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Species Database

Cystoseira osmundacea - Chainbladder Kelp

Chainbladder Kelp image

Geographic range:

Seaside, Oregon to Ensenada, Baja California, Mexico

Key features:

The blades are shaped like the leaf of a coastal oak. When reproductive, the chain of bladders (i.e. floats) emerging from the tip of the blade and rising to the surface is unique.


bay (rocky shore), exposed rocky shore, kelp forest, protected rocky shore

Primary common name:

Chainbladder Kelp

Synonymous name(s):

Cystoseira expansa

General grouping:

Brown seaweed/algae

ITIS code:


Geographic Range

Range Description:

Cystoseira osmundacea can be found from Seaside, Oregon to Ensenada, Baja California, Mexico.

Intertidal Height

Lowest intertidal height:

0 meters OR -2 feet

Highest intertidal height:

0 meters OR 0 feet

Intertidal height notes:

Cystoseira osmundacea occurs occasionally in the lower intertidal but is more common in the subtidal of kelp forests.

Subtidal Depth Range

Minimum depth:

0 meters OR 0 feet

Maximum depth:

10 meters OR 33.3 feet

Subtidal depth notes:

Cystoseira osmundacea is common in the subtidal.


bay (rocky shore), exposed rocky shore, kelp forest, protected rocky shore

Habitat notes:

Cystoseira osmundacea can be found in pools and on rocks in the low intertidal and shallow subtidal down to 10 m. It is frequently mingled with Macrocystis pyrifera beds. It thrives most proliferately in a well-illuminated habitat, yet cannot withstand much desiccation. Therefore most plants are found on the sublittoral rocky sea floor between 4 and 8 m depth. Some plants can grow in deeper water, but their growth is stunted. They are rare in shallow waters likely due to severe wave force removing them during violent winter storms.


Relative abundance:

Cystoseira osmundacea is common, especially in the subtidal.

Species Description

General description:

Cystoseira osmundacea is a brown alga in the family Cystoseiraceae, the order Fucales, the class Phaeophyceae and the phylum Heterokontophyta. This species is one of the only fucalean species that grows large enough to form a surface canopy. The genus Cystoseira is one of the most widely distributed genera of the Fucales order and as such, provides essential habitat for many epiphytes, invertebrates, fish and even humans. Cystoseira osmundacea intergrades gradually with the other two species in California, Cystoseira neglecta and Cystoseira setchellii. Though intermediates occur, these latter two species represent different morphological types that are characteristic of certain environments.

Distinctive features:

The thallus of Cystoseira osmundacea is blackish-brown below, light tan above and can grow to be 8 m tall. The thallus consists of the holdfast, stipe, blades and floats. The holdfast is disc-shaped and gives rise to a short, tough stipe that is triangular in cross-section. The stipe has a light midrib arising radially and bears flattened, pinnate blades. The holdfast, lower stipe and flattened blades are all considered the basal region of the plant. The basal region is retained year round and makes up the entire plant during the winter dormant season. The top, or apical, region of the plant is where pneumatocysts, or floats, arise from both new apical branches and the bases of old lateral blades. These floats may be spherical or ellipsoid and occur in chain-like formations on the upper branches. Apical branches tend to be cylindrical versus the flattened blades of the lower branches. During fertile growth, reproductive structures, called receptacles, can be found distal to the pneumatocysts on the upper branching fronds. This top region of the plant is appropriately referred to as the apical region and is shed at the onset of winter.


Cystoseira osmundacea can reach heights of about 8 m and 30 m in length as part of a surface canopy.

Natural History

General natural history:

Cystoseira osmundacea exhibits dramatic seasonal variations in its morphology. The apical region, which contains the reproductive structures and is responsible for the formation of the surface canopy, breaks off in the winter, leaving behind the short basal region. The shorter, smaller version of Cystoseira osmundacea can better survive violent winter storms and wave stress. It is believed this alga may live for many years.

Dense stands of Cystoseira osmundacea, along with Macrocystis pyrifera, and their surface canopies result in regions of high productivity and essential habitat. The canopies provide protection from the sun for understory algae and also serve as a foraging area, as well as providing protection, for many invertebrates. Fish also use these stands for protection and for finding prey. Cystoseira osmundacea also directly provides habitat for the epiphytes Coilodesme californica, Coilodesme plana, and Rhodophysema elegans. In experimental removals of Cystoseira osmundacea canopies, there were significant losses to regional diversity.

Although it is called a kelp, it is not a member of the kelp Order Laminariales but is instead in the Order Fucales, Family Cystoseiraceae.


A variety of invertebrates, such as sea urchins, feed on Cystoseira osmundacea. Young plants are especially susceptible to predation.


Cystoseira osmundacea nourishes itself through photosynthesis, converting the energy of light to the energy of carbohydrate molecules.

Feeding behavior


January - November


By early spring, reproductive fronds appear on Cystoseira osmundacea and may be male or female, but not both. This plant is dioecious, meaning that male and female reproductive parts are on different plants of the same species and reproduces annually. By August, the reproductive fronds have reached the surface, meiotically produce and then release eggs or biflagellate sperm. Fertilization and germination lead directly to another diploid thallus. In contrast to most algae, there are no sporophyte or gametophyte phases in Cystoseira osmundaceaís life cycle. Instead, its life cycle is more animal like. By late autumn or winter, the reproductive fronds will be shed or ripped away by storms.
Click on an image below to view a larger version in the SIMoN Photo Library. You will also be able to view important information on each photo such as photographer, date, caption and more.
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O'Clair, R.M. 2000. North Pacific Seaweeds. Plant Press, Auke Bay, Alaska. 162 p.
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