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Seaweeds of Kenya

Updated: Oct 23, 2019

Published in Swara Magazine Sept/Oct 1990


Very little has been published about the seaweeds of the Kenyan Coast, but they are fascinating to study – even for an amateur

To most people this is something that rots on otherwise pristine beaches; or is carefully sorted out of the salad in Chinese restaurants and left on the side of the plate; and the Welsh fry it, don't they? Apart from that most people manage to spend years of their lives without ever giving it a passing thought. Yet without it there would be no seafood; no oysters, blue marlin, hammerhead sharks, octopus or Moses sole, no life at all in the sea. The seaweeds absorb light rays from the sun. They use the energy to split water molecules and join the hydrogen to carbon dioxide from the sea to form sugars, then forming the millions of complex molecules which make up a living plant. This chemical energy is passed along the food chains to every marine animal from sponge to dolphin. Even the piles, of brown, rotting and reeking seaweed that are thrown up by the high spring tides support a dynamic community of animals. Just turn a damp pile over and thousands of tiny sand fleas will spring out like popcorn from a hot frying pan. These are mainly amphipods, a group of crustaceans with many thousands of species, which feed on the dead plants. They have powerful jumping legs and effective nipping claws, which can make their presence felt on delicate human skin.

Many of the species living here and burrowing in the sand beneath are probably unknown to science — as yet unnamed. Ghost crabs scavenge at night for anything they can find, including seaweed. Fiddler crabs are found in the mangrove swamps, waving their enormous pink claws at their rivals. They eat sand and mud mixed with particles of decaying seaweed. This cocktail is whisked around by special hairs in the mouth and separated, the seaweed swallowed, and the sand spat out. Nothing is wasted, marine fungi and bacteria complete the decay process, ensuring that valuable minerals are recycled for new growth, maintaining a healthy reef.

The lagoons behind the reefs are shallow, calm and free from large predators. It is here that many fish lay their eggs and the young begin their development; even many of the normally deep-water oceanic fish do this. Protection and food is provided by dense stands of Sargassum seaweeds and extensive seagrass meadows. The latter are not seaweeds but flowering plants (although not, confusingly, grasses). They have true roots, stems, leaves and flowers; pollen is carried by the currents to other flowers, rather like wind carries pollen on land, and seeds are formed. Seaweeds have none of these features, although the structures that hold some of them to the rock, called the stipe and holdfast, can look superficially stem- and root-like. Seaweeds reproduce by means of spores. Many of the small fish do not eat the large fleshy seaweeds directly, but nibble at the microscopic plants and animals that grow on the surface of their leaves (called epiphytes). Thus, the seaweed provides shelter for the fish, while the fish help by reducing fouling and allowing the sunlight to reach the plants. As the lagoons empty of water during low tides small stagnant pools can be left, which quickly heat up in the sun. This is doubly dangerous for the animals which live or have been stranded there as not only can the heat be lethal but also warm water holds little oxygen. Seaweeds in the pools prevent asphyxiation as they produce large amounts of oxygen; it is this process that made the world habitable thousands of millions of years ago.

The earth's atmosphere was moulded by seaweeds, especially the microscopic types which make up the phytoplankton. The Kenyan sea may look transparent — visibilities of 30 metres or more are common — yet the open water has a luxuriant profusion of tiny living organisms. These include marine bacteria and blue-green algae, which are primitive cells similar to those of the very first living things in the primeval soup of 3.5 billion years ago; they even lack a true nucleus. The most significant planktonic plants are the diatoms, microscopic, box-like and usually golden brown. The single cell is made from two interlocking sections like a pill-box, fashioned from silica and finely etched with lines and patterns. Despite their minute size they produce more organic matter (therefore food) than any other marine plant; most marine food chains depend on them.

The next most prolific type of plants are the dinoflagellates, single celled organisms which have two fine hairs (flagellae) to give them motility. The dinoflagellates are well known for their luminescence; when disturbed by waves or the movements of a swimmer a magical iridescent bluish-green radiance appears like stars in a galaxy. Others can form 'red tides' when they become so multitudinous as to colour vast areas of the sea. The Red Sea was probably named because of this phenomenon.Unfortunately, red tides are usually poisonous to fish and many die. Some fish may become poisonous on eating another dinoflagellate. This is known as `ciguatera' poisoning and can be lethal to humans.

Many reef organisms are filter feeders. They have a selection of fine nets and mucous traps in which to sieve out plank-tonic organisms from the sea water. They are nearly all fixed to the reef itself and often nearly buried within it, like the giant clams Tridacna sp, and the boring bivalves Lithophaga sp, which inhabit living coral colonies. Most other reef herbivores feed on the algal turf, which is a richly varied assemblage of filamentous seaweeds coating every available space on the reef surface. The turnover rate is stupendous: it grows more quickly than almost any other plant community on earth, probably only exceeded by the plant plankton. In heavily grazed areas it will appear to the snorkeller as a green filmy layer over the rock, with occasional thicker patches in crevices and on the tips of dead branched corals. It is not very dramatic, but essential to the well-being of sea urchins such as the black long-spined hat-pin urchins Diadema sp, the boring Echinometra matthaei, the heavy red-brown slate-pencil urchinsHeterocentrotus and the deadly Toxopneustes, as well as a multitude of molluscs such as top-shells, chitons and limpets, and many fish. As fast as it grows it is grazed down by specialised rasping teeth. If the balance is altered by, for example, removal of grazers, the destruction of the corals or an increase in the nutrients dissolved in the sea, then the filamentous turf thickens and traps sediments, and the rocks become covered by a dense, deep green coat. This can lead to the destruction of the 'natural' reef through smothering with sediment and the production of the poisonous gas hydrogen sulphide deep within the turf. If grazing remains low the larger fleshy seaweeds are able to grow and thus it turns into a seaweed forest.

Another important seaweed type does not resemble a seaweed at all. The coralline algae are hard, pink, purple or grey and cement like. Each cell is encased in a layer of rock-hard calcium carbonate with a rather unstable surface. These seaweeds grow where the waves pound most powerfully, on the seaward side of the reef. They grow out against the continual onslaught as aka. well as cementing loose rubble and sand into place in reef crevices. Without them the reef would crumble and only surfers would gain as ocean rollers would crash directly on to the beaches. Coralline algae cannot compete with the rapidly growing algal turf and actually need to be regularly grazed otherwise they would quickly be overgrown and would die. Most of the easily visible seaweeds which live in these different parts of the reef belong to four groups, which are conveniently distinguished by their colour: blue-greens, greens, browns and reds. It is worth considering each group separately as each has special features that relate to their ecology and to the part they play within the reef community.

The green colour of the green algae is due to the presence of chlorophyll pigments identical to those of land plants. It is from green algae that land plants first evolved. This colour is suitable for absorbing red and blue light, both of which are found in sunlight. Water, however, absorbs red light very rapidly and a few metres below the surface everything appears blue or green, as no red light penetrates. Colour photography requires a flash gun if reds and yellows are to be seen. Clearly, green algae are shallow water seaweeds. They are able to make the best use of sunlight and grow more efficiently than other algae. Thus, it is in the shallows that you will find the bright green sheets of sea lettuce Ulva sp. One species, Ulva pertusa, has continuous sheets while others are delicately perforated and net-like (eg Ulva reticulata).

As they are much sought after by grazing fish they are often found at the water's edge at low tide. Another seaweed with the same preferences is Enteromorpha. The bright green sheets form hollow tubes, sometimes appearing filamentous, but others are more like deflated balloons (eg Enteromorpha flexuosa). Both Ulva and Enteromorphaare highly resistant to changes in salinity and temperature and can easily cope with estuarine conditions. The seaweed Chaetomorpha crassa is common in rock pools. It resembles a tangled mass of thick, bright green nylon fishing line. The ball is made from a single filament, which may be up to one metre long, although unravelling it would be problematic. The green seaweed that has exerted the greatest effect on the Kenyan coast, apart from merely providing food, is undoubtedly Halimeda. There are several species, but they all have rather oval fleshy leaves impregnated with calcium carbonate. In the lagoons one species forms thick greyish green mats ten or more centimetres deep, which crunch as you walk over them. This is the heavily calcified Halimeda opuntia (the name opuntia alludes to its shape, which is similar to the lobes of the prickly pear cactus Opuntia sp). The calcification is, presumably, to make them less attractive to herbivores. It would make a gritty mouthful compared to the succulent sea lettuce. Even if they are eaten the calcium carbonate is not digested and passes out as a fine white sand. Similarly, the dead lobes soon bleach white and if you bury your hands deep in the rock pool sand near a Halimeda bed you will find it is all dead skeletons. In fact, the so-called `white coral sands' should be 'Halimeda sands’, but it doesn't have the same ring to it.

The seaweed Dictyosphaeria, is also found here. Its dark green, hollow rounded domes devastated the corals of a Hawaiian bay, leaving the reefs dead and poisonous. Its growth was stimulated by increased nutrients from the town's sewage and fertiliser run-off. The tragic result should be remembered as a warning of what can happen. The brown seaweeds are often more robust and tend to be larger than the greens. Their colour is designed for intermediate depths as the yellow brown pigment that they have in addition to chlorophyll makes the absorbtion of blue light more efficient while not totally cutting out the red. Typically, they grow from the low tide mark and in the deeper lagoon pools and channels.

The largest type is Sargassum, which can form strikingly beautiful stands, especially with the sunlight filtering through the yellowish holly-like 'leaves'. Sargassum is related to the enormous masses of floating seaweeds that circulate in the doldrums of the Sargasso Sea, and to the temperate kelps, which can grow to hundreds of metres in length. Many brown seaweeds, including some Sargassum species, have air bladders in their fronds to hold them vertically above their holdfasts on the rock. This is for the same reason that land plants evolved woody stems, so that they can compete successfully for sunlight.

Turbinaria is a very distinctive seaweed with tetrahedral lobes, which are often hollow. It is found in areas where strong currents might occur, so its heavy structure allows it to hang on despite buffeting. Just try and pull a piece off and you will appreciate its powerful grip.

A brown seaweed that stands out on the edges of the rock pools is Padina. The rounded lobes of Padina boryana, with its concentric tracery of creams and browns and a slight iridescence, can be delightful. Other species are rather more gross, but all have light bands on the leaves made by crystals of calcium carbonate. This is one of the few calcified browns. Brown algae such as Ectocarpus and Giffordiaare an important constituent of the filamentous algal turf which covers every available space in lagoons and on reefs. Finally, the red seaweeds. Their colour is the next best thing to black for effective light absorbtion in the deep sea. Many nocturnal and deep-sea creatures are red as the lack of red light at depth makes them nearly invisible. Red pigments will absorb blue light very efficiently but reflect red and yellow. Thus, the red algae are adapted to living at depth. The deepest known living plant is a red seaweed: one of the coralline algae found growing at 265 metres off the Bahamas in what is near total darkness. This ability does not confine red seaweeds to the depths — a profusion will be found in rock pools and reef channels — but it means they can grow under overhangs and in dimly lit caves where other seaweeds would perish. There are probably more species of reds than browns and greens put together in the Kenyan shallows so identification can be frustrating. Yet when you concentrate on the smaller scale and linger around rocks, you will find that the red seaweeds can be exquisite; only under the microscope can their true beauty be fully appreciated.

They range from the gelatinous Laurenciato the fragile and brittle articulated corallines such as Amphiroa fragilissima. None are very large; some live as microscopic epiphytes on brown seaweeds. Some are rock-like, such as Porolithon onkodes, others are leafy like Amansia glomerata, and yet others finely filamentous like Ceramium. But a list is of little use. To appreciate seaweeds, it is essential to go out and look at them in their natural habitat; there is no need to identify them, although the effort involved does concentrate the mind. Not only do seaweeds provide food for the reef ecosystem, they also, as noted above, cement and build the reef, which maintains a complex community of other organisms.

Several seaweeds have even closer relationships with animals: they live within the cells of the animal, obtaining protection and minerals and giving back organic food. This type of symbiotic relationship is common in the reef environment where a small advantage over others can mean life, a small disadvantage, extinction. All the following contain single-celled green algae: hard corals (which contain a type of dinoflagellate), soft corals, giant clams, Convoluta worms (which are found on damp sand at low tide as a green film, which vanishes if you rub the surface) and the sponge Spongocladia vaucheriaeformis (often coating rocks with green, rubbery and spongy sheets).

Seaweeds do have a number of commercial applications, although I am not aware of any systematic exploitation in Kenya. Apart from their use as a food in their natural state, thickening and gelling agents are extracted from seaweeds. The most important are alginates, which are used in soft cheeses and soft ice-cream, and agar, which is also used in ice-cream, but has a much more important application in the preparation of microbiological media for use in laboratories to grow bacteria and fungi. These substances have no food value. Halogens (iodine and bromine) are concentrated by seaweeds from the sea-water and are easily extracted. Seaweeds are regularly spread on to fields as a mulch in some countries and to contribute to the soil humus as they decay; similarly, coralline algae, dredged from the sea-bed as marl, have been used to lime fields. It' is not easy for a layman to find literature on the identification of tropical seaweeds; much of the work is published in scientific journals. The best guide is that by Dr Erik Jaasund (then at the University of Tromsoe, Norway) entitled Intertidal Seaweeds in Tanzania — A Field Guide. In his preface he promises a second and more complete edition, but sadly, he died of Parkinson's disease before this was accomplished. There is no doubt that the study of seaweeds can be very rewarding and the relative paucity of information on Kenyan species would make their study doubly so.


E. Jaasund Intertidal seaweeds in Tanzania — A Field Guide University of Tromsoe, Norway.

W. E and F. M. Isaac 'Marine Botany of the Kenya Coast'. Journal of the East African Natural History Society and National Museum, Vol. XXVII, No. 1 (116) Jan 1968.

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