Spinning eggs for Easter

I am going to render a normal conversation that often happens when I talk about seaweed and what I do.

– So, you work with seaweed. Nice! What do you do with it?
– Well, among other things I try to cross different species with one another in order to understand how speciation occurs.
-That sounds interesting. When does the seaweed bloom, then? Or….does it have flowers?
-Nope, it has eggs and sperm just like us. Bladderwrack and narrow wrack have male and female plants and actually have an almost identical lifecycle to humans.
-It has eggs and sperm?! But…is it an animal, then?

Suddenly you realize that what you learned during biology class in school was just a rough cut, simplified picture of reality. Nature and evolution is so much more than that, with more imagination and concepts than we humans are able to name.

I think it is fantastic that algae, some of the planet’s first living organisms, have used eggs and sperm for a long time. Maybe longer even than humans have been around. A flick on the nose at us when we think we are evolutionarily advanced.

Here’s a video of how eggs from bladderwrack start spinning by all the sperm swimming around them, hoping to fertilize. Beautiful!


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.