Sargassum
Sargassum:-
1. Classification:-
Division:- Phaeophyta
Class:- Phaeophyceae
Sub-class:- Cyclosporeae
Order:- Fucales
Family:- Sargassaceae
Genus:- Sargassum
2. Habit and Habitat:-
> Sargassum is a seaweed (macroalgae). 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.
> 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.
> The common species found in India include:
i. S. tennerimum
ii. S. cinereum
iii. S. wightii
iv. S. ilicifolium
3. Thallus 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.
4. Reproduction:- Sargassum reproduces vegetatively and sexually. Asexual mode of reproduction is absent.
a. 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.
b. 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.
i. 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.
ii. 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.
5. 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.