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.
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.
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.
New Zealand has 850 native seaweeds, a third of which are endemic – they are not found anywhere else. Seaweed is particularly plentiful in three areas: the warm northern waters around the Kermadec and Three Kings Islands, the Cook Strait–Kaikōura coast region in central New Zealand, and the south, in an area encompassing Fiordland, Stewart Island and the Otago coast. Distribution varies according to species. Some, like the red seaweed Gelidium longipes, only grow in a few places. Others, such as giant kelp (Macrocystis pyrifera), are widespread in cool southern hemisphere waters.
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.