the green cushion alga of NZ's warm northern rock pools
- Size
- Height: 5–15 cm
- Lifespan
- 2–5 years
- Diet
- Photosynthetic. Grows on rocky reefs in low intertidal and shallow subtidal zones. Requires clean water, stable rock surfaces, and good water flow. Forms grape-like clusters of small, bead-like fronds that are green to brownish-green.
- Habitat
- Grows in sheltered bays and estuaries of northern New Zealand. Forms clusters of small, round, grape-like green beads.
- Range
- Found throughout the North and South Islands on rocky reefs in low intertidal and shallow subtidal zones. Most common in clear, sheltered waters with good water flow. Also found in tropical and subtropical regions worldwide.
- Endemism
- Native
- Main Threats
- None significant. This species is common and widespread. Localised threats include coastal development, pollution, and climate change affecting water temperature.
- Population
- Not Threatened. Sea grape is common in sheltered coastal waters of northern New Zealand, particularly in the Hauraki Gulf, the Bay of Islands, and other warm, shallow bays. It is a tropical and subtropical species that reaches its southern limit in New Zealand.
- Conservation Status
- Not Threatened
The one that looks like a tiny bunch of green grapes has fronds that are made of a central stem with many small, round, bead-like side branches, each one a perfect sphere, packed tightly together. The beads are bright green, shiny, and plump, about the size of a peppercorn. They look like they should be edible, like they should burst with sweet juice when you bite them. It is the alga of the false fruit, the one that looks good enough to eat but is not.
What makes it special is the resemblance. The sea grape is a master of mimicry. It looks like a cluster of grapes, but it is not a grape. It is a single-celled organism, the entire plant is one giant cell with many nuclei. The beads are not separate structures. They are lobes of a single cell, connected by a network of internal tubes. It is a living sculpture, a single cell that has organised itself into a complex shape. It is the alga of the giant cell, the one that is not what it seems.
The sea grape is a member of the Caulerpa genus, which includes some of the most invasive seaweeds in the world. The Mediterranean strain of Caulerpa taxifolia has devastated seagrass beds and reefs across the Mediterranean. But our native sea grape is a good citizen, staying where it belongs, growing slowly in the warm northern bays. It is the alga of the good neighbour, the one that does not overstay its welcome.
Biologically, the sea grape reproduces by fragmentation, a piece broken off by a wave can grow into a whole new plant. It also reproduces sexually, releasing gametes into the water. The creeping stem (stolon) runs along the bottom, sending up new fronds at intervals.
To find sea grape is to find a cluster of tiny green beads in the shallow water. They look like grapes, like something you could eat. But you should not. Some Caulerpa species are toxic, and even the non-toxic ones taste terrible, bitter and salty, with a texture like soggy plastic. It is the look-but-don't-touch of the seaweed world, the one that proves that not everything that looks good is good for you.