Brown algae are more colourful than their name suggests

As a species group, the brown algae are very diverse in colour, size, and shape. This group includes the most prominent algae on our coast, such as bladder wrack and sea-lace. The sheer variety of this group is crowned by brown filamentous algae, which are difficult to distinguish from their green and red cousins.

Typically, brown algae are easy to identify with the following memory rule: the more specialised the algae looks the more likely it belongs to the brown algae.

Sea-lace can even attach itself to a crumb of rock

Although brown algae usually grow attached to the hard sea bottom, the whip-like sea-lace alga is content with any surface, be it a crumb of stone or the shell of a blue mussel. Sometimes these algae appear to be rooted in a sandy or silty bottom but in reality, they are attached to a hard object and no roots are found.

 Brown algae species dead man's rope sway in clear beautiful water
The aptly named sea-lace (Chorda filum) belongs to the brown algae.

The large-sized bladder wrack is easy to notice

The bushy bladder wrack (Fucus vesiculosus) is a seaweed familiar to most Finns and is easy to identify. In the Finnish language, it was formerly known as rakkolevä, literally “bladder seaweed”. However, with the discovery of a new closely-related species of wrack, i.e. Fucus radicans, which is found mainly in the Kvarken sea area, it was decided to give new names in Finnish to both species to help keep them separate. Now Fucus vesiculosus is called rakkohauru (bladder wrack) and the smaller F. radicans is known as itämerenhauru, literally “Baltic wrack”.

Both wracks appear as bush-like growths which adhere to hard surfaces. Under favourable conditions, the bushes of bladder wrack can become massive. They often have gas-filled swellings at the tips of the fronds that support the plant, allowing it to remain upright in the water.

Wrack seaweeds growing on exposed shores are smaller in size. Such plants have only tubular spore sacs required for reproduction along their fronds. In eutrophicated areas, small and low growing wracks can be difficult to see, because fast-growing filamentous algae often prefer to attach themselves to the bladder wrack’s broad leafy fronds.

The low-growing brown alga Elachista fucicola (Fin. haurunturkki, literally “wrack fur”) only grows on the surface of bladder wrack fronds, forming golden-brown furry tufts. In contrast, the common brown filamentous algae, i.e. Ectocarpus siliculosus (Fin. litupilvilevä) and Pylaiella littoralis (Fin. lettiruskolevä) can grow so rapidly, they can completely bury the surrounding rock surfaces, as well as smothering wrack seaweeds, red algae, and blue mussels.

Wrack seaweeds offer shelter for fish and invertebrate animals

Wrack seaweeds are an important part of the underwater biodiversity of the Baltic Sea. These large algae provide shelter and food for adult and juvenile fish, as well as many invertebrates.

Animals feed on the wrack seaweeds and the algae growing on them, hide among their fronds, as well as prey on other small hidden creatures. If you put a whole bush of wrack seaweed in a bucket and shook it, you might be surprised by the number of animals teeming inside it!

 Bladder wrack seaweed reaches towards the sunlight.
A forest of bladder wrack (Fucus vesiculosus) forms one of the key habitats of the Baltic Sea.

Brown algae:

  • Sea-lace and Furry rope-weed (Chorda filum, Halosiphon tomentosum)
  • Bladder wrack (Fucus vesiculosus)
  • “Baltic wrack” (Fucus radicans)
  • Haurunturkki, ”Wrack fur” (Elachista fucicola)
  • Litupilvilevä ”Pod cloud alga”(Ectocarpus siliculosus)
  • Lettiruskohahtu ”Braided brown fluff”(Pyaiella littoralis)
  • Isopods (Idotea spp.)