Reproductive development and classification of the red algal genus Ceratodictyon (Rhodymeniales, Rhodophyta)

Abstract

Abstract Ceratodictyon spongiosum Zanardini (Rhodymeniales, Rhodophyta) is a widely distributed tropical Indo-Pacific species, the thalli consisting of a reticulate network of algal axes surrounded and covered by a sponge symbiont. Several important reproductive features of the algal partner are described in detail for the first time. Procarp organization, early gonimoblast and fusion cell development, mature cystocarp structure, and cruciately divided tetrasporangia, as well as the occasional presence of vesicular (‘gland’) cells on medullary filaments, relate the alga to the family Rhodymeniaceae of the order Rhodymeniales, rather than to the Gracilariaceae or the Gigartinales as has been previously considered. It is argued that Ceratodictyon and the non-sponge-associated Gelidiopsis are closely related genera that should be maintained as distinct taxa, rather than combined into the single genus Ceratodictyon as has been advocated recently.