The Southern Bullkelps Durvillaea Antarctica and D willana

The southern bull kelp Durvillaea is a member of the Fucales [13]. Its morphology is typical for large brown seaweeds with a holdfast, a stalklike stipe, a transitionary palm zone at the apical end of the stipe, and a large blade. Unlike other members of the Fucales, growth in Durvillaea is not restricted to a small apical meristematic zone but is diffuse [14]. The distribution of Durvillaea is confined to the Southern hemisphere where it grows on temperate rocky shores [15].

Durvillaea is the largest intertidal seaweed in the world. Individuals with a length of greater than 13 m [16] and a mass of more than 80 kg (C. Hurd, unpublished data) have been recorded. This genus can thrive even in the harsh conditions of the wave-swept surf zone. Moreover, it needs at least a moderate wave exposure for the successful establishment at a particular site [14].

Durvillaea antarctica occurs along the coasts of New Zealand, Chile, and some sub-Antarctic islands [15]. Its size and morphology are highly dependent on the ambient wave climate [15,17]. Three morphotypes can be identified [15]

FIGURE 3.1 The morphology of Durvillaea antarctica is highly dependent on wave exposure. (A) At comparatively sheltered sites, the blade becomes broad and undulating. (B) If wave exposure is more severe, the blade is subdivided into many whip-like thongs. The overall length of the blade is approximately 5 to 7 m in both photographs.

FIGURE 3.1 The morphology of Durvillaea antarctica is highly dependent on wave exposure. (A) At comparatively sheltered sites, the blade becomes broad and undulating. (B) If wave exposure is more severe, the blade is subdivided into many whip-like thongs. The overall length of the blade is approximately 5 to 7 m in both photographs.

FIGURE 3.2 (A) The blade of D. antarctica is positively buoyant so the lamina is floating at the water surface, whereas (B) the blade of D. willana is neutrally buoyant, so that the lamina is upright in the water column. (C) The medulla of D. antarctica contains honeycomb-shaped, gas-filled sacs.

(Figure 3.1). At wave-sheltered sites, the overall morphology of the blade is broad and cape-like, with undulating edges (Figure 3.1A, left). At more wave-exposed sites, the blade becomes flatter and subdivided into many whip-like thongs (Figure 3.1B, right). At extremely wave-exposed sites, the stipe becomes longer, the blade shorter, and the overall morphology is stunted [15]. The morphology of D. antarctica is therefore a qualitative measure of the predominant wave exposure at a particular site.

The medulla of the blade of D. antarctica consists of gas-filled sacs [14], which make the whole blade positively buoyant (Figure 3.2C). At low tide, the photosyn-thetically active area can therefore be maximized as the blade floats at the surface while minimizing self-shading [18]. The thickness of the medulla is not uniform but is dependent on a variety of factors such as wave exposure, age, and overall morphology (C. Hurd, unpublished data). The thallus of D. antarctica can consequently be very voluminous at a comparatively low weight.

The congeneric species D. willana is endemic to New Zealand. In general, the stipe is larger and stiffer and bears lateral secondary blades of smaller size in addition to the apical main blade [19]. If the main blade is lost as a result of failure, one of the lateral blades can increase in size considerably. Durvillaea willana commonly form a belt in the intertidal-subtidal zone just below the belt of D. antarctica. Sometimes stands of the two Durvillaea species will be mixed. The ecological range of D. willana, however, seems to be more restricted than for D. antarctica, because this species is absent at sites of very severe wave exposure and also at sites of moderate wave exposure, where populations of D. antarctica can still exist.

The main morphological-anatomical difference between the two species of Durvillaea is the makeup of the blade. With D. antarctica, the blade is positively buoyant and has the tendency to float at the water's surface (Figure 3.2A). Unlike many other seaweeds, e.g., Macrocystispyrifera or Ascophyllum nodosum, the entire medulla of the blade of D. antarctica is gas-filled rather than only the pneumatocysts. The blade of D. willana lacks the honeycomb-shaped, gas-filled sacks of the medulla. As a consequence, the blade of D. willana is neutrally buoyant and floats upright in the water column if no wave action or currents are present (Figure 3.2B) and is generally not as bulky as the blade of D. antarctica. A difference in the way these two species react to flow-induced loading can therefore be expected.

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