Shapes of seaweed

Its all about communication, the science of identifying algae, or any organism for that matter revolves around identifying the different parts. Therefore my guide is useful in conjunction with identification guides or taxonomic keys.

Seaweeds suffer greatly in the harsh environment of the intertidal zone. This environmental pressure from dessication, wave action and predation has lead to a large variety of different forms or shapes appearing on the shore. In this section, we look at the different forms from the foliose and leathery macrophyte groups.

So, what are the individual parts called? First lets study the structure of each of these groups leathery and foliose.

Leathery (Fucus sp) Foliose (Ulva sp.)

Leathery algae.Foliose algae.

Leathery algae are large with a complex structure, comprising of many adaptations to its environment such as bladders and the large claw holdfasts of Laminaria sp. Foliose algae are basically sheets of tissue, this forms the frond attached to substrate by a small discoid holdfast.

And the terms explained…

Laminaria digitata.Thallus The thallus is basically the entire body of the plant, from the very tip to the bottom. The shape of the thallus allows us to simply classify the algae. For example the foliose algae are all sheet like, and very fragile (see Ulva above right). The filamentous algae are all thin lines of cells and the leathery macrophytes are large complex tough algae.

So the thallus varies greatly between the different groups, but what does it do. Basically the thallus could be described as the unit that encompasses the entire plant, the reproductive organs, holdfast and all.

The size of the thallus can vary greatly. Some algae are so small they can only be seen through a microscope, others are so large they can hardly be picked up by a human (see Laminaria pic right).

Blade/Frond Following on from the thallus, the frond is often used to describe the main structure of seaweed. Frond describes the branches that originate from the stipe or holdfast. The pattern of branching that the fronds exhibit can be grouped into four different types (summariesed below). Dichotomous branching is the most common, with species such as the fucoids have. Whorled generally occurs among the filamentous algae.

Stipe The stipe joins the frond to the holdfast. Without this length of extremely tough tissue (in comparison to the stength of the fronds) it would be difficult to exist in wave swept areas.

Air Bladders Bladders are a common feature of fucoids, but are lacking among most species of seaweed’s, the spherical bladders are filled with gas allowing the algae to remain upright in the water column during submergence.

Receptacles The reproductive organs consisting of concepticles and receptacles, a little difficult to explain concisely (more info on way..)

Holdfasts.Holdfasts Self explanatory really. Seaweed’s are algae, which are entirely different to terrestrial plants which absorb water and nutrients through a root system. Algae doesn’t have a root system to absorb water, water is absorbed through the cellular structure. In the absence of a root which also provides a fixing point for plants, algae have substituted roots for holdfasts. There are 3 main types varying between species, the environment that they are found in and the size of the plant.