Sometimes you are just in the right place at the right time to witness and record natural phenomena that are rarely seen. This happened one morning in early April at Spaniard Rocks, Rhossili Bay in Gower. I had photographed the dark olive green Egg Wrack on the rocks of the causeway last year and noticed the yellowy-green, raisin-like, fruiting bodies hanging down on short stalks. However, this spring, after just over a year on the plant, they were ripe and ready for their big moment.
Perhaps I should recap for a bit. Egg Wrack or Knotted Wrack, Ascophyllum nodosum (Linnaeus) La Jolie, belongs to the same group of brown seaweeds or Phaeophyceae as Bladder Wrack and Toothed Wrack but, unlike these other Fucoids, it has no midrib and it has male and female plants – it is dioecious.
Egg Wrack attaches to rocks around the mid tide level by means of a many-fingered holdfast or hapteron. A short rounded stalk develops from this into the flattish, irregularly serrated, frond along the sides of which are notches or axils every 2 to 5 cm. The axils are growing points from which either another ordinary branch of frond arises or a bunch of thin stalks about 1 to 5 cm long with an oval or ellipsoid fruiting body on the end: the receptacles. The receptacles about 1 to 2.5 cm long. On male plants they are said to be a golden yellow colour while on the female plant they are yellowish olive. In reality, it is a bit difficult to distinguish them on this basis. There is a shape difference too.
Each receptacle contains a large number of small superficial cavities called the conceptacles and each of these opens outwards through a minute pore or ostiole. The gametes or sex cells are produced in these special cavities. The gametes are produced by a type of cell division known as meiosis which results in cells with half the full complement of genetic material – they are haploid. The plant itself is diploid and has a full complement of chromosomal material.
The female gametes, the eggs, are produced by oogonia developing in the female conceptacles on the female Egg Wrack plant. The male gametes, antherozoids or sperm, are produced by antheridia in the male conceptacles on the male plant.
Sexual reproduction in Egg Wrack and other Fucoids is oogamous, meaning that the eggs are released from the parent plant before fertilisation. Fertilisation takes place in the surrounding seawater. The eggs and the antherozoids are released into the water at the same time. The activity is synchronised. The antherozoids can swim. They have two minute flagellae attached to the side of the cell that move the cell along by a whip-like action. The egg is non-motile but has enough buoyancy to remain suspended in the water rather than sinking to the sea bed.
Release of the gametes is triggered by the number of daylight hours and exposure of the ripe receptacles to air overnight. The first gametes are released in April and the last in June. After fertilisation, the zygote settles within 10 days. The spent receptacles are shed in June.
Once the gametes are released, the antherozoids are attracted to the egg cell by a chemical it contains and they swim along a gradient of the chemical concentration in the water until huge masses of antherozoids surround each egg and spin it around in their attempts to penetrate it. Finally, one male gamete fertilises the egg, joining the two haploid chromosomal halves, and produces a zygote, that settles down on a rock surface into a new diploid plant.
What I was fortunate enough to witness was the moment immediately prior to the synchronised release of the reproductive products from the mature specimens of Egg Wrack (plants reach maturity over a five year period and can live from 10 to 20 years). The receptacles take from 12 to 14 months to ripen. The photographs here show the small swollen yellow conceptacles glistening on the surface of the receptacles – resembling minute yellow blisters. These burst when touched. The yellow liquid containing the gametes formed a wet coating over some of the plants and stained my hands like nicotin where I had handled the fronds. The incoming tide was just seconds away as I took these shots, my outstreched feet behind me getting wet as I positioned myself low enough to capture the moment that the pustules started to burst.
Click here for more pictures and posts on Egg Wrack in Jessica’s Nature Blog.
More Egg Wrack information online at the Marine Life Information Network
Lowson’s Textbook of Botany (Revised by Simon, Dormer, and Hartshorne), 1977, University Tutorial Press, London, ISBN 0 7231 0614 2, pp 513-516.
British Seaweeds, Carola I Dickinson, The Kew Series, Eyre & Spottiswoode, 1963, Catalogue no, 6/2372/1, pp 108-109.
A handbook of British Seaweeds, Lily Newton, The Trustees of the British Museum, London, 1931, pp 219 – 222.
Revision of a post first published 17 May 2009
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