little hard fern

Blechnum penna-marina features a wiry rhizome, which gives rise to long creeping and sparing branches. The fronds of this species exhibit strong dimorphism, with sterile fronds growing up to 400 mm when semi-erect in manner, while fertile fronds are longer and held erect, often reaching twice the length of the sterile fronds. Sterile fronds have a yellow-brown stem, whereas fertile fronds have a purplish-black stipe. The laminae of both sterile and fertile fronds are pinnatisect, linear to narrowly elliptic, tapering to a pinnatifid apex, and coriaceous, with free veins. The sterile fronds bear 12–44 pairs of triangular or oblong pinnae, while the fertile fronds have 10–36 pairs of linear to narrowly oblong pinnae, which are replaced by tiny sterile flanges at the base of the lamina.

The species is widely distributed throughout the global south, including New Zealand, Australia, and South America. It is native to Marion Island, a subantarctic island halfway between Africa and Antarctica. Within New Zealand, this fern species can be found throughout the country, spanning from the North Island to the South Island, including the Chatham, Stewart, Auckland, Campbell, and Antipodes Islands. It exhibits a broad altitudinal range, from sea level up to 1900 meters above sea level. In the North Island, Blechnum penna-marina is found in the lowlands to alpine regions, extending from Hamilton and East Cape to south Wellington. Similarly, in the South Island and Stewart Island, it occurs in the lowland to alpine areas. Notably, it has been observed growing at high elevations of up to 1900 meters above sea level in the Spenser Mountains, Nelson. The wide geographic distribution of Blechnum penna-marina highlights its adaptability to diverse habitats and climates across the southern hemisphere.

There are few identified enemies of Blechnum penna-marina. Blechnum penna-marina is subject to herbivory by several endemic grasshopper species in New Zealand. The grasshoppers Brachaspis nivalis, Paprides nitidus, and Sigaus australis are known to feed on the pinnae of Blechnum penna-marina. These grasshoppers inhabit areas with vegetation covering the ground and can be found in alpine environments where Little Hard Fern grows well. While these grasshoppers are considered endemic to New Zealand, their presence indicates a potential interaction between the fern and native herbivores in other ecosystems. The impact of these grasshoppers on the population dynamics and growth of Blechnum penna-marina warrants further investigation, as it may play a role in shaping the fern's distribution and abundance in its natural habitat. No fungal parasites or diseases have been recorded for this species as of March 30, 2023.