ECTOCARPUS - STRUCTURE OF VEGETATIVE BODY AND REPRODUCTION

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A. CLASSIFICATION:
Division – Phaeophyta
        Class – Isogeneratae
                 Order – Ectocarpales
                           Family – Ectocarpaceae
                                   Genus – Ectocarpus
Ectocarpus is the most primitive of all brown algae and comprises of many species. A few of them (about 6) have been reported to occur in fresh water. The marine species is world-wide in distribution but are found in abundance in colder seas of temperate and polar regions. The plants occur attached to the rocks and stones along coast in both littoral and sublittoral zones. Some species occur in shallow water on the sides of the tidal pools. Some species grow as epiphytes on other algae especially along with Fucuales and Laminariales (E. coniferus and E. breviarticulatus). E. fasciculatus is found to grow on the fins of certain fishes in Sweden.
About 16 species of Ectocarpus have been reported from India. Some of the common Indian species are – E. coniferus, E. indicus, E. arabicus, and E. dermonematus.
B. STRUCTURE OF THE VEGETATIVE BODY:
The vegetative body i.e., the thallus of Ectocarpus is a small septate, heterotrichous filament, differentiated into two parts – the prostrate or creeping portion and the erect or projecting portion.
The prostrate portion in epiphytic species is irregularly branched and sticks to the substratum by rhizoids. In aquatic species, the erect system arises from the prostrate part and waves freely in water. The branching of the thallus is always lateral. The branches arise just beneath the septa. Growth of the prostrate portion is apical but the growth of erect portion is intercalary.
In a filament, cells are joined end to end in a single series. The cells of Ectocarpus are semi-oval to rectangular in outline. It consists of two walls – outer wall composed of pectin and inner wall composed of cellulose. The characteristic gelatinous substance algin and fucoidan is also present in the cell wall of Ectocarpus.
Each cell is uninucleate and consists of few chromatophores and pyrenoid-like bodies. Mature cell also contains numerous large vacuoles and also some small vesicles. The pigments present are – fucoxanthin (brown pigment), chlorophyll-a, chlorophyll-b and β-carotene.
C. REPRODUCTION: Ectocarpus reproduces both by asexual and sexual methods of reproduction.
1. Asexual Reproduction:- Asexual reproduction takes place by the formation of biflagellate zoospores produce within the zoosporangia, borne singly within the tip of lateral branchlets of the sporophytic plant body. Zoosporangia are of two types – unilocular and multilocular zoosporangia.
(a) Unilocular Zoosporangia:- It is produced by the enlargement of terminal cell of lateral branchlet. The single nucleus of the terminal cell divides and redivides producing 32 or 64 nuclei. The first division of is meiotic, followed by repeated mitotic divisions. Next the cytoplasm of the young sporangium undergoes cleavage into uninucleate daughter protoplasts, each with a single chromatophore. Each daughter protoplast is then metamorphosed into haploid, uni-nucleate, pear-shaped, biflagellate zoospores (zoomeiospores).
At maturity the zoospores escape all at one time through a small opening formed at the distal end of the sporangial wall.
            (b) Multilocular Zoosporangia:- It is multi-chambered body developed on the terminal cell of the young branchlets of diploid sporophytic thallus. The terminal cell becomes long and enlarged and numerous chromatophores accumulate. It then undergoes repeated transverse and longitudinal divisions to form a series of small, cubical cells forming multilocular sporangia. The protoplast of each cell is then directly metamorphosed into single uni-nucleate, biflagellate, diploid zoospores (2n). 
At maturity the diploid zoospores escape all at one time through a small opening formed at the distal end of the sporangial wall.
Both haploid and diploid zoospores take a short period of rest after liberation. After the rest period is over they start germinating. The germination of haploid zoospores produces haploid gametophytic plant, while the germination of diploid zoospores produces diploid sporophytic Ectocarpus plant.
2. Sexual Reproduction:- Sexual reproduction is of both isogamous and anisogamous type. Isogamy occurs in homothallic species and anisogamy occurs in heterothallic species. Oogamy is absent. Gametes are produced in large numbers in elongated, multilocular bodies called gametangia. The gametangia are formed only on haploid gametophytic thallus, which developed as a result of germination of haploid zoospores formed in unilocular sporangium.
The gametangia resemble that of multilocular zoosporangia. They are either sessile or short stalked. Each cubical cell of the gametangium contains a single haploid nucleus, which metamorphoses into a single haploid gamete.
            The gametes are pear-shaped, biflagellate structure. The flagella are unequal in length. Liberation of gametes take place through the terminal pore formed in the gametangium wall.
(c) Fertilization:- Fusion takes place between two gametes of morphologically similar structure (isogamy) or gametes of morphologically dissimilar structure (anisogamy). In isogamy, the identical gametes are produced in the same gametangium of homothallic species (E. golifer). In anisogamy, the differential gametes are produced in different gametangia of heterothallic species (E. secundus).
            As a result of fusion, a diploid zygote(2n) is formed, which directly germinates into a new diploid sporophytic Ectocarpus plant which bears both unilocular and multilocular sporangia.
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