Narrow wrack (Fucus radicans) and bladderwrack (Fucus vesiculosus) have separate male and female plants. The reproductive organs are called receptacles and are placed at the tips of the plant. They are easily recognised by their warty structure.

When we try to “sow” seaweed”, we start with collecting ripe plants from the field, determine which sex they have, and cut of the receptacles.

Cut receptacles of bladderwrack

Cut receptacles of bladderwrack

The picture to the left shows a bladderwrack ready to sow. Note that the pile of cut receptacles to the right in the photo is from three plants.

In order to separate between males and females, one has to cut a mm-thin slice of the receptacle and (with a little magnifying help from a loupe or similar) see if there are oogonia (8 eggs in a small sack) or antheridia (64 sperm in an even smaller sack). This can only be done on ripe receptacles, or else it is very hard to see.

Round oogonia contains 8 eggcells. Some are beginning to open up.

Round oogonia contains 8 eggcells. Some are beginning to open up.

Each receptacle consists of several small chambers, conceptacles. The opening pore of these conceptacles are what causes the warty structure of the receptacle. Each conceptacle openes onto the receptacle surface, and this is where oogonia and antheridia (eggs and sperm in bags) are ejected out into the water mass during fertilization. When the oogonia and antheridia have reached the water, the bag keeping them contained, begins to dissolve.

The female oogonia looks like a collection of small green peas, and can be seen with the naked eye if they are very ripe.

It's a girl! Lots of ripe, round oogonia in the receptacles.

It’s a girl! Lots of ripe, round oogonia in the receptacles.

A ripe male, packed full of orange sperm.

A ripe male, packed full of orange sperm.

Antheridia are too small to see, even with a loupe. You need a microscope for them. On a receptacle cut, they give an impression of orange balls along the inside of the receptacle edge (see picture). The colour comes from the eyespot of the sperm, which is orange. With this, the sperm can tell light from dark.

Reproduction occurs around full moon, when it is much darker down towards the bottom than up towards the surface. The sperm “knows” that it should swim towards darkness. The reason for this is that the heavy eggs are sinking in order to attach to the bottom once they become fertilized.

Fucus evanescens

Fucus evanescens, is an introduced species to the Baltic Sea. That means that it did not enter the Baltic Sea by itself, but was brought here, possibly by humans. A piece of seaweed could easily have stuck to an anchor or similar. This is quite common nowadays, where boats unknowingly transport species in their ballast water from one place to another.

More information will follow.

Fucus serratus – Serrated wrack

Serrated wrack, Fucus serratus, is easily recognized by its serrated edges. In the Baltic Sea we can find serrated wrack along the Swedish coast up to the Gryt archipelago in Östergötland, where the salinity is approximately 7 psu. We have not found any observations of its distribution along any other Baltic countries coastlines. If you know of any such, please let us know.

Serrated wrack (Fucus serratus) on the Swedish coast

Serrated wrack (Fucus serratus) on the Swedish coast

Serrated wrack is sensitive to dehydration and is not as tolerant as i.e. bladderwrack to air exposure. Where we find baldderwrack and serrated wrack living together, the serrated wrack is often found slightly deeper thatn the bladderwrack. Outside the east coasts of Öland and Gotland, for example, there are wonderful forrests of serrated wrack at about 8 m depth.

Serrated wrack thrown ashore by waves often gets a more brown-orange colour before it dries and turns almost black.
In the winter, you can find frost covered seaweed when you walk along the shore.

Frozen Fucus serratus

Frozen Fucus serratus

More information will follow.

Fucus radicans – Narrow wrack

The scientific name of narrow wrack is Fucus radicans which means that it belongs in the Fucus family, together with bladderwrack, serrated wrack and spiral wrack (Fucus spiralis).

Narrow wrack is recently discovered, since it was long belived to be a dwarf morph of bladderwrack. It is not unusual that organisms are smaller in the low salinities of the Baltic Sea. Using genetic tools, narrow wrack was identified as a new species in 2005. Since then, researchers at Stockholm and Gothenburg University have studied the ecology, reproduction and genetics of the narrow wrack.

Narrow wrack (right) is thinner and more bushy than bladderwrack (left)

Narrow wrack (right) is thinner and more bushy than bladderwrack (left)

Narrow wrack is found along the Swedish coast from Öregrund to Umeå, from around Vaasa area down to Poori/Björneborg on the Finnish coast and around the Estonian island Ösel/Saaremaa (see map under The Baltic Sea fact). Narrow wrack has not been found outside the baltic Sea, as far as we know.

Narrow wrack is clonal, wich means that it reproduces by fragmentation, but it also has sexual reproduction. The individuals that have formed by fragmentation, where small branches from the plant falls off, drift away and then reattach to a rock or boulder, all have the same genetic variation as their “mother” plant. There is one plant in particular that have been very successful along the Swedish coast, where almost 80% of all individuals are one clone.

Fucus vesiculosus – Bladderwrack

The most common wrack in the Baltic Sea is the bladderwrack, Fucus vesiculosus. Fucus is from the greek phykos, meaning seaweed. The species name vesiculosus refers to the gas filled bladders that are common in the species.
It is belived thet the bladderwrack has been present in the Baltic Sea for about 8000 years, from the period known as the Yoldia Sea. Bladderwrack and serrated wrack (Fucus serratus) are both marine species that have, more or less, adapted to the brackish waters of the Baltic Sea. The best adapted species is bladderwrack, that we can find all along the Swedish coast almost as far north as Umeå.

Bladderwrack can vary very much in shape and size, depending on whether it is living on rocks that are exposed to strong waves or living in sheltered bays. Where it is sheltered it can grow to over one metre in height, with broad thallus and plenty of bladders.
In exposed sites, the bladderwrack often lacks bladders so that the waves cannot tear it away. Both height as well as thallus width is much less on exposed sites.

Bladderwrack has the same lifecycle as humans. The plants are either male or female. They produce eggs and sperm in special organs, called receptacles. The receptacles are located in the tips, with a warty structure.


Eggs and sperm are released into the water column on still nights around full moon. The negatively buoyant eggs sink to the sea floor, followed by actively swimming sperm. New seaweed is made.

Seaweed in general

What is seaweed?

The word ”seaweed” is often used to describe perennial macroalgae. So, what are perennial macroalgae?

The term perennial is used in botany to describe how long a plant lives. Perennial means that the individual lives for several years. The opposite, only living for one year, is called annual.

A macroalgae is an algae we can see with the naked eye (macro – large) as opposed to microalgae (micro – small) where we need a loupe or a microscope to be able to see them.

Finally, we must define the term “algae”. (Hint: There is a very neat page on this on Wikipedia for those of you who wants a longer explanation.)

One of the major diffrences between algae and plants is that algae lack vascular tissue.

Plants have developed an equivalent to our bloodstream in order to transport nutrients and water that is taken up by the roots from the soil to all the parts of the plant. The vasciular system also transports nutrients that is produced in the leaves by photosynthesis to other parts of the plant.

In algae, water and nutrient uptake is directly over the thallus (body), making roots superfluous. Some algae have rootlike hapteres, but these only function as attachment to substrate.
Photosynthesis in algae occurs over the entire thallus, so there is no need for any advanced nutrient transportation system, beyond certain connectivity between nearby cells.

The flying insects evolved on dry land, wich favoured plants with flowers. Some land plants also use the wind to spread their pollen.

Algar do not have flowers. Instead, they reproduce with spores, gametes (egg and sperm) or in some cases cloning (a part of the algae is torn off then re-attaches to another surface, thus forming a new individual).

Some plants have returned to a life in water, but have kept their roots, vascular tissue and flowers. Amongst these are sea grass, water lilies and European bur weed.

The Baltic Sea

The Baltic Sea is unique because of its low salinity. In the northernmost parts, the water is almost considered freshwater, whilst the salinity at the entrance/exit in Öresund lies around 15 psu (practical salinity unit, roughly equal to ‰ and denotes salinity). The low salinity of the Baltic Sea makes it a stressful environment for both freshwater and marine species, both groups lives on the very edge of their tolerance of either too high or too low salinity.

Salinity and Fucus distribution in the Baltic Sea

Salinity and Fucus distribution in the Baltic Sea

The map show surface water salinity and the distribution of the three species from the genus Fucus that can be found in the baltic Sea.
Green denotes bladderwrack (Fucus vesiculosus)
Orange denotes narrow wrack (Fucus radicans)
Yellow denotes serrated wrack (Fucus serratus) Note that for this species only the distribution along the Swedish coasline is shown, since we have not found any data on its distribution in any other countries along the Baltic Sea.

More information will follow.