Shifts in morphological and mechanical traits compensate for performance costs of reproduction in a wave-swept seaweed

Transcription

1 Journal of Ecology 2013, 101, doi: / Shifts in morphological and mechanical traits compensate for performance costs of reproduction in a wave-swept seaweed Kyle W. Demes 1 *, Christopher D. G. Harley 1, Laura M. Anderson 2 and Emily Carrington 3 1 Department of Zoology, University of British Columbia, 6270 University Blvd., Vancouver BC V6T 1Z4, Canada; 2 Department of Botany, University of British Columbia, 6270 University Blvd., Vancouver BC V6T 1Z4, Canada; and 3 Friday Harbor Laboratories, University of Washington, 620 University Blvd., Friday Harbor, WA 98250, USA Summary 1. In addition to metabolic costs associated with reproduction, morphological and mechanical changes accompanying reproductive effort can affect an organism s performance. Reproductive effort may have unavoidable costs in plants and plant-like taxa (i.e. extra mass or drag); however, the extent to which an organism can ameliorate these consequences by additional modifications of shape or tissue properties remains unknown in seaweed and plant taxa. 2. We investigated mechanical and morphological changes associated with reproduction and how these shifts influence functional performance in the winged kelp, Alaria marginata. 3. Compared to vegetative blades, reproductive blades were similar in width but were longer and had greater surface area. Reproductive sporophylls were also thicker and less ruffled. Tissue extensibility and breaking stress were not different in reproductive vs. vegetative blades. However, reproductive tissue exhibited greater tensile stiffness, flexural stiffness and force to break. 4. Reproductive blades experienced greater drag (despite decreased flapping) than did vegetative blades, but did not experience greater size-specific drag. Tissues cut into experimental blades of the same size and shape experienced greater drag when cut from reproductive tissue suggesting that the change in shape associated with the onset of reproduction ameliorates the cost of increased tissue stiffness. Nonetheless, increased blade breaking force in reproductive individuals resulted in increased blade safety factors (breaking force/drag experienced) in reproductive compared to non-reproductive sporophylls. 5. Synthesis: In A. marginata, decreased flexibility and increased surface area are mechanical costs associated with reproduction. Decreased blade ruffliness and increased strength associated with the onset of reproduction in A. marginata ameliorate the concomitant mechanical costs of decreased flexibility and increased size. Shifts in mechanical and morphological traits among plants and plant-like taxa may allow them to increase reproductive output without decreasing functional performance. Key-words: Alaria marginata, biomechanics, drag, flexural stiffness, kelp, material properties, reproductive ecology Introduction Understanding how reproductive processes influence an organism s performance has been a perennial goal in the field of evolutionary ecology: What limits reproductive output? This question has been approached through the framework that reproductive effort poses some cost to the individual and that reproductive strategies develop as a means to optimize trade-offs between growth, reproduction and mortality (Bell *Correspondence author. demes@zoology.ubc.ca 1980). These trade-offs stem from interacting metabolic and performance costs. The production of offspring requires energetic investment, which may be syphoned away from investment in growth and/or defences. Such metabolic costs of reproduction are well documented in diverse taxa including plants (e.g. Calow 1979; Obeso 2002), invertebrates (e.g. Taylor & Leelapiyanart 2001), squamates (e.g. Angilletta & Sears 2000) and mammals (e.g. Randolph et al. 1977). Performance costs to reproduction arise from decreased performance during the reproductive phase relative to the non-reproductive phase, and usually result from morphological 2013 The Authors. Journal of Ecology 2013 British Ecological Society

2 964 K. W. Demes et al. and/or mechanical consequences of reproductive effort. The most intuitive examples of performance costs to reproduction are the decreased speed and mobility of gravid animals, notably mammals (e.g. Noren, Redfern & Edwards 2011), squamates (i.e. Shine 2003a,b; Brown & Shine 2004), crustaceans (Cromarty, Mello & Kass-Simon 1998) and spiders (Pruitt & Troupe 2010). Decreased speed and agility pose a cost to the individual because they adversely affect the individual s ability to capture prey and/or escape predation. In order to compensate for such performance costs to reproduction, gravid animals often alter their anti-predator behaviour, for example by increasing their sensitivity to predator cues or shifting their behaviour away from antipredator strategies that rely on escape velocity (e.g. Schwarzkopf & Shine 1992; Pruitt & Troupe 2010). Reproduction may impose performance costs, even in nonanimal taxa. For instance, the survival and functioning of plants and seaweeds is commonly dependent on the ability of their supporting structures to resist the forces they experience (via drag or self-loading weight), for example the production of fruits and large inflorescences by terrestrial plants adds non-trivial loads, which branches and stems must resist to avoid breaking (Peters et al. 1988; Etnier & Vogel 2000). For some plant species, an even greater cost is incurred when the fruit serves as an invitation to grazing by large animals, which in turn damage or break the plant (Hemborg & Bond 2006). In addition to tissues needing to bear increased weights associated with reproduction, plants and seaweeds may experience threatening increases in drag forces in areas of high wind and water disturbance (Esechie, Rodriguez & Al-asmi 2004). The wave-swept intertidal zone is often referred to as the most mechanically hostile environment on the planet, with crashing waves posing a lethal threat to intertidal seaweeds (Denny 1988). In this environment, seaweeds survival is dependent upon their ability to resist the hydrodynamic forces they experience. Drag forces experienced by seaweeds attached to rocks are dictated by the density and velocity of the water, the amount of surface area of the seaweed perpendicular to the direction of flow and the drag coefficient of the seaweed (related to its shape; Denny 1988). However, because seaweeds are composed of non-rigid tissues, they reconfigure in flowing water, and their surface area perpendicular to flow and drag coefficient may decrease substantially as water velocity increases (e.g. Boller & Carrington 2007; Demes et al. 2011; Martone, Kost & Boller 2012). The extent to which seaweeds can reduce drag forces by reconfiguring into smaller and/or more streamlined shapes is dependent upon the flexibility of their tissues; more flexible tissues reconfigure into smaller and/or more streamlined shapes in flow. Flexibility is therefore important to the survival of individuals on the wave-swept shore (Harder, Hurd & Speck 2006). Variation in tissue flexibility within and among species of seaweeds is dependent upon both tissue thickness and dimensionless material properties (e.g. Demes et al. 2011, 2013), both of which can potentially change during the onset of reproduction. For instance, spore production in the kelps (Laminariales, Phaeophyceae) occurs with the addition of a layer of sterile protective cells (Fritsch 1959). This layer of cells increases the overall thickness of the tissue and, because of its anatomical distinction from the other tissue layers, may change the dimensionless material properties of the tissue as well. Kelps are of particular mechanical interest because they thrive in wave-swept environments despite their enormous size. Thus, obstruction of drag reducing/resisting strategies of kelp by reproductive efforts could have drastic consequences. Nonetheless, kelps survive and reproduce in the most wave-exposed of environments, suggesting adaptations to ameliorate the mechanical costs to reproduction. Previous studies have reported mechanical adaptations and strategies among kelps that are responsible for their success in hydrodynamically stressful environments. For instance, high extensibility of kelp tissues allows them to go with the flow and minimize drag forces (Koehl & Wainwright 1977), allometric growth (change in shape with increases in size) has been reported to reduce drag in Nereocystis luetkeana, Eisenia arborea and Pterygophora californica (Johnson & Koehl 1994; Denny, Gaylord & Cowen 1997; Gaylord & Denny 1997) and conspecifics can exhibit variable mechanical traits that are presumed adaptive in their respective hydrodynamic regimes (Armstrong 1987; Kraemer & Chapman 1991). Furthermore, some species adapt to hydrodynamic regimes by changing the ruffliness of their blades. Individuals are less ruffled (more flat) in higher flow environments, which serves to reduce drag via a reduction in blade flapping (Koehl et al. 2008). Despite considerable attention to kelp mechanical design, whether and how reproduction might influence their mechanical traits and/or hydrodynamic performance remains unclear. Previously, the only attempt to address mechanical costs of reproduction in kelp occurred as an ancillary test in a series of experiments investigating physiological costs of reproduction (Pfister 1992). In this study, the presence of reproductive blades (hereafter referred to as sporophylls) did not significantly increase drag on Alaria marginata plants: controls with reproductive blades experienced similar drag to plants whose reproductive blades had been experimentally removed. This attempt to characterize mechanical costs of reproduction was limited by the researcher s ability to account for differences in size and shape of experimental plants and to control and replicate hydrodynamic treatment among plants. Furthermore, because A. marginata always possesses sporophylls, irrespective of its reproductive status, we argue that a more appropriate metric of mechanical costs of reproduction would be to measure how reproductive state of sporophylls impacts upon mechanical performance. Additionally, while only a few kelp species possess sporophylls, all kelp species produce spores on blades, so understanding how spore production impacts upon mechanical performance is more broadly applicable to other kelp species. Likewise, we sought to investigate whether and how the drag reducing/resisting strategies of kelps are changed during spore production. Here, we measure functional traits and hydrodynamic performance of reproductive and non-reproductive A. marginata

3 Mechanical costs of reproduction in a kelp 965 Postels et Ruprecht (Laminariales, Phaeophyceae) sporophylls to assess mechanical costs associated with spore production. Specifically, we compared vegetative and reproductive blades with respect to (i) morphological traits: size, shape, thickness and degree of ruffliness, (ii) tissue mechanical properties: breaking stress, blade breaking force and tensile and flexural stiffness and (iii) hydrodynamic performance: drag on entire sporophylls, size-specific drag, drag on sporophylls of artificially constant shape and size and degree of flapping at six different velocities ( m s 1 ). Because of the addition of sterile protective cells during reproduction, we predict that reproductive blades will be thicker and stiffer than nonreproductive fronds, resulting in decreased flexibility among reproductive fronds. We hypothesize increased size and stiffness will increase drag on reproductive fronds, but that changes in morphological traits may, to some degree, mediate these biomechanical costs of reproduction. Testing such hypotheses is vital to our understanding of evolutionary ecology because mechanical traits are some of the fundamental determinants of seaweed hydrodynamic performance and optimizing reproductive effort with functional performance may be an important component of an individual s fitness. 20 cm Materials and methods STUDY SPECIES AND SAMPLE COLLECTION Alaria marginata is an abundant kelp in a variety of intertidal and shallow subtidal habitats from Alaska to central California. Individuals consist of a single, main blade, which may grow to >4 m in length and multiple smaller (< 30 cm) reproductive blades, or sporophylls, located near the plant s holdfast (Fig. 1). The successful completion of the A. marginata life cycle requires the production and subsequent release of competent spores from sporophylls. Theoretically, dislodgement of a ripe sporophyll could serve as a dispersal vector. However, because most drift accumulates at or above the high tide mark (where recruits could not survive) as beach wrack, sporophyll dislodgement is probably not a common dispersal method (although no evidence currently exists in favour or against this hypothesis). Although sporophylls are present year-round, sori (reproductive patches on sporophylls where spores are produced) are most abundant between May and July (McConnico & Foster 2005). Reproductive and non-reproductive sporophylls were collected randomly from A. marginata individuals growing on docks at the Friday Harbor Laboratories on San Juan Island, WA ( N, W) between May and July, 2011 to assess the effects of reproductive status on mechanical traits and hydrodynamic performance. Specimens were placed in a flow through water table until biomechanical analyses, which were performed within 48 h of collection. MORPHOLOGICAL PROPERTIES Because sporophylls are roughly ovular (Fig. 1), length and width describe most of the variability in shape. Length and width were measured to the nearest 0.1 cm and the length:width ratio was used to describe sporophyll shape. Size of each sporophyll was measured from digital scans of specimens in IMAGEJ photo analysis software (U.S. National Institutes of Health, Bethesda, MD, USA) as the total surface area of a sporophyll (cm 2 ). Sporophyll thickness was measured to the Fig. 1. Mature, adult Alaria marginata sporophyte showing main vegetative blade and sporophylls: dark patches within sporophylls are soral (reproductive) patches which, when mature, take up more than 75% of sporophyll surface area. nearest 0.1 mm using digital calipers 5 cm from above the petiole (point of attachment between the seaweed stipe and sporophyll). Sporophyll ruffliness index (detailed in Johnson & Koehl 1994) was measured as the ratio of actual sporophyll surface area (after deruffling) to projected sporophyll blade area (while still ruffled). MECHANICAL PROPERTIES Dumbbell shaped working sections were cut from 5 cm above the petiole to determine tissue mechanical properties of samples. The dumbbell working section provides grips that are necessary for tensile testing and more accurately controls strain localization, increasing the precision of material testing (Mach 2009; Demes et al. 2011). Working sections were cut just above the petiole in an attempt to standardize tissue age in vegetative vs. reproductive blades because kelp blades grow basally, the youngest tissue occurs near the base of the blade. Working sections were then attached to a tensometer (5565; Instron, Norwood, MA, USA) via pneumatic clamps (90 psi). Strain was applied to tissues at a rate of 10 mm min 1 until failure occurred and the resisting force was measured at a frequency of 5 Hz. Tissues which failed at or near the clamps were discarded from analyses. The working section constrained sample length and width to 55 and 5 mm, respectively. Tissue thickness of each sample was measured to the nearest 0.1 mm using digital calipers. From each tensile test, breaking strain and breaking force (N) were extracted. These variables, along with tissue thickness, were then used to calculate breaking stress (Force per initial cross-sectional area, MPa), tensile stiffness (initial linear slope of stress vs. strain curve, MPa) and flexural stiffness (product of tensile stiffness and second moment of inertia, ln*m 2 ).

4 966 K. W. Demes et al. HYDRODYNAMIC PERFORMANCE Drag was measured on intact sporophylls in a recirculating flume (Boller & Carrington 2006) at six water velocities: 0.18, 0.75, 1.00, 1.71, 2.00 and 2.73 m s 1. Although A. marginata can occur in areas with much greater maximum velocities, the site from which experimental specimens were collected is current dominated and velocities do not likely exceed 3 m s 1. Reproductive (n = 45) and non-reproductive (n = 43) sporophylls were attached directly to a pre-calibrated force transducer (detailed in Boller & Carrington 2006), which measured force at 10 Hz for 10 s at each velocity (the average was used for all drag measurements). Drag force is influenced by size, shape and flexibility (Carrington 1990; Demes et al. 2011), all of which were predicted to vary with reproductive state. In addition to measuring differences in drag between reproductive and vegetative sporophylls, we attempted to disentangle the contributions of variation in size, shape and flexibility to differences in drag. First, we measured drag on entire sporophylls. Next, we calculated size-specific drag (drag per surface area, N*cm 2 ) of reproductive sporophylls to correct for differences in size while allowing for variation among individuals in shape and ruffliness to influence drag. Finally, we cut vegetative and reproductive sporophylls into standardized working sections (detailed in Demes et al. 2011) of the same size and shape (differing only in tissue mechanical traits). Sporophyll flapping will increase the overall drag force experienced by a flexible body in flow as well as the variance in drag. Because we measured drag at 10 Hz for 10 s, we were able to calculate a dimensionless coefficient of variation (standard deviation/ mean * 100) in drag forces for each sporophyll at each speed. We used this coefficient of variation as a measure of sporophyll flapping. A subset (N = 46) of the sporophylls used in hydrodynamic performance measurements (from July collections only) were also used in materials testing to allow the calculation of sporophyll safety factors (breaking force/drag force experienced) and compared among reproductive status groups at the six velocities tested. STATISTICAL ANALYSES One-way analysis of variance was used to test for differences in morphological and mechanical properties in reproductive vs. non-reproductive sporophyll tissues. Drag, flapping and sporophyll safety factors were measured on the same sporophylls at multiple velocities, requiring the use of repeated measures ANOVA to account for the nonindependence of samples, while testing for differences in drag and sporophyll safety factor among individual samples with respect to reproductive status. Repeated measures ANOVA was also used to test the effects of sporophyll L : W ratio on size-specific drag in vegetative vs. reproductive sporophylls. The assumption of sphericity was violated in all analyses, necessitating correction of degrees of freedom by the Greenhouse Geisser test statistic (Greenhouse & Geisser 1959). Sporophyll safety factor and drag per surface area data were log-transformed before analyses to meet the assumption of a normal distribution. All statistical analyses were implemented in SPSS 17 (SPSS Inc. Chicago, IL, USA) with a = Results MORPHOLOGICAL PROPERTIES Reproductive sporophylls had significantly more surface area than vegetative sporophylls (F 1,84 = 10.70, P = 0.002). Although sporophyll width was not different among reproductive status groups (F 1,84 = 1.17, P = 0.283), reproductive sporophylls were significantly longer (F 1,84 = 12.50, P =0.001), resulting in greater length:width ratios (F 1,84 = 18.95, P < 0.001) in reproductive vs. vegetative sporophylls. Reproductive sporophylls were also significantly thicker (F 1,84 = , P < 0.001) than vegetative sporophylls. Ruffliness indices were marginally significantly (F 1,45 = 4.022, P = 0.051) lower in reproductive vs. vegetative sporophylls. Results from statistical comparisons of differences in morphological traits between reproductive and vegetative sporophylls are graphically depicted in Fig. 2a d. MECHANICAL PROPERTIES Although we did not detect significant differences associated with reproduction in breaking stress (F 1,63 = 2.08, P = 0.154), reproductive tissue tended to have a somewhat lower mean breaking stress. We did, however, detect higher tensile stiffness (F 1,85 = 24.25, P < 0.001) in reproductive sporophylls. Furthermore, tissue from reproductive sporophylls exhibited significantly greater force required to break (F 1,63 = 74.26, P 0.001) and flexural stiffness (F 1,85 = , P < 0.001) than vegetative sporophylls. Associations of tissue mechanical traits with reproductive state are shown in Fig. 2e h. HYDRODYNAMIC PERFORMANCE Intact reproductive sporophylls experienced greater drag than did vegetative sporophylls and to an increasing extent with increasing water velocity (Fig. 3a). However, vegetative and reproductive sporophylls experienced similar size-specific drag (N cm 2 ; Fig. 3b). When sporophylls were cut into hydrodynamic working sections of the same shape and size (only differing in material), drag increased with increasing velocity, was higher in working sections cut from reproductive materials and the rate of increase in drag with increasing water velocity was higher among reproductive working sections (Fig. 3c). Statistical results from repeated measures ANO- VA testing the effects of reproductive state, water velocity and their interaction on these hydrodynamic performance traits are presented in Table 1. Sporophyll shape (L:W ratio) and interactions with water velocity and reproductive state were not significant predictors of size-specific drag in Repeated Measure ANOVA analyses (Table 2). Flapping decreased with water velocity (F 1.93,77.01 = , P < 0.001); flapping was generally lower in reproductive sporophylls (F 1,40 = 6.318, P = 0.016) and the difference between reproductive status groups was dependent upon water velocity (F 1.93,77.01 = , P < 0.001) (Fig. 4). Post hoc tests between reproductive groups at each velocity revealed that vegetative sporophylls experienced greater flapping at 0.18 and 1.71 m s 1 ; there were not significant differences in vegetative vs. reproductive sporophylls at the other velocities tested. Sporophyll safety factor decreased with increasing velocity for all sporophylls (F 1,33 = , P < 0.001), but was significantly higher (F 1,33 = 4.787, P = 0.036) for

5 Mechanical costs of reproduction in a kelp (a) 4.5 (b) 0.9 (c) (d) Surface area (cm 2 ) Length : width ratio Thickness (cm) Ruffliness index (e) 5.5 (f) 8.0 (g) 0.14 (h) Breaking stress (MPa) Tensile stiffness (MPa) Breaking force (N) Flexural stiffness (µn*m 2 ) Fig. 2. Comparison of (a d) morphological and (e h) mechanical properties between vegetative, that is, non-reproductive (empty circles) and reproductive (filled circles) Alaria marginata sporophylls. Values are mean SE. Statistical results are summarized in Table 1. reproductive sporophylls (Fig. 5). We did not detect a significant interaction between water velocity and reproductive state on sporophyll safety factor (F 1,33 = 2.593, P = 0.117). Discussion Reproduction is essential for population and species persistence, especially in environments characterized by high individual mortality, such as the wave-swept intertidal zone. However, reproductive investment is usually non-trivial for individuals and often involves metabolic and performance costs. For instance, reproductive investment can delay or reduce growth (Ang 1992) and in some cases growth to a larger size can be more beneficial than early reproduction since it increases survivorship (Aberg 1996). The optimization of reproductive strategies is therefore an important component of an organism s fitness. Metabolic costs of reproduction have received much attention in phylogenetically disparate taxa (e.g. Calow 1979; Bell 1980; Rose & Bradley 1998; Obeso 2002); but mechanical costs to reproduction are much less well characterized and are described almost exclusively in animal taxa. Nonetheless, mechanical traits important to survival in most plants and algae have the potential to be affected by the onset of reproduction. Variation in hydrodynamic performance of seaweeds has been tightly linked to mechanical (Koehl 1986; Gaylord & Denny 1997; Boller & Carrington 2007; Koehl et al. 2008; Demes et al. 2011, 2013) and morphological (e.g. Carrington 1990; Koehl et al. 2008) traits. In Alaria, the costs of reproduction are decreased flexibility and increased overall surface area. Reproductive sporophylls of A. marginata experienced greater drag than non-reproductive sporophylls. However, after correcting for differences in size among sporophylls, there was no difference in (size-specific) drag between reproductive states, suggesting that increased size is an important component of the added drag experienced by reproductive A. marginata sporophylls. This lack of difference in size-specific drag exists in spite of differences in mechanical and morphological traits known to influence drag: flexural stiffness and ruffliness. When sporophylls were cut into the same size and shape (varying only in flexural stiffness), they experienced greater drag when cut from reproductive tissue than from vegetative tissue, reconfirming that increased flexural stiffness increases drag (e.g. Demes et al. 2011) and suggesting that some morphological aspect compensates for it in entire sporophylls. Given their marked differences in flexural stiffness, how then did size-specific drag not vary between reproductive states? Perhaps the most straightforward interpretation is that any increased drag arising from increased flexural stiffness was compensated for by another mechanism, two of which are readily apparent: (i) a nonlinear relationship between surface area and drag and/or (ii) variation in morphological traits (ruffliness and length:width ratio) between reproductive states. Contrary to the former hypothesis, relationships between surface area and drag at each velocity tested were strongly linear (P < 0.001, r 2 = ). Consistent with the latter hypothesis, reproductive sporophylls displayed

6 968 K. W. Demes et al. Drag (N) Drag per surface area (N mm 2 ) Drag (N) (a) (b) (c) Vegetative sporophyll Reproductive sporophyll Velocity (m s 1 ) Different shape, size, & flexbility Different shape & flexibility Same size Different flexibility Same shape & size Fig. 3. Comparison of hydrodynamic performance between reproductive and vegetative sporophylls: (a) Drag force on intact, whole sporophylls, (b) Drag force per unit surface area of whole sporophylls and (c) Drag force on standardized working sections (same shape and size) cut out of reproductive and non-reproductive sporophyll tissue. In all graphs, vegetative, that is, non-reproductive, samples are represented by empty circles and dashed lines while reproductive samples are denoted with filled circles and solid lines. Values are mean SE. decreased ruffliness and were significantly longer than vegetative sporophylls. Assessing the contribution of differences in sporophyll shape (i.e. length:width ratio) is complicated by a correlation between length:width ratios and surface area, which is strongly positively associated with drag. Likewise, we addressed the effect of sporophyll shape on drag by testing how the length:width ratio of sporophylls influenced sizespecific drag but did not find evidence that differences in shape among reproductive groups explained variation in size-specific drag (Table 2). Decreased sporophyll ruffliness seems more likely to be the morphological factor offsetting the cost of increased flexural stiffness among reproductive sporophylls. Although ruffling, and therefore flapping, may be beneficial in low-flow environments by increasing photosynthetic surface incident to sunlight, ruffling can also increase drag (Koehl & Alberte 1988) by increasing sporophyll flapping (thereby increasing both area perpendicular to flow and turbulence downstream of the sporophyll). We found decreased ruffliness and a concomitant decrease in flapping among reproductive sporophylls. Therefore, the decreased ruffliness among reproductive sporophylls could have offset the cost of decreased flexibility in reproductive A. marginata tissues. Variation in ruffliness has been observed among conspecifics at different sites (Koehl et al. 2008), but this is the first report of ruffliness varying among developmental states at the same site. Although changes in morphology appear to have compensated for the increased flexural stiffness among reproductive sporophylls, whole reproductive sporophylls were larger and consequently experienced proportionately greater drag. Despite the observed increase in drag, sporophyll safety factor was actually significantly higher at two velocities, owing to the increased breaking force associated with the thicker, reproductive sporophylls. Plants and plant-like taxa cannot behaviourally modify their interactions with their environment in the same way that mobile animals can (Bradshaw 1972; Huey et al. 2002). For instance, spiders that experience decreased sprinting speed while gravid may mitigate increased predation risks associated by hiding more frequently while gravid (Pruitt & Troupe 2010). Instead, plants and seaweeds are firmly anchored to substrate and cannot change their physical environment. They must rely, instead, on shifts in functional traits to mitigate costs of reproduction. In windy areas, the added drag on corn Table 1. Results from repeated measures anova testing the effects of reproductive state, water speed and their interaction on (a) whole sporophyll drag, (b) drag per surface area and (c) drag on sporophylls cut into the same shape and size (a) Drag (N) (b) log[drag per surface area (N cm 2 )] (c) Drag (N) when same shape and size Source d.f. F P Source d.f. F P Source d.f. F P Speed 1.1, < Speed 2.4, < Speed 1.6, < State 1, < State 1, State 1, Sp*St 1.1, Sp * St 2.4, Sp * St 1.6, Results are visually presented in Fig. 3a c, respectively.

7 Mechanical costs of reproduction in a kelp 969 Table 2. Statistical output from 3-way repeated measures anova testing how reproductive state, water speed and sporophyll shape (i.e. length:width ratio) affect log-transformed drag per surface area of Alaria marginata sporophylls Source d.f. F P Speed (Sp) 1.4, < State (St) 1, Length : Width (L : W) 1, St*Sp 1.4, Sp*L : W 1.4, St*L : W 1, St*Sp*L : W 1.4, Coefficient of variation in drag Reproductive sporophylls Vegetative sporophylls Velocity (m s 1 ) Fig. 4. Degree of sporophyll flapping, measured as the coefficient of variation in drag, in reproductive (filled circles) vs. vegetative (empty circles) sporophylls. Values are mean SE. Fig. 5. Sporophyll safety factor (breaking force/drag force experienced, on a log scale) at six water velocities. Values for reproductive and vegetative sporophylls (represented by filled and empty circles, respectively) are mean SE. Dotted line represents the value at (and below) which sporophylls break. crops from the production of reproductive structures can result in lodging (stem breaking or root dislodgement; Berry et al. 2003). Differential lodging among terrestrial crops during reproduction has been directly related to mechanical traits, such that stronger and thicker individuals were less likely to lodge (Esechie et al. 2004). Our results in a marine seaweed similarly suggest that parent morphological and mechanical traits may mitigate performance costs of reproduction. Conclusion Optimizing reproductive effort to maximize fitness requires balancing metabolic and performance costs to the parent. Because reproductive output scales positively with size for most plant-like taxa (e.g. Peters et al. 1988; Weiner et al. 2009), increasing size should increase fitness. However, because increasing size also increases the forces an individual must withstand to survive (e.g. increased drag, increased weight for branches to support in fruiting trees, etc.), the size at which an individual optimizes its fitness is the largest size it can support metabolically and mechanically without increasing risk of mortality to the parent. Alaria marginata sporophylls mitigate the reproductive costs of increased stiffness and surface area via decreased ruffliness and increased strength among reproductive sporophylls. We argue that shifts in morphological and mechanical traits in plant-like taxa may allow them to increase reproductive output without significantly decreasing functional performance. Acknowledgements We are indebted to Matthew George, Hilary Hayford, Jaquan Horton and Mike Nishizaki for their help in the laboratory and Jonathan Pruitt for constructive comments on early drafts of the manuscript. This work was funded by a Stephen and Ruth Wainwright fellowship to Kyle Demes and National Science Foundation Grants OCE and EF References Aberg, P. (1996) Patterns of reproductive effort in the brown alga Ascophyllum nodosum. Marine Ecology Progress Series, 138, Ang, P.O. (1992) Cost of reproduction in Fucus distichus. Marine Ecology Progress Series, 89, Angilletta, M.J. & Sears, M.W. (2000) The metabolic cost of reproduction in an oviparous lizard. Functional Ecology, 14, Armstrong, S.L. (1987) Mechanical properties of the tissues of the brown alga Hedophyllum sessile (C. Ag.) Setchell: variability with habitat. Journal of Experimental Marine Biology and Ecology, 114, Bell, G. (1980) The costs of reproduction and their consequences. American Naturalist, 116, Berry, P.M., Sterling, M., Baker, C.J., Spink, J. & Sparkes, D.L. (2003) A calibrated model of wheat lodging compared with field measurements. Agricultural and Forest Meteorology, 119, Boller, M.L. & Carrington, E. (2006) The hydrodynamic effects of shape and size during reconfiguration of a flexible macroalga. Journal of Experimental Biology, 209, Boller, M.L. & Carrington, E. (2007) Interspecific comparison of hydrodynamic performance and structural properties among intertidal macroalgae. Journal of Experimental Biology, 210, Bradshaw, A.D. (1972) Some of the evolutionary consequences of being a plant. Evolutionary Biology, 5, Brown, G.P. & Shine, R. (2004) Effect of reproduction on the antipredator tactics of snakes (Tropidonophis mairii, Colubridae). Behavioral Ecology, 56, Calow, P. (1979) The cost of reproduction- a physiological approach. Biological Reviews, 54, Carrington, E. (1990) Drag and dislodgement of an intertidal macroalga: consequences of morphological variation in Mastocarpus papillatus K utzing. Journal of Experimental Marine Biology and Ecology, 39,

8 970 K. W. Demes et al. Cromarty, S.I., Mello, J. & Kass-Simon, G. (1998) Comparative analysis of escape behaviour in male, and gravid and non-gravid, female lobsters. Biological Bulletin, 194, Demes, K.W., Carrington, E., Gosline, J. & Martone, P.T. (2011) Variation in anatomical and material properties explains differences in hydrodynamic performances of foliose red macroalgae (Rhodophyta). Journal of Phycology, 47, Demes, K.W., Pruitt, J.N., Harley, C.D.G. & Carrington, E. (2013) Survival of the weakest: increased frond mechanical strength in a wave-swept kelp inhibits self-pruning and increases whole-plant mortality. Functional Ecology, 27, Denny, M.W. (1988) Biology and Mechanics of the Wave-Swept Environment. Princeton University Press, New Jersey. Denny, M.W., Gaylord, B.P. & Cowen, E.A. (1997) Flow and flexibility II. The roles of size and shape in determining wave forces on the bull kelp Nereocystis luetkeana. Journal of Experimental Biology, 200, Esechie, H.A., Rodriguez, V. & Al-Asmi, H. (2004) Comparison of local and exotic maize varieties for stalk lodging components in a desert climate. European Journal of Agronomy, 21, Etnier, S.A. & Vogel, S. (2000) Reorientation of daffodil (Narcissus: Amaryllidaceae) flowers inwind: drag reduction and torsional flexibility. American Journal of Botany, 87, Fritsch, F.E. (1959) The Structure and Reproduction of the Algae, Vol. II. Cambridge University Press, New York. Gaylord, B. & Denny, M.W. (1997) Flow and flexibility I. Effects of size, shape, and stiffness in determining wave forces on the stipitate kelps Eisenia arborea and Pterygophora californica. Journal of Experimental Biology, 200, Greenhouse, S.W. & Geisser, S. (1959) On methods in the analysis of profile data. Psychometrika, 24, Harder, D.L.D., Hurd, C.L.C. & Speck, T.T. (2006) Comparison of mechanical properties of four large, wave-exposed seaweeds. American Journal of Botany, 93, Hemborg, A.M. & Bond, W.J. (2006) Do browsing elephants damage female trees more? African Journal of Ecology, 45, Huey, R.B., Carlson, M., Crozier, L., Frazier, M., Hamilton, H., Harley, C., Hoang, A. & Kingsolver, J.G. (2002) Plants versus animals: do they deal with stress in different ways? Integrative and Comparative Biology, 42, Johnson, A.S. & Koehl, M.A.R. (1994) Maintenance of dynamic strain similarity and environmental stress factor in different flow habitats: thallus allometry and materials properties of a giant kelp. Journal of Experimental Biology, 195, Koehl, M.A.R. (1986) Seaweeds in moving water: form and mechanical function. On the Economy of Plant Form and Function (ed. T.J. Givnish), pp Cambridge University Press, New York. Koehl, M.A.R. & Alberte, R.S. (1988) Flow, flapping, and photosynthesis of Nereocystis luetkeana: a functional comparison of undulate and flat blade morphologies. Marine Biology, 99, Koehl, M.A.R. & Wainwright, S.A. (1977) Mechanical adaptations of a giant kelp. Limnology and Oceanography, 22, Koehl, M.A.R., Silk, W.K., Liang, H. & Mahadevan, L. (2008) How kelp produce blade shapes suited to different flow regimes: a new wrinkle. Integrative and Comparative Biology, 48, Kraemer, G.P. & Chapman, D.J. (1991) Biomechanics and alginic acid composition during hydrodynamic adaptation by Egregia menziesii (Phaeophyta) juveniles. Journal of Phycology, 27, Mach, K.J. (2009) Mechanical and biological consequences of repetitive loading: crack initiation and fatigue failure in the red macroalga Mazzaella. Journal of Experimental Biology, 212, Martone, P.T., Kost, L. & Boller, M. (2012) Drag reduction in wave-swept macroalgae: alternative strategies and new predictions. American Journal of Botany, 99, McConnico, L.A. & Foster, M.S. (2005) Population biology of the intertidal kelp, Alaria marginata Postels and Ruprecht: a non-fugitive annual. Journal of Experimental Marine Biology and Ecology, 324, Noren, S.R., Redfern, J.V. & Edwards, E.F. (2011) Pregnancy is a drag: hydrodynamics, kinematics and performance in pre-post parturition bottlenose dolphins (Tursiops truncatus). Journal of Experimental Biology, 214, Obeso, J.R. (2002) The costs of reproduction in plants. New Phytologist, 155, Peters, R., Cloutier, S., Dube, D., Evans, A., Hastings, P., Kohn, D. & Sawer- Foner, B. (1988) The allometry of the weight of fruit on trees and shrubs in Barbados. Oecologia, 74, Pfister, C.A. (1992) Costs of reproduction in an intertidal kelp: patterns of allocation and life history consequences. Ecology, 73, Pruitt, J.N. & Troupe, J.E. (2010) The effect of reproductive status and situation on locomotor performance and anti-predator strategies in a funnel-web spider. Journal of Zoology, 281, Randolph, P.A., Randolph, J.C., Mattingly, K. & Foster, M.M. (1977) Energy costs of reproduction in the cotton rat, Sigmodon hispidus. Ecology, 58, Rose, M.R. & Bradley, T.J. (1998) Evolutionary physiology of the cost of reproduction. Oikos, 83, Schwarzkopf, L. & Shine, R. (1992) Costs of reproduction in lizards: escape tactics and susceptibility to predation. Behavioral Ecology and Sociobiology, 31, Shine, R. (2003a) Locomotor speeds of gravid lizards: placing costs of reproduction within an ecological context. Functional Ecology, 17, Shine, R. (2003b) Effects of pregnancy on locomotor performance: an experimental study on lizards. Oecologia, 136, Taylor, H.H. & Leelapiyanart, N. (2001) Oxygen uptake by embryos and ovigerous females of two intertidal crabs, Heterozius rotundifrons (Belliidae) and Cyclograpsus lavauxi (Grapsidae): scaling and the metabolic costs of reproduction. Journal of Experimental Biology, 204, Weiner, J., Campbell, L.G., Pino, J. & Echarte, L. (2009) The allometry of reproduction within plant populations. Journal of Ecology, 97, Received 20 November 2012; accepted 26 March 2013 Handling Editor: Christer Nilsson