BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. DIVERSITY OF LIFE ON EARTH

2.6. Kingdom Protoctista

2.6.5. Phylum Phaeophyta (Brown Algae)

Fucus

Fucus is a relatively large brown alga with a fairly complex Structure. Its body is a thallus differentiated into a holdfast, a basal disc, and a frond (it should be noted that these are not true stems, roots, and leaves). This marine alga is commonly found on the rocky shores of the British coastline. It is well adapted to harsh coastal conditions, where it is alternately exposed to the air and submerged by the tides.

There are three most common species of Fucus, which typically occupy three distinct zones, or tidal heights, along the shore — a phenomenon known as zonation (section 10.6.4). The vertical distribution of these Algae is related primarily to their tolerance of exposure to air. The main features for identifying them, along with their typical shore habitats, are listed below.

F. spiralis (flat wrack) — found washed ashore near the high-tide mark. When submerged, the thallus is slightly twisted in a spiral.

F. serratus (serrated wrack) — found in the mid-shore zone. The margins of the thallus are toothed.

F. vesiculosus (bladder wrack) — found near the low-tide mark. It possesses air bladders that provide buoyancy for the alga. The external features of F. vesiculosus are shown in Fig. 2.31.

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Fig. 2.31. External structure of Fucus vesiculosus; characteristic features are indicated, including environmental adaptations.

ADAPTATIONS TO THE ENVIRONMENT. Before examining how Fucus is adapted to its habitat, a few words should be said about The Nature of the environment itself, which is rather inhospitable. As intertidal organisms, various algal species are exposed to the atmosphere to varying degrees during low tide. Consequently, they must be protected against desiccation. Furthermore, temperatures can fluctuate drastically when cold seawater rushes into warmed tide pools left behind by the receding tide. These plants must also be adapted to another factor: wide fluctuations in salinity, whether increasing due to evaporation in small tide pools or decreasing during rainfall. To withstand forces such as tides, ebb currents, surf, and wave action, a certain degree of mechanical strength is required. Large waves can roll boulders over, which may crush the algae and cause severe damage.

MORPHOLOGICAL ADAPTATIONS (GENERAL STRUCTURE). The algal thallus is firmly anchored to the substrate by a holdfast, or basal disc (Fig. 2.31). The attachment to the substratum—usually rocks—is so strong that it is extremely difficult to detach the alga. In fact, the rock itself usually fractures before the basal disc gives way.

The thallus branches dichotomously (i.e., splitting into two equal branches at each junction). This minimizes resistance to the flow of Water rushing between the branches. In addition, the thallus is tough and resilient rather than rigid, while the midribs of the fronds are strong and flexible.

F. vesiculosus features air bladders that provide buoyancy; this keeps the fronds close to the water's surface, thereby maximizing light capture for Photosynthesis.

METABOLISM/14.html">Chloroplasts are located in the outer layers of the alga, ensuring maximum exposure to the light required for photosynthesis.

PHYSIOLOGICAL ADAPTATIONS. The brown pigment fucoxanthin is the dominant photosynthetic pigment. This represents an adaptation to underwater photosynthesis, as fucoxanthin strongly absorbs light in the blue region of the visible spectrum, which penetrates much deeper into the water Column than longer-wavelength light, such as red light.

The thallus secretes large amounts of mucus, which fills all internal cavities of the alga and oozes onto the surface. This mucus helps retain water, thereby preventing dehydration during low tide.

The osmotic potential of the algal Cells is higher (less negative) than that of seawater, preventing Water Loss via osmosis.

ADAPTATIONS FOR SEXUAL REPRODUCTION. Gamete release in Fucus is synchronized with the tides. During low tide, the thallus dries out slightly and extrudes its reproductive Organs, which are protected from desiccation by mucus. As the tide comes in, the walls of the reproductive organs dissolve, releasing the Gametes.

Male gametes are motile and exhibit positive chemotaxis, directing their movement toward the chemical attractant secreted by the female gametes.

Zygote development occurs immediately after Fertilization, minimizing the risk of being washed out into the open ocean.