Until recently seaweeds were described as simple plants that grew in the sea, but scientists now consider that the structure and chemistry of seaweeds is so distinctive that they warrant their own classification. They are simple in structure, usually consisting of a holdfast that anchors them to a surface, and a blade which may be divided into fronds. Some of the larger seaweeds have a flexible stalk or stipe connecting the blade to the holdfast. Unlike plants, seaweeds do not have roots or internal tissues to conduct water. They absorb minerals and gases directly from sea water through the surface of their blades.
Seaweed groups
Seaweeds can be grouped into three types, based on colour – green, red or brown. They all contain the light-absorbing pigment chlorophyll, which is necessary for photosynthesis. Brown and red seaweeds have additional pigments that enable them to photosynthesise at depths where little light penetrates. These extra pigments mask the green colour of chlorophyll. Brown seaweeds can be yellow-brown to dark olive. Red seaweeds have the greatest range of tone – pink to purple, red, and brown to nearly black.
Life cycles
Many seaweeds only live, or only grow, for a single season; others, especially the large kelps, grow year-round and may live for many years. Seaweeds have complex life cycles involving both sexual and asexual stages. Their appearance may change markedly between these stages. Red seaweeds have the most complicated life cycle. One edible species, karengo (Porphyra species), includes a phase that bores into the surface of shellfish and rocks.
Role in the marine ecosystem
Seaweeds play a major role in marine ecosystems. As the first organism in marine food chains, they provide nutrients and energy for animals – either directly when fronds are eaten, or indirectly when decomposing parts break down into fine particles and are taken up by filter-feeding animals. Beds of seaweed provide shelter and habitat for scores of coastal animals for all or part of their lives. They are important nurseries for many commercial species such as the rock lobster, pāua (abalone) and green-lipped mussel.
Types of seaweed
Green seaweeds
Green seaweeds are usually found in the intertidal zone (between the high and low water marks) and in shallow water where there is plenty of sunlight. About 140 species have been recorded around the coast. One of the easiest to recognise is sea lettuce (Ulva lactuca), which forms bright green sheets up to 30 centimetres in diameter. As its common name suggests, it is edible, although prolific growth often indicates sewage pollution. Sea lettuce can become a problem when large quantities are washed ashore and begin to rot, giving off an offensive sulphurous smell. Gut weed (Enteromorpha intestinalis), a tubular green seaweed, also favours high-nutrient sites. Another common green seaweed is sea rimu (Caulerpa brownii), also edible, and looking very much like the foliage of the large tree rimu.
Brown seaweeds
These medium to giant-sized seaweeds typically grow at depths below the greens and above the reds. Neptune’s necklace (Hormosira banksii) is well known to most people who have visited the rocky shore. Its branching chains of water-filled bladders help it withstand periods of exposure when the tide goes out. Many seaweeds produce mucilage or slime to protect against drying out. Of the brown group, Gummy weed (Splachnidium rugosum) takes mucilage production to the extreme – its swollen tubes ooze profuse quantities of sticky slime when touched.
Red seaweeds
There are 550 species of red seaweed, making them the largest group. In the clear waters around the Kermadec Islands red seaweeds may be found at depths greater than 200 metres. In the nutrient-rich coastal waters of New Zealand’s main islands very few survive below 25 metres.
One of the best-known reds is the edible karengo (Porphyra species), which grows on rocks near high-tide level and resembles sheets of light purple cellophane. It is a close relative of the Japanese nori, used for sushi. Another familiar red is the fern-like agar weed (Pterocladia lucida) which has been harvested for agar production in New Zealand since 1943. The coralline seaweeds are a group of reds that deposit calcium carbonate in their cell walls, forming pink skeletons or paint-like crusts on coastal rocks. Scientists have discovered that some crust-forming seaweeds release chemicals that encourage pāua (abalone) larvae to settle and mature.
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What is seaweed?