Turtlenut

The turtlebush grows only upon the island archipelago enclosed by the Wyrm Straits. The bush gains its name from the distinctive nuts, known as turtlenuts, with shells that resemble two turtle carapaces pressed together.
It is a coastal plant, growing along the coastline and in the salty mouths of estuaries where the rivers meet the sea. Their extensive root system weaves through the rich silt, stabilising it against the action of the waves, and their waxy leaves are rimed with the salt they excrete to survive the seawater they grow in. It has also been successfully grown a little inland in brackish water, but will not grow in fresh water.
When the turtlebush is completely submerged in a king tide, its buoyant nuts float upwards. Wave currents break ripe nuts away from the bush and carry them away to propagate elsewhere when they wash up on a suitable stretch of land.
The turtlenut itself is the size of a flattened hand, and the shell can be cracked into two halves to reveal the succulent nut-meat. This meat is quite delicious, healthy, and filling, allowing shipwrecked sailors to survive for months on a diet which was principally composed of turtlenuts, although the taste palled quickly.
The turtlebush is commonly planted or encouraged by families living near the sea, as it forms a cheap and ready additional food source. It is notoriously difficult and labour-intensive to farm at any scale, however.