the spiny green alga of NZ's warm northern coasts
- Size
- Height: 10–20 cm
- Lifespan
- 2–5 years
- Diet
- Photosynthetic. Grows on rocky reefs in intertidal and shallow subtidal zones. Requires clean water, stable rock surfaces, and good water flow. Forms cactus-like, branching fronds with spine-like projections.
- Habitat
- Grows in sheltered bays and estuaries of northern New Zealand. Forms upright, cylindrical, segmented fronds that look like a cactus.
- Range
- Found throughout the North and South Islands on rocky reefs in intertidal and shallow subtidal zones. Most common in exposed, rough-water habitats. Also found in tropical and subtropical regions worldwide.
- Endemism
- Native
- Main Threats
- None significant. This species is common and widespread in exposed habitats. Localised threats include coastal development, pollution, and climate change affecting water temperature.
- Population
- Not Threatened. Sea cactus is common in sheltered coastal waters of northern New Zealand, particularly in the Hauraki Gulf and the Bay of Islands. It is a tropical and subtropical species that reaches its southern limit in New Zealand.
- Conservation Status
- Not Threatened
The one that looks like it belongs in the desert, not the sea, has fronds that are upright, cylindrical, and segmented, growing up to 20 centimetres tall. They are bright green and covered in small, spine-like projections. It looks like a prickly pear cactus that decided to go for a swim. But the spines are not sharp. They are soft and fleshy, made of the same tissue as the rest of the frond. It is the alga of the desert disguise, the one that is not what it seems.
What makes it special is the resemblance. The sea cactus is a master of mimicry, but it is not mimicking anything. It just happens to look like a cactus, a case of convergent evolution, where two unrelated organisms evolve similar shapes because they work. The cactus shape works on land, where it stores water and deters herbivores. It also works in the sea, where it creates surface area for photosynthesis and confuses grazers. It is the alga of the convergent shape, the one that proves that good design is universal.
The sea cactus is a green alga, a member of the Udoteaceae family. It is calcified, it deposits calcium carbonate in its tissues, making it stiff and crunchy. This calcification helps protect it from grazing by fish and sea urchins. It is the armour-plated cactus of the sea, the one that is soft inside but hard outside.
Biologically, the sea cactus reproduces by releasing spores from specialised structures on its fronds. It also reproduces by fragmentation, a piece broken off can grow into a new plant. The creeping stem (stolon) runs along the bottom, sending up new fronds at intervals.
To find sea cactus is to find a tiny cactus garden on the sandy bottom. The upright fronds sway in the current, their soft spines waving. It is the desert plant of the sea, the one that proves that a good shape works anywhere, the one that reminds us that evolution has a sense of humour.