This study deals with a population of Undaria pinnatifida (Harvey) Suringar, established in the coastal area around St Malo. In another study, during a monthly monitoring program, growth rate was estimated. Simultaneous morphological observations permitted the identification of two morphs, one resembling f. typica and the other f. distans. Anatomical characters of the two morphs are the same. The punched-holes method and the induced bifurcation of thalli permitted estimation of the contribution of the stipo-frondal region to overall growth. The structure of the midrib, pinnules and sporophylls are described and new data are given about the formation of Yendo cells. These latter serve as an early distinctive character between Undaria pinnatifida and Saccorhiza polyschides (Lightfoot) Batters which does not possess such cells. This criterion was used to identify juvenile blades in the study of monthly recruitment. Thallus morphology and bifurcation are discussed.
As a result of the deliberate introduction of Undaria pinnatifida (Harvey) Suringar into the Rance Estuary for aquaculture purposes, this kelp has now established itself naturally in the coastal area around St Malo. A population of U. pinnatifida in the Dinard area was subjected to a monthly monitoring programme, conducted in the lower eulittoral zone during low water of Spring tides. Tagging of thalli and punching of holes on the blades were methods used to estimate growth rate. Five parameters were measured: total length, stipe length, midrib width, sporophyll number and sporophyll width. During the 20 months of this study, 4 series of thalli (two per year) developed successively, in spring and autumn. Thalli appeared throughout the year, although very few were present during a period of three weeks, from the end of August to mid-September. Maximal daily growth rates occurred in March–April (21 mm/day) for spring thalli appearing from January to March, and in November (18 mm/day) for those appearing in October. The highest growth rate was observed both in the spring rise and autumnal fall of temperature and illumination. Thalli that appeared in autumn and winter produced the widest sporophylls. Thalli that appeared in spring and early summer gave rise to sporophylls less wide but more numerous. The maximal life-span noted in the field was 222 days (about 7.5 months); but most had a life-span of about 6 months. Maximal size observed was 1.90 m, with 1.470 g wet weight and 190 g dry weight.