The Sex Life of Brown Seaweeds

The arrival of spring brings a sudden burst of reproductive growth on the seashore intertidal zone, just as it does in woodlands. The big difference is that on the seashore it's algae - seaweeds - that are switching into reproductive mode, not flowering plants like bluebells. The picture above shows chanelled wrack Pelvetia canaliculata on the shore at Low Newton on the Northumberland coast and ....



.... this is knotted wrack Ascophyllum nodosum. The swollen tips of the channelled wrack and those yellowish egg-shaped objects on the knotted wrack contain the reproductive structures.

Brown seaweeds in the genus Fucus are common in the intertidal zone. Two species are visible here - saw wrack Fucus serratus with a saw-tooth edge to the fronds and bladder wrack F. vesiculosus with smooth frond edges and paired flotation bladders. In spring they make rapid new growth and enter their reproductive phase, producing swollen receptacles at the end of the fronds

The receptacles are covered in large numbers of small swellings called conceptacles, each of which opens via a minute pore called an ostiole. 

This is a transverse section of the conceptacle of a brown seaweed Fucus sp., viewed under a fluorescence microscope. It has been stained with a fluorescent dye called anilino-naphthalene-sulphonic acid to reveal the detail in its structure. 

This is a section through a receptacle showing two conceptacles developing inside. This is from a female conceptacle. The radiating, elongated filament-like structures are sterile hairs (paraphyses) and the club-shaped structures are oogonia, each of which produces eight eggs (oospheres)....

...... and here is an egg (oosphere) being liberated from an ostiole into the surrounding sea water. Inside the conceptacle some oogonia are still dividing to produce oospheres - you can see the cell walls forming (click on the picture for a larger image).

The clusters of small bright yellow structures that you can see above amongst the rounded oospheres are the antheridia that produce the antherozoids - this conceptacle is hermaphrodite, showing that it came from spiral wrack Fucus spiralis; saw wrack and bladder wrack have conceptacles that are either male or female.

When the conceptacles are mature eggs and vast numbers of swimming male cells (antherozoids) are liberated into the water of the rising tide - most prolifically during spring tides  - and at high water the eggs are fertilised, if they are lucky, and carried away by the falling tide. If they're luckier still the fertilised zygotes attach to a rock and develop into a new seaweed. 

 The pictures above and below show fertile fronds of a Fucus species attached to the harbour wall at St.Peter's marina at the mouth of the river Wear in Sunderland at high tide. That calm water will be seething with countless seaweed eggs and antherozoids, engaged in the frantic business of reproduction. 

You can find more detailed information on the structure and life cycle of seaweeds here  

For soothing movie of seaweeds swaying in the tide click here

The Seaweed Collector

Last week's visit to the Yorkshire coast fired my enthusiasm for the forthcoming rock-pooling season. These days, with motorised transport, a trip to the coast by motor vehicle is easy but it was the coming of the railways in Victorian Britain that made the pleasures of the seashore accessible to the majority of the population. That in turn triggered the publication of a host of seashore guides, advising visitors on what to look for in rock pools. 

This charming book, The Seaweed Collector by Shirley Hibberd, is a classic example. Then, as now, visitors to the seaside wanted to bring home a souvenir of their experiences of the wonders of the deep, as well as simply enjoying their day out; a pressed seaweed had considerable aesthetic appeal and was likely to be less smelly then a decaying winkle. 

Hibberd's book is full of information on collecting and identifying seaweeds and also includes some helpful sartorial advice for the would-be seashore naturalist: "To be suitably dressed is one of the most important matters in setting forth to gather seaweeds" declared the author. "Above all things it is desirable to protect the feet with stout boots, the so-called seaside boots of canvas or white leather being quite unsuitable for clambering over boulders to find rock-pools, or for wading in the marshy parts of a sandy shore where fresh water streams come down. Serviceable woollen garments that fit somewhat close are to be preferred to fashionable “fly away” things which the wind will sport with unkindly, and which are sure to get well wetted when you begin to stoop and “potter” about. A stout stick is a good friend, or, if that not be genteel enough, a strong umbrella on which you can lean as a stick, and the hooked handle of which may be serviceable to catch something the hand cannot possibly reach. I took a lesson from an old ratcatcher in my employ, who always went about the garden with a stout stick tipped with a broad chisel-like point. This he used for probing into holes and crannies when determining the “run” of a rat. With a heavy stick of this sort I have secured many a specimen with a sharp thrust that I must have laboured hard for on my knees with hammer and chisel, at the risk, perhaps, of a too intimate acquaintance with the rock-pool in which it grew".

Shirley Hibberd. The Seaweed Collector: A Handy Guide to the Marine Botanist. Groombridge and Son, London. 1872.

Shirley Hibberd, incidentally, was a man - James Shirley Hibberd (1825-1890). He was a very successful horticultural writer, author and editor of gardening magazines including Amateur Gardening, which is still published today.

It's easy to understand the urge to preserve seaweeds which, even at this time of year when they are in the early stages of spring growth, can be very attractive objects. Pressing them was the method favoured by Hibberd and by seaweed collectors ever since. The technique is simple: (1) Wash away all traces of animal marine life from the specimen with clean water (you might be amazed how much microscopic animal life lives amongst those fronds - see here and here, for example); (2) float the seaweed in a bowl of water above a submerged sheet of paper then raise the paper, teasing out the seaweed fronds as you go;(3) cover with a layer of muslin; (4) press between layers of newspaper until dry, when - with luck - you'll find that the natural agar in the seaweed will have stuck it to the paper. You can find a detailed on-line guide to seaweed pressing here.

I first became acquainted with the technique when I went on a school field club expedition to the Isle of Wight, in 1962. Here I am (in the middle) with a group of school mates, with an overambitious candidate for pressing - what looks like a very large specimen of kelp Laminaria saccharina.

 Amazingly, half a century later, I still have some of the specimens that we pressed when we took them back to our Youth Hostel at Whitwell  (I'm an inveterate hoarder of natural history artefacts). This one has lost its original label and it has faded but I think it's dulse Rhodymenia palmata. 

 Preserved between the pages of a book, this sea lettuce, Ulva lactuca has retained its colour remarkably well.

 Caragheen Chondrus crispus.

Podweed Halydris siliquosa.

It's inevitable that pressed specimens scarcely do justice to the beauty and delicacy of seaweeds swaying in a rock pool, so the outcome of pressing is always a bit disappointing. 

In Victorian times people produced albums of their pressed seaweeds collected on trips to the seaside and occasionally these turn up in antiquarian bookshops, although they are seldom well preserved unless they have been kept in very dry conditions in the proverbial cool, dark place. .... but in the middle of the 19th. century someone had already developed an excellent method of preserving the beauty of seaweeds for posterity.........

The technique was called Nature Printing and involved making impressions of specimens that had been dipped in ink. The process was developed and refined by an Austrian called Alois Auer and an English printer called Henry Bradbury went to Vienna to learn his techniques.

These involved placing the specimen between soft lead and hard steel plates which were pressed together, leaving an imprint of the specimen on the lead sheet which was then inked and used as a printing plate.

Using this technique Bradbury produced exquisite illustrations for natural history books, notably on ferns and seaweeds, which are masterpieces of his craft. The Nature Printed British Seaweeds by W.G. Johnstone and A. Croall, published in four volumes in 1859, contains detailed accounts of all the known seaweeds around the British Isles illustrated with exquisite nature printed images of each.

The images of the seaweeds were nature-printed on the lead plates which were then separately engraved with microscopic identification features, so the completion of these volumes was a major task.

The printed images are as highly detailed and as fresh today as on the day they were printed, unlike real pressed seaweeds which inevitably decay.

The most successful images are those of the delicate red and green seaweeds, which responded particularly well to the nature printing process.

There is still a thriving Nature Printing Society - check out their web page at http://www.natureprintingsociety.org/