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.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.
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.
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.