Brittle bladderfern is a small fern, found in rock crevices scree slopes and thickets from the lowlands to the high alpine zone, mostly in the mountains. It grows from a stout dark rhizome, tufted, 10-30 cm tall. Leaves are twice-pinnately divided, the leaflets pinnately lobed. The overall outline of the leaf is lanceolate, the uppermost pinnae decreasing in size to a point. The leaf stalks are mostly scale-less, with only a few at the base. Almost all leaves are fertile. An oval-shaped, white indusium covers the sporangia on the back of the leaves. The indusium often falls off, revealing lumpy clusters of brown sporangia. The other member of the genus Cystopteris in Denali, C. montana has fronds triangular in outline, nearly as wide as it is long, not lanceolate in outline as in C. fragilis. Other lanceolate ferns lack the hood-shaped indusium unique to the genus Cystopteris.
Cystopteris fragilis is perennial and deciduous; the stipes are persistent in a clump above the rhizome. Spore production occurs in mid-summer.
Brittle bladderfern is, like all fern species, spore producing with haploid and diploid life stages. A study in Oregon found high germination rates of the species from forest soil samples and a preference for shade (Strickler and Edgerton 1976). Fronds have high sporangial opening (<1% of examined sporangia were closed), and it was one of the few fern species with measurable establishment rates in the wild in studies from central Iowa (Farrar 1976). Plants can also reproduce without fertilization, growing as buds from leaves (Lawton 1936). If tetraploid plants are generated this way, the plants are sterile.
Cystopteris fragilis is wide-ranging, cosmopolitan, circumpolar species, and also found in mountainous regions of South America, southern Africa, and New Zealand. In Denali, C. fragilis occurs in hills and mountains on both sides of the Alaska Range, with a occasional low elevation localities on both sides of the Park.
Brittle bladder fern is an alpine species in Denali, found most commonly on elevations of 900-1100 m. It has a marked preference for steep slopes, the majority of occurrences found on slopes above 20 degrees.