Red algal exotics on North Sea coasts

A total of ten red seaweed species are recognized as introduced into the No rth Sea from other parts of the world. These are Asparagopsis armata and Bo nnemaisonia hamifera (Bonnemaisoniales), Grateloupia doryphora (Halymeniale s), Antithamnionella spirographidis, Antithamnionella ternifolia, Anotrichi um furcellatum, Dasya baillouviana, (sic)Dasysiphonia sp., Polysiphonia har veyi and Polysiphonia senticulosa (Ceramiales). The oldest of these is B. h amifera, introduced prior to 1890, while the most recent, (sic)Dasysiphonia sp., was first found in 1994 and still requires taxonomic investigation. A variety of distribution patterns is seen, with geographical ranges varying from general within the North Sea to very restricted. The diversity of int roduced red algae on eastern coasts of the North. Sea is much greater than in the west. The most likely explanation for this pattern is that French co asts were the initial site of introduction for many of the seaweeds, which were then distributed northwards by the residual surface currents. Their in creasing success in the Netherlands has probably been promoted by the drast ically changed local hydrodynamic conditions which have also permitted the recent introduction of many native European species. Of the biological feat ures of species that may favour their success as introductions, clonal vege tative propagation, often with specialized propagules or fragmentation mech anisms, is almost ubiquitous. Low-temperature tolerances can be inferred, b ut data are sparse. Many of the alien red algae in the North Sea contain an ti-grazing compounds such as bromophenols, which may contribute to their in vasive potential by deterring grazing sufficiently to permit establishment of an inoculum.