' relatedness. There was a significant negative relationship between drag avoidance and tolerance strategies, even after incorporating phylogeny. Kelps that were more tenacious were less able to reduce drag, resulting in a continuum from "tolerators" to "streamliners," with some species demonstrating intermediate, mixed strategies. Drag and tenacity were correlated with geometric properties (i.e., second moment of area) of the stipe in large kelps. Results presented in this study suggest that kelps are either strong or streamlined, but not both. This continuum is consistent with avoidance and tolerance trade-offs that have been documented in many different biological systems and may have widespread implications for the evolution of large macroalgae, perhaps driving morphological diversity within this group.

Resolving cryptic species of Bossiella (Corallinales, Rhodophyta) using contemporary and historical DNA

PREMISE OF THE STUDY

Phenotypic plasticity and convergent evolution have long complicated traditional morphological taxonomy. Fortunately, DNA sequences provide an additional basis for comparison, independent of morphology. Most importantly, by obtaining DNA sequences from historical type specimens, we are now able to unequivocally match species names to genetic groups, often with surprising results.

METHODS

We used an integrative taxonomic approach to identify and describe Northeast Pacific pinnately branched species in the red algal coralline genus Bossiella, for which traditional taxonomy recognized only one species, the generitype, Bossiella plumosa. We analyzed DNA sequences from historical type specimens and modern topotype specimens to assign species names and to identify genetic groups that were different and that required new names. Our molecular taxonomic assessment was followed by a detailed morphometric analysis of each species.

KEY RESULTS

Our study of B. plumosa revealed seven pinnately branched Bossiella species. Three species, B. frondescens, B. frondifera, and B. plumosa, were assigned names based on sequences from type specimens. The remaining four species, B. hakaiensis, B. manzae, B. reptans, and B. montereyensis, were described as new to science. In most cases, there was significant overlap of morphological characteristics among species.

CONCLUSIONS

This study underscores the pitfalls of relying upon morpho-anatomy alone to distinguish species and highlights our likely underestimation of species worldwide. Our integrative taxonomic approach can serve as a model for resolving the taxonomy of other plant and algal genera.

Environmental correlates of phenotypic variation: do variable tidal regimes influence morphology in intertidal seaweeds?

Seaweed morphology is often shaped by the hydrodynamic environment. However, exposure to air at low tide represents an additional factor potentially affecting the morphology of intertidal species. Here, we examined the relationships between the morphology of Hormosira banksii, an important intertidal habitat-forming seaweed in southern Australia, and environmental factors across multiple spatial scales around the island of Tasmania, Australia. Tasmania is surrounded by a diverse coastline with differences in wave exposure, tidal parameters, and temperature. We sampled Hormosira from four regions (100s km apart), three sites (10s km apart) within each region, and two zones (meters apart; eulittoral and sublittoral) at each site, and measured multiple morphological variables to test for differences in morphology at those different spatial scales. Thirteen environmental variables reflecting wave exposure, tidal conditions, and temperature for each site were generated to assess the relationship between Hormosira morphology and environmental variation. Morphology varied at all spatial scales examined. Most notably, north coast individuals had a distinct morphology, generally having smaller vesicles and shorter fronds, compared to other regions. Tidal conditions were the main environmental factors separating north coast sites from other sites and tidal regime was identified as the best predictor of morphological differences between regions. In contrast to other studies, we found little evidence that wave exposure was associated with morphological variation. Overall, our study emphasizes the role of tidal conditions, associated with emersion stress during low tide, in affecting the morphology of intertidal seaweeds.

The tarani mutation alters surface curvature in Arabidopsis leaves by perturbing the patterns of surface expansion and cell division

The leaf surface usually stays flat, maintained by coordinated growth. Growth perturbation can introduce overall surface curvature, which can be negative, giving a saddle-shaped leaf, or positive, giving a cup-like leaf. Little is known about the molecular mechanisms that underlie leaf flatness, primarily because only a few mutants with altered surface curvature have been isolated and studied. Characterization of mutants of the CINCINNATA-like TCP genes in Antirrhinum and Arabidopsis have revealed that their products help maintain flatness by balancing the pattern of cell proliferation and surface expansion between the margin and the central zone during leaf morphogenesis. On the other hand, deletion of two homologous PEAPOD genes causes cup-shaped leaves in Arabidopsis due to excess division of dispersed meristemoid cells. Here, we report the isolation and characterization of an Arabidopsis mutant, tarani (tni), with enlarged, cup-shaped leaves. Morphometric analyses showed that the positive curvature of the tni leaf is linked to excess growth at the centre compared to the margin. By monitoring the dynamic pattern of CYCLIN D3;2 expression, we show that the shape of the primary arrest front is strongly convex in growing tni leaves, leading to excess mitotic expansion synchronized with excess cell proliferation at the centre. Reduction of cell proliferation and of endogenous gibberellic acid levels rescued the tni phenotype. Genetic interactions demonstrated that TNI maintains leaf flatness independent of TCPs and PEAPODs.

Models for elastic shells with incompatible strains

The three-dimensional shapes of thin lamina, such as leaves, flowers, feathers, wings, etc., are driven by the differential strain induced by the relative growth. The growth takes place through variations in the Riemannian metric given on the thin sheet as a function of location in the central plane and also across its thickness. The shape is then a consequence of elastic energy minimization on the frustrated geometrical object. Here, we provide a rigorous derivation of the asymptotic theories for shapes of residually strained thin lamina with non-trivial curvatures, i.e. growing elastic shells in both the weakly and strongly curved regimes, generalizing earlier results for the growth of nominally flat plates. The different theories are distinguished by the scaling of the mid-surface curvature relative to the inverse thickness and growth strain, and also allow us to generalize the classical Föppl-von Kármán energy to theories of prestrained shallow shells.

Brown algal morphogenesis: atomic force microscopy as a tool to study the role of mechanical forces

Over the last few years, a growing interest has been directed toward the use of macroalgae as a source of energy, food and molecules for the cosmetic and pharmaceutical industries. Besides this, macroalgal development remains poorly understood compared to other multicellular organisms. Brown algae (Phaeophyceae) form a monophyletic lineage of usually large multicellular algae which evolved independently from land plants. In their environment, they are subjected to strong mechanical forces (current, waves, and tide), in response to which they modify rapidly and reversibly their morphology. Because of their specific cellular features (cell wall composition, cytoskeleton organization), deciphering how they cope with these forces might help discover new control mechanisms of cell wall softening and cellulose synthesis. Despite the current scarcity in knowledge on brown algal cell wall dynamics and protein composition, we will illustrate, in the light of methods adapted to Ectocarpus siliculosus, to what extent atomic force microscopy can contribute to advance this field of investigation.

Ecophysiology of photosynthesis in macroalgae

Macroalgae occur in the marine benthos from the upper intertidal to depths of more than 200 m, contributing up to 1 Pg C per year to global primary productivity. Freshwater macroalgae are mainly green (Chlorophyta) with some red (Rhodophyta) and a small contribution of brown (Phaeophyceae) algae, while in the ocean all three higher taxa are important. Attempts to relate the depth distribution of three higher taxa of marine macroalgae to their photosynthetic light use through their pigmentation in relation to variations in spectral quality of photosynthetically active radiation (PAR) with depth (complementary chromatic adaptation) and optical thickness (package effect) have been relatively unsuccessful. The presence (Chlorophyta, Phaeophyceae) or absence (Rhodophyta) of a xanthophyll cycle is also not well correlated with depth distribution of marine algae. The relative absence of freshwater brown algae does not seem to be related to their photosynthetic light use. Photosynthetic inorganic carbon acquisition in some red and a few green macroalgae involves entry of CO(2) by diffusion. Other red and green macroalgae, and brown macroalgae, have CO(2) concentrating mechanisms; these frequently involve acid and alkaline zones on the surface of the alga with CO(2) (produced from HCO(3) (-)) entering in the acid zones, while some macroalgae have CCMs based on active influx of HCO(3) (-). These various mechanisms of carbon acquisition have different responses to the thickness of the diffusion boundary layer, which is determined by macroalgal morphology and water velocity. Energetic predictions that macroalgae growing at or near the lower limit of PAR for growth should rely on diffusive CO(2) entry without acid and alkaline zones, and on NH(4) (+) rather than NO(3) (-) as nitrogen source, are only partially borne out by observation. The impact of global environmental change on marine macroalgae mainly relates to ocean acidification and warming with shoaling of the thermocline and decreased nutrient flux to the upper mixed layer. Predictions of the impact on macroalgae requires further experiments on interactions among increased inorganic carbon, increased temperature and decreased nitrogen and phosphorus supply, and, when possible, studies of genetic adaptation to environmental change.

Plant Proteus: brown algal morphological plasticity and underlying developmental mechanisms

Brown algae are multicellular photosynthetic marine organisms, ubiquitous on rocky intertidal shores at cold and temperate latitudes. Nevertheless, little is known about many aspects of their biology, particularly their development. Given their phylogenetic distance (1.6 billion years) from other plant organisms (land plants, and green and red algae), brown algae harbor a high, as-yet undiscovered diversity of biological mechanisms governing their development. They also show great morphological plasticity, responding to specific environmental constraints, such as sea currents, reduced light availability, grazer attacks, desiccation and UV exposure. Here, we show that brown algal morphogenesis is rather simple and flexible, and review recent genomic data on the cellular and molecular mechanisms known to date that can possibly account for this developmental strategy.

VARIATION IN ANATOMICAL AND MATERIAL PROPERTIES EXPLAINS DIFFERENCES IN HYDRODYNAMIC PERFORMANCES OF FOLIOSE RED MACROALGAE (RHODOPHYTA)(1)

Over the last two decades, many studies on functional morphology have suggested that material properties of seaweed tissues may influence their fitness. Because hydrodynamic forces are likely the largest source of mortality for seaweeds in high wave energy environments, tissues with material properties that behave favorably in these environments are likely to be selected for. However, it is very difficult to disentangle the effects of materials properties on seaweed performance because size, shape, and habitat also influence mechanical and hydrodynamic performance. In this study, anatomical and material properties of 16 species of foliose red macroalgae were determined, and their effects on hydrodynamic performance were measured in laboratory experiments holding size and shape constant. We determined that increased blade thickness (primarily caused by the addition of medullary tissue) results in higher flexural stiffness (EI), which inhibits the seaweed's ability to reconfigure in flowing water and thereby increases drag. However, this increase is concurrent with an increase in the force required to break tissue, possibly offsetting any risk of failure. Additionally, while increased nonpigmented medullary cells may pose a higher metabolic cost to the seaweed, decreased reconfiguration causes thicker tissues to expose more photosynthetic surface area incident to ambient light in flowing water, potentially ameliorating the metabolic cost of producing these cells. Material properties can result in differential performance of morphologically similar species. Future studies on ecomechanics of seaweeds in wave-swept coastal habitats should consider the interaction of multiple trade-offs.

FLOW-INDUCED MORPHOLOGICAL VARIATIONS AFFECT DIFFUSION BOUNDARY-LAYER THICKNESS OF MACROCYSTIS PYRIFERA (HETEROKONTOPHYTA, LAMINARIALES)(1)

In slow mainstream flows (<4-6 cm · s(-1) ), the transport of dissolved nutrients to seaweed blade surfaces is reduced due to the formation of thicker diffusion boundary layers (DBLs). The blade morphology of Macrocystis pyrifera (L.) C. Agardh varies with the hydrodynamic environment in which it grows; wave-exposed blades are narrow and thick with small surface corrugations (1 mm tall), whereas wave-sheltered blades are wider and thinner with large (2-5 cm) edge undulations. Within the surface corrugations of wave-exposed blades, the DBL thickness, measured using an O2 micro-optode, ranged from 0.67 to 0.80 mm and did not vary with mainstream velocities between 0.8 and 4.5 cm · s(-1) . At the corrugation apex, DBL thickness decreased with increasing seawater velocity, from 0.4 mm at 0.8 cm · s(-1) to being undetectable at 4.5 cm · s(-1) . Results show how the wave-exposed blades trap fluid within the corrugations at their surface. For wave-sheltered blades at 0.8 cm · s(-1) , a DBL thickness of 0.73 ± 0.31 mm within the edge undulation was 10-fold greater than at the undulation apex, while at 2.1 cm · s(-1) , DBL thicknesses were similar at <0.07 mm. Relative turbulence intensity was measured using an acoustic Doppler velocimeter (ADV), and overall, there was little evidence to support our hypothesis that the edge undulations of wave-sheltered blades increased turbulence intensity compared to wave-exposed blades. We discuss the positive and negative effects of thick DBLs at seaweed surfaces.