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 sulfurous 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.
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. The largest brown seaweeds are known as kelps, and are prominent in the cooler coastal waters of southern New Zealand.
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.