Experimental Ecology of Gigartina stellata (Rhodophyta) at Roscoff, France, Using an in situ Culture Method

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Abstract
Transplantation from laboratory to field conditions of early crustose stages of G. stellata grown on removable artificial substrates permitted monthly detailed observations on the complete development of this tuft-forming species in its natural habitat. Crust growth variations were investigated with respect to elevation from shore level, seasons and age. Growth rate of this crustose material responds sharply to even slight environmental variations. Recruitment of fronds on crusts shows 2 seasonal peaks, one in spring and another in autumn. Spring and autumn-recruited fronds are synchronously reproductive during the reproduction season (late May-early June) of the following year, but at different sizes. Only formation of cystocarps on blade apices definitely stops growth and leads to a gradual degeneration. The optimum growth level for cultured crusts and blades lies in infralittoral conditions, out of the low intertidal biomass optimum level of the wild population suggesting that factors other than physiological are involved in the limitation of the lower extension of this species in this area. Fastest growth for fronds of all ages occurs around May and in Sept.-Oct.; for young crusts, in July and Oct., and for older crusts in early Aug. and Oct. In late summer (Aug.) fronds of all ages show a definite arrest of growth, and young crusts show only a growth rate decrease. Several mechanisms may be involved in this summer growth limitation, but long exposure to high irradiance appears the major one. Older cultured crusts seem to be able to withstand much better this growth limitation and defer it until early Sept. Thus, crustose and erect thalli of this species reproducing through a heteromorphic life history, show different growth strategies.

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