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Sargassum is a seaweed (macroalgae) that belongs to the class Phaeophyceae (brown algae). It is commonly known as gulfweed and sea holly. Almost 150 species of the seaweed are known that commonly inhabit the tropical seas and oceans. The common species found in India include: S. tennerimum, S. cinereum, S. wightii and S. ilicifolium.
They are found free floating on the water bodies forming an island. This island houses various aquatic species, one example being, the Sargassum fish. The Sargasso sea in the Atlantic ocean is filled with floating masses of S. fluitans and S. natans and hence the water body is named after the genus.
Classification of Sargassum
|
Kingdom |
Chromista |
|---|---|
|
Phylum |
Phaeophyta |
|
Class |
Phaeophyceae |
|
Order |
Fucales |
|
Family |
Sargassaceae |
|
Genus |
Sargassum |
Sargassum Structure
This algae grows to a length of several metres. The plant body is dark green to brown in colour and consists of a holdfast, stipe and frond. The thallus is branched and has berry-like structures called pneumatocysts. The pneumatocysts are filled with oxygen and help in buoyancy of the plant.
The fronds are numerous, they are small and leaflike and have toothed edges. The plant is a diploid sporophyte. The stipe consists of a main axis and lateral branches. The plant is attached to the substratum by holdfast which is broad and discoid. Mature plants are however free-floating.
The main axis of the stipe has two types of branches: primary laterals and secondary laterals. The primary laterals arise from the main axis and have unlimited growth. The secondary laterals arise from the primary laterals and have limited growth.
Some of the lateral branches are repeatedly branched and are referred to as fertile branches. The fertile branches are known as receptacles. The receptacles bear flask-shaped cavities called conceptacles that contain the sex organs.
Refer: Algae – Definition, Characteristics, Types and Examples
Reproduction
Sargassum reproduces vegetatively and sexually. Asexual mode of reproduction is absent.
- Vegetative Reproduction: Two species of Sargassum, S. natans and S. fluitans are holopelagic, meaning they never attach to the seafloor and reproduce vegetatively by the means of fragmentation.
- Sexual Reproduction: Sexual reproduction in Sargassum is of oogamous type. Both the antheridium and oogonium are borne on different fleshy conceptacles. The conceptacles have multilayered walls that contain chromatophores.
- Antheridium: The inner layers of the conceptacle have fertile and branched paraphyses that contain the antheridium. Each antheridium is round, oval and double-layered and gives rise to 64 antherozoids after meiotic division.
The antherozoids are biflagellate and pear-shaped. The flagella arise sub-terminally and are unequal in size. The wall of the antheridium breaks and all the antherozoids are released in water.
- Oogonium: The oogonial initials are also formed by the inner layers of the female conceptacle. The oogonium is round and triple layered: outer exochite, middle mesochite and inner endochite.
The nucleus of the oogonia is diploid which divides by meiotic divisions to form 8 haploid nuclei. The haploid nuclei form 8 ova. Of these, 7 ova are aborted and one ovum goes further for fertilisation. The oogonia remains attached to the inner walls of the conceptacle by a mucilaginous stalk.
- Fertilisation: After fertilisation of the antherozoid and ovum, a zygote is formed. The oogonia remains attached to the conceptacle until the first transverse division happens in the zygote. Of the two cells formed in the zygote, the lower cell makes the attaching organs and the upper cell works as an apical cell and forms an adult plant.
- Life Cycle: The main plant body is a diploid sporophyte. The formation of antherozoids and ovum is the only representation of the haploid gametophyte stage in the life cycle of Sargassum.
Ecology
The pelagic Sargassum islands or mats in the Sargasso sea serve as a habitat for the development of the ecosystem because the sea does not have any land boundaries. The Sargassum mats are home to many endemic species that mimic the algae through specialised patterns to camouflage themselves. Over 100 different endemic species reside on these islands.
Additionally, 81 species of fish that live on the mats utilise them in their life cycles as well. Sea turtles live on the Sargassum mats and utilise their food and shelter until they are mature enough to reside by themselves. The European eel, the American conger eel and the American eel migrate to the Sargasso sea for spawning and laying eggs.
The Sargassum species is being wiped out due to human activities such as pollution, overfishing, boat traffic and trash.
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