Abstract
The morphological relationship between the glomeriferous and flabelloid aspects of Tydemania expeditionis, a rarely collected siphonous green algae, is presented and discussed in relation to the ecological niche it occupies and the phylogenic position of the genus within the Codiaceae. Using SCUBA techniques, the alga was observed and extensively collected at depths in the 40-65-m range. It is suggested that seasonal mean in situ light intensities exceeding 0.03 g cal/cm2/min inhibit the development of dense stands of the alga, and that Tydemania is a primitive member of the Codiaceae.

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