Dasycladales

(botany) An order of lime-encrusted marine algae in the division Chlorophyta, characterized by a thallus composed of nonseptate, highly branched tubes.

An order of green algae (Chlorophyceae) in which the plant body (thallus) is composed of a nonseptate axis, attached by rhizoids and bearing whorls of branches. The walls utilize a partially crystalline mannan (polymer of mannose) as the skeletal component and usually are impregnated with aragonite, a form of calcium carbonate, causing these algae to be easily fossilized. Among green algae, only the Dasycladales have a fossil record sufficient to permit meaningful phylogenetic speculation. About 140 extinct genera have been described from limestones as old as the Ordovician, with three peaks of abundance and diversity—Carboniferous, Jurassic-Cretaceous, and Eocene. Only about 11 genera comprising 50 species are extant, with 3 of the genera extending from the Cretaceous. All are confined to warm marine waters except Batophora, which occurs as a relict in brackish sinkholes in New Mexico in addition to having a normal Caribbean distribution.

The extant Dasycladales are usually placed in a single family, Dasycladaceae, but those forms in which fertile and sterile whorls alternate or only one fertile whorl is produced may be segregated as the Acetabulariaceae (or Polyphysaceae). See also Algae; Chlorophyceae.