Comparative study on reproduction by Phymatolithon calcareum (Pallas) Adey& McKibbin and Lithothamnion corallioides (P-& H-Crouan) P-& H-Crouan (Corallinales, Rhodophyta), and ne considerations on the definition of genera

The two Corallinales Phymatolithon calcareum and Lithothamnion corallioides are the main components of European maerl communities. A large majority of the thalli are free-living branches borne on juvenile crusts that are rare ly found. A detailed investigation of these juvenile stages has allowed the discovery of their reproductive features. Sexual reproduction of P. calcar eum, the type-species of the genus, is described here for the first time an d thus gives fundamental data for the generic definition. On the floor of t he female conceptacle, a set of carpogonial branches is formed. Each fertil ized carpogonium then fuses with the supporting cell of the carpogonial bra nch, which is also its auxiliary cell. A small fusion cell is thus formed, which produces upwards a small gonimoblast reduced to a short chain of carp osporangia. In a mature conceptacle, the surface of the fertile floor thus appears entirely covered with small gonimoblasts. In the male conceptacle, mature spermatangia produced on the floor are typically dendroid; those pro duced on the walls are of a more simple type. The organization of the asexu al conceptacle is briefly reviewed. The reproduction of L. corallioides is also newly described and compared with that previously reported for L. muel leri, the type-species of the genus.