The Octopus on the Sand

The spiny forest of southwest Madagascar covers more than 14,000 square kilometres, a xerophytic ecosystem found nowhere else on Earth, shielded from the cyclones that batter the east coast by the island’s central highlands. Its defining trees belong to the family Didiereaceae, four genera, Alluaudia, Alluaudiopsis, Didierea, and Decaryia, in a striking case of convergent evolution with cacti that stops short of being cacti at all.

It looks like a cactus. It never was one.

Rather than the swollen green stems true cacti rely on, Didiereaceae carry small deciduous leaves on grey, woody branches studded with enormous spines, giving the whole plant a scraggly, unkempt look from any distance. Alluaudia ascendens takes that same basic plan and builds it into massive V-shaped structures ten to fifteen metres tall, alongside baobabs of its own, Adansonia rubrostipa and Adansonia za.

Didierea madagascariensis, the octopus tree proper, grows on the red coastal sands between Morondava and Toliara, its long spine-armored leaves arranged in rosettes atop an upright trunk. Its close relative, Didierea trollii, takes root further south, from Lake Tsimanampetsotsa to the Mandrare river, but starts life in a form that gives the whole family its nickname: a juvenile stage that crawls flat along the ground, its radiating spiny arms genuinely resembling an octopus laid out on the sand.

It is an easy plant to mistake for something violent at first glance and nearly impossible to mistake for anything else once its shape registers. Nothing else in the spiny forest, or arguably anywhere else on the island, grows quite like it.

On the Mahafaly plateau, part of this same dry domain, annual rainfall runs under 500 millimetres across a dry season lasting ten months, with an average of just seven to nine days of rain a year. Plants here survive by storing water in bottle-shaped trunks and swollen underground tubers; animals survive by shutting down entirely, with the common tenrec, Tenrec ecaudatus, and the grey-brown mouse lemur, Microcebus griseorufus, both entering torpor or hibernation to wait out the worst of it.

Despite the hardship, or because of the isolation it enforces, the region supports real diversity: 159 recorded bird species, ten of them, along with three entire genera, found nowhere else on the planet. Roughly 95 percent of the plant species in Madagascar’s south are endemic to it, a level of specialization that only this kind of prolonged, punishing scarcity tends to produce.

Outside its handful of formally protected parcels, the spiny forest, often called the bush épineux, carries little legal protection of its own. Near towns and settlements, Didierea madagascariensis is cut directly for firewood and charcoal production, a use that steadily erodes exactly the specialized, slow-growing vegetation the region’s endemic wildlife depends on.

What makes the loss especially hard to reverse is the ecosystem’s own arithmetic: a landscape built around scarcity, low rainfall, minimal soil, brutal heat, grows back slowly even under the best conditions. An octopus tree cut for a cooking fire today is not a resource on a renewable timeline. It is closer to a withdrawal from an account that took centuries to fill, in a region that can least afford the loss.