Callithamnion Genus

Callithamnion is a genus of marine red algae (Rhodophyta) in the family Callithamniaceae, order Ceramiales. It was first described by Danish botanist Hans Christian Lyngbye in 1819, with Callithamnion corymbosum (Smith) Lyngbye designated as the type species.

Species in the genus are monaxial, meaning each filament grows from a single central axis, with thalli typically forming small, erect tufts up to about 10 cm tall. Branching is irregular, and the cells are multinucleate — a feature later used to distinguish Callithamnion from related genera. Gametophyte and sporophyte generations can be found throughout the year, though plants are usually only reproductively fertile in late summer and autumn.

The genus has a complicated taxonomic history. In 1861, Carl Nägeli transferred species lacking alternate branchlets out of Callithamnion into Antithamnion, Rhodochorton and Acrochaetium. Then in 1941, Genevieve Feldmann-Mazoyer erected the genus Aglaothamnion for species with uninucleate cells, zig-zag carpogonial branches and lobed carposporangia clusters, re-circumscribing Callithamnion in the process. Aglaothamnion is today sometimes treated as a synonym of Callithamnion, as the evidence for distinct evolutionary lineages between the two remains inconclusive.

Callithamnion has a cosmopolitan distribution, with species recorded from Europe (including Norway and Great Britain), Australia, the Americas (including Massachusetts, New York, North Carolina and Georgia), Newfoundland in Canada, Sri Lanka and South Africa.