Herbarium press of Plocamium pacificum

Plocamium pacificum is a beautiful red alga that can be found in both intertidal and subtidal salt water habitats. It is dark red in color and has an elastic, rubbery feel to it. Its height can range from 4-25 cm. The branches are arranged to form a dense and delicate framework. All of the branches come off of the main axes in an alternate pattern. The branchlets are small and are arranged on the branches like the teeth of a comb.

Systematics is the study of classification. More specifically, it is the classification of algae into taxonomic groups. Classification is done by examining organisms and placing them in groups based on shared characteristics.


Rhodophyta (Red Algae)






Gigartinales (Plocamiales*)





* The Order Plocamiales replaces Gigartinales, as the newest proposed Order for the Family Plocamiaceae.

Division Rhodophyta Characteristics

These are discussed in considerable detail in the "Red Background" section.

Class Rhodophyceae Characteristics

The Division Rhodophyta has only one class, which is the Rhodophyceae. Therefore, all red algae fall in the Class Rhodophyceae and share common class characteristics, which are the same as the division characteristics.

Subclass Florideophyceae Characteristics

There are two subclasses of red algae, Bangiophyceae and Florideophyceae. All algae within the Florideophyceae subclass have specific shared characteristics. The plant body of an alga is referred to as the thallus, and in this subclass it is filamentous with pseudoparenchymatous development. This means that the cells that compose the thallus are in a linear arrangement and those that are next to each other are not necessarily the result of the same cellular division.

Florideophyceae algae are multi cellular, when they grow they do so by producing new cells at the end of branches, apical growth. The algae of the other subclass can grow by producing new cells anywhere within the thallus. All of the cells within the florideophyceae thallus have disc shaped chloroplasts, energy producing photosynthetic organelles.


Order Gigartinales Characteristics

(No Plocamiales description was available for reference, therefore the presumably similar Gigartinales description is given.)

The filamentous thalli of the algae within this Order form crusts, bushes, or blades. This Order holds some of the largest red algae known. They are often described as fleshy due to the many erect branches or large blades. If they have a crustose form it is due to the tightly compacted filaments.


Family Plocamiaceae Characteristics

Growth pattern of Plocamium pacifcum

The thalli of Plocamiaceae algae are directed upward and the fronds are flattened. The branches are arranged in two rows on opposite sides of the thallus or they can also have a cushion like appearance where the branching patterns are unseen. The main thallus, often referred to as the main axis, has a sympodial form. This means that the apparent main axis developed so that it is a series of diverging branches. When a new branch forms off of the apparent main thallus it grows in the direction of the apparent main thallus, while the apparent main thallus curves off to the side and becomes a secondary branch. This occurs continuously as the alga grows.

Comb pattern on ultimate branches

All of the branches are singly arranged at consistent intervals on the main axis. The branchlets coming off of these branches are in a close series like the teeth of a comb. All of the branches are visibly flattened in shape.

Cell of Central unaiaxial filament in P. pacificum

Within the thallus there is a single, centrally located, longitudinal filament called a uniaxial. This filament is the chain of apical cells and is therefore the site of main algal growth. The apical cells are larger than the other cells and can be distinguished with a microscope.

The tetrasporangia, reproductive structures, are zonately divided.

Genus Plocamium Characteristics

The thalli are directed upward in relation to their attachment, or holdfast. A holdfast is a structure that attaches an alga to the substrate (any surface it lives on). The holdfast of Plocamium looks like a small disc on a substrate or on its own stolons. Stolons grow out from the bottom of some algae and are capable of producing offshoots.

Cross section of Plocamium pacifcum thallus

The comb like branches each have two to five branchlets of their own. Within the thallus the cells get smaller farther away from the centrally located apical cell. They are compacted around the edges of the thallus.

Shown at left is a cross section of the thallus of P. pacificum.


Plocamium pacificum

(Plocamium cartilagineum, as found on the Pacific coast of North America)

The distinguishing characteristic for Plocamium pacificum in many dichotomous keys in that the branchlets are not strongly incurved. Instead they are either straight, slightly incurved, or curving outward towards the parent branch. Another species of Plocamium, P. violaceum, has strongly incurved branchlets that appear to be almost claw shaped. It is hard to distinguish between the two Plocamium species without taking a close look at the shape of the branchlets.

Plocamium pacificum ranges from 4-25 cm in height and its axis is 1-2 mm in diameter. There may be more than one main axis arising from each holdfast. The main axes and branches appear slightly zigzagged. The more submerged they are found in the ocean the less dense the branches are.

The thallus is somewhat translucent and is deep red, purplish, or pink in color. It looks firm and feels elastic, and is therefore called cartilaginous. It is these key features that catch the eye. Once pressed its color ranges from a deep maroon to a light pink. P. pacificum is a visually beautiful alga that makes elegant herbarium presses.


Similar Species

There are several algal species found in the same locations as P. pacificum that look very similar. These species can often be confused with and mis-identified as P. pacificum. The key to distinguishing the species is to look for a few of the following characteristics.

Microcladia spp.

Plocamium spp.

-second set of branches always with closely set, equal length branchlets (comb-like in shape)

-not all of secondary branches have comb-like branches

-mostly found growing epiphytically on other algae

-usually on a rocky substrate that is covered by sand

Branching pattern of Microcladia

Branching pattern of Plocamium pacificum


Ptilota and Neoptilota spp.

Plocamium spp.

-branching always oppositely arranged

-branching alternate or irregular along the branches

-main axis is straight

-main axis has slight zig-zag

Branching pattern of Ptilota filicina

Branching pattern of Plocamium pacificum

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© 1999 Nilauro Markus, Marla Ranelletti, Christopher Loo