No abstract

: Algal and fauna1 assemblages living on soft bottom sediments were studied in the Bay of Brest, France, using quantitative descriptions of the distribution, diversity and biomass of epibenthic species. Similar classification and ordination analyses were used on floral and animal data. Dense assemblages of macrophytes, mainly Rhodophyceae, were found to grow on sandy mud sediments. Sediments were essentially covered by maerl beds of the calcareous red alga Lithothamnion corallioides and various shell substrates. Five maln algal assemblages extended along a west-east decreasing gradient of hydrodynamic activity (i.e. tidal currents, waves and turbidity). The high frequency of storms during the year is the main feature which disturbs both flora and epifauna. Interactions between flora and fauna were either direct, e.g. grazing and spatial competition, or indirect, most of the substrata occupied by macrophytes being shells of dead bivalves and gastropods. Moreover attachment of algae was largely dependent on population dynamics of shell species living in the area. As a consequence, in the most disturbed area, the macrophytes found were opportunistic species (Polysiphonia fibrillosa and Polysiphonia urceolata). In areas where the assemblage was unstructured, a fact which prevented the development of the herbivorous species, the animal assemblage was then dominated by suspension feeders (FicuLina ficus and Phallusia marnmillata) which competed for space with the macrophytes. In the least disturbed area, the herbivorous species dominated the fauna assemblage while flora assemblage had a high level of organization. However, in those places eutrophication and high turbidity prevented the establishment of many algal species, causing the ubiquitous ones to domInate (Ulva sp., Ceramium rubrum). Between these extreme areas, the living Lithotharnnion beds were more developed and were defined as a characteristic assemblage of diversified Rhodophyceae species such as Chyloclad~a verticillata, Halarachnion ligulaturn, Crytopleura ramosa. Finally, the role of wave disturbance as the main factor controlling algal assemblages of shallow soft-bottom sedlments is discussed.

Lithothamnion corallioides Crouan et Crouan (Rhodophyta, Corallinales) is the main constituent of the maerl beds of the Bay of Brest (Atlantic coast of western Brittany) . Its growth rate was measured monthly in situ during one year . Growth rates were obtained by an adaptation of the buoyant weight technique . The highest daily growth rate was observed in July and reached 0 .26 % d -' (S .D. = 0 .06), when expressed as the increase of calcium carbonate weight . The average daily growth rate was 0 .12% d -' (S .D. = 0 .04) for a period of 275 days (summer and autumn 1988, winter 1989) . Using this preliminary data, the calcium carbonate accretion rate can be estimated provisionally : 876 gm-2 year -', a rate much lower than that of tropical reef coralline algae, but higher than that of Lithophyllum incrustans, the well known temperate European reef-builder . N 1 1 D J

Since its first introduction into the Atlantic for farming purposes in 1983, the brown alga Undaria pinnatifida has been found at several sites along the European coasts . The present study deals with an ecological experiment that was performed at the island of Ushant (Brittany, France) in order to check the ability of Undaria to recruit to a rocky substratum in situ . On the one hand, the authors were unsuccessful in getting Undaria to colonize a previously denuded area of shore, which was subsequently colonized by other native species, a result that may speak for a competition between Undaria and the local kelps . On the other hand, evidence is presented that Undaria pinnatifida has become a member of the Atlantic flora for good . The sporophytes can be found growing from low water mark of neap tide (+ 1 .5 m) down to at least -15 m below lower water mark of spring tide . Its propagation is thought to occur in at least two ways : either step by step from cultivation sites along the coasts, or from the hulls of ships sailing from harbour to harbour . The preference of Undaria for settlement is on artificial structures .

Seven samples of Chondrus crispus Stackhouse, representing widely contrasting forms from both sides of the North Atlantic, were compared by restriction digestion of their plastid DNA. The similar banding patterns confirmed that the seven forms were conspecific and distinct from Chondrus ocellatus Holmes f. ocellatus from Japan, used as an outgroup. Nucleotide sequences of the internal transcribed spacers (ITS1 and ITS21 and the intervening 5.8s rRNA gene of the nuclear rRNA operon were investigated as a potential indicator of genetic divergence among morphological variants of C. crispus. The combined ITS regions were relatively short in Chondrus (between 719 and 731 base pairs [bpl in C. crispus and 724 bp in C. ocellatus f. ocellatus), and the sequence of the 5.8s rDNA fragment (152 bp) was identical in both species. In the aligned ITS regions, there were 0-18 base pair differences (0-2.18% divergence) in paitwise comparisons of the seven forms of C. crispus but no consistent pattern of variation according to gross morphology or geographic origin. However, the ITS sequence differed at 41-54 sites (6.22-7.56%) between C. crispus and C. ocellatus f. ocellatus, again illustrating the genetic distinctiveness of the latter species.