190 K
.L.D. Milligan, R.E. DeWreede J. Exp. Mar. Biol. Ecol. 254 2000 189 –209 Keywords
: Biomechanics; Holdfast; Kelp; Macroalgal ecology; Wave-exposure
1. Introduction
Recent studies on mechanisms of wave-induced mortality in macroalgal populations have demonstrated that biomechanical approaches can explain species distributions and
morphological patterns Gaylord et al., 1994. For example, Shaughnessy et al. 1996 demonstrated that differences in distribution of two closely related species Mazzaella
linearis and M . splendens, Rhodophyta in relation to wave-exposure could be predicted
by a biomechanical survivorship model. Many of the studies to date focus on variations in thallus attributes, such as tissue mechanics Koehl and Wainwright, 1977; Biedka et
al., 1987; Denny et al., 1989 and thallus shape Koehl, 1984; Carrington, 1990; Dudgeon and Johnson, 1992, which may resist and minimize wave-induced stresses,
and attachment forces that resist wave stress Carrington, 1990; Friedland and Denny, 1995.
In general, survival in wave-exposed sites is dependent on the total forces transferred from the wave to the thallus attachment and the resistance force of the attachment. If the
total hydrodynamically-induced force exceeds the attachment force, then the alga or part of the alga will be torn from the substratum and dislodged Carrington, 1990. The
majority of field measurements of attachment forces resulted in thallus breaks above the holdfast at either the stipe or the stipe blade junction Carrington, 1990; Dudgeon and
Johnson, 1992; Shaughnessy et al., 1996. This results in partial thallus loss, with the holdfast or holdfast and stipe remaining for potential regrowth. In this case, biomass loss
before reproduction reduces the number of propagules available for dispersal and recruitment. In cases of holdfast failure, the whole thallus is dislodged, and space is
opened for recolonization.
Despite increased attention towards understanding effects of wave force on intertidal organisms e.g. Denny, 1988, 1995, there is a lack of empirical studies documenting the
variation in attachment properties among and within species, and across gradients such as wave-exposures, seasons, and substratum types. Past model approaches to predict
algal survival along wave-exposure gradients have been based on assumptions that a species’ attachment properties do not significantly vary with season or site e.g. Gaylord
et al., 1994; Denny, 1995. However, it is unknown whether or not these assumptions represent natural populations. Biomechanical attributes, such as tissue locations most
prone to breakage, forces to break, and tissue strengths, that are observed in the field reflect the complex interactions between intrinsic such as evolved tissue traits and
extrinsic for example, herbivore and wave damage factors which directly affect individual survival in wave-exposed sites. These factors may or may not result in similar
attachment properties between seasons or across wave-exposure gradients.
A species’ evolved tissue mechanics contribute to ways in which it resists physical damage. For example, to increase attachment force, either stronger strength is measured
as force per unit area tissue or increased cross sectional area of break junction is needed. For example, Mazzaella splendens, M
. linearis, Shaughnessy et al., 1996 and Mastocarpus papillatus Carrington, 1990 increase resistance to breakage by increasing
stipe cross sectional area while maintaining a constant tissue strength.
K .L.D. Milligan, R.E. DeWreede J. Exp. Mar. Biol. Ecol. 254 2000 189 –209
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Combinations of herbivore- and wave-induced damage to a thallus, such as flaws and cracks in stipes, blades, or holdfasts, are common, causing failure-prone locations
Koehl and Wainwright, 1977; Denny et al., 1989; DeWreede et al., 1992; Tegner et al., 1995. Depending on the severity of wounding from extrinsic factors, the thallus will
ultimately break at locations different from those that occur in non-damaged thalli Biedka et al., 1987.
Holdfast failure has not been studied in detail even though it is a commonly observed occurrence, especially in some kelp species Koehl and Wainwright, 1977; Tegner et al.,
1995. Holdfast loosening is a natural progression in kelp maturity, resulting from individual haptera being detached from the substratum, usually by invertebrate burrows
McLay and Hayward, 1987, age specifically, by increased sedimentation and shading; Ghelardi, 1971, and wave forces Dayton et al., 1984.
The objectives of this study were to quantify the variation in Hedophyllum sessile’s holdfast attachment properties and interpret the results in the context of H
. sessile’s survival. Hedophyllum sessile is a dominant kelp species found in moderately to highly
wave-exposed sites in the lower intertidal zone in the Northeastern Pacific Ocean Abbott and Hollenberg, 1976.
Hedophyllum sessile sporophyte recruitment occurs in late winter through spring and successful recruitment differs on different substrata types Milligan, 1998. Juveniles are
characterized by a holdfast, stipe, and blade Widdowson, 1965. Holdfasts are made up of finger-like haptera that are attached to the substratum by microscopic tissue-
extensions into crevices Tovey and Moss, 1978. Juvenile mortality has been primarily attributed to herbivore damage by the dominant grazing chiton, Katharina tunicata
Duggins and Dethier, 1985. Adult mortality by holdfast dislodgment is primarily from winter storm damage Dayton, 1975; Paine, 1980; Duggins and Dethier, 1985, and there
is some evidence that K
. tunicata will loosen adult holdfasts, making dislodgment more likely during winter storm swells Markel and DeWreede, 1998. If thalli are cropped
and tattered as a result of winter storms, the basal meristem and holdfast survive and new blade material will regenerate Armstrong, 1987.
We investigated how attachment forces vary within Hedophyllum sessile at different developmental stages juveniles vs. adults, wave-exposures, substratum types, and
season. Since hydrodynamic force is bi-directional shoreward and seaward in the intertidal Koehl, 1984, the holdfast mechanics in each of these directions were
compared for adults. In addition, H . sessile’s holdfast removal force and strength were
contrasted to those of other species found in similar environments to establish if H .
sessile displays either a strategy of strength or increased attached holdfast surface area to resist wave forces. Causes for these patterns were not isolated in these experiments.
2. Materials and methods
