Life history of a generalised kelp

Reproductive cycle of a generalised kelp

The life histories of species of the Laminariales are composed of two morphologically distinct phases (heteromorphic), similar in pattern to the life history of pteridophytes (ferns and fern allies). The large, obvious plants are the sporophyte generation (diploid or 2N) which persist for various lengths of time and produce spores. These plants may be regarded as K-strategists since they take optimal advantage of the environmental resources and use a greater part of their annually fixed organic carbon to build up their thallus rather than allocate it to reproduction as the annual algae do. The sporophytes grow rapidly at certain times of the year, fixing enormous amounts of carbon from the coastal seawater while continually releasing DOM and POM.

Microscopic stages in the life history of a kelp

(from Bold & Wynne, 1978).

Within the spore producing tissue (see table below), sporangia develop (a. in figure above) and haploid spores (b.in figure above) are produced in vast numbers, but these represent a very small percentage of the annual biomass production of an individual plant. The zoospores (about 5 µm diameter, b. in figure above) may be transported some distance (at least 200 m) from the parent plant but lose their flagella after about 24 h (Sjøtun, 1995) and settle out onto any available substratum (c. in figure above). Male and female kelp spores must settle at a high density (within 1 mm of each other) if the maturing gametangial egg is to be fertilised (Reed, 1990).

The zoospores develop into the dioecious haploid gametophytes, male plants producing sperm and female plants producing eggs (d,e & f. in figure above). The gametophytes of all kelp species are microscopic and may consist of only a few cells before the plants become fertile (e & f. in figure above), which can occur in 10 days under optimal conditions. The male gametophyte plants develop antheridia which produce and release sperm; the female gametophyte plants develop oogonia in which the egg develops and on discharge this is fertilised and the young sporophytes start to grow in situ (f, g & h in figure above). Under sub-optimal conditions in the laboratory, the gametophytes may be held in a vegetative state for years and this ability may explain why kelp sporophytes are apparently able to "re-colonise" cleared areas so rapidly. On maturation of the gametophyte plants, motile antherozoids (sperm) will fertilise the sessile egg and a new sporophyte plant will start to grow from the zygote.

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References