Reinstatement of the genus Mastocarpus Kutzing (Rhodophyta)

Morphological, life history and biochemical characters are used to show why the genus Mastocarpus should be reinstated for four widely distributed species of Gigartina (Gigartinaceae). Mastocarpus species have channelled thalli on which the female reproductive structures and carposporophytes are formed on specially formed papillate structures, have heteromorphic-type (and associated direct-type) life histories involving an alternation with tetrasporophytes previously referred to the genus Petrocelis, and tetrasporangia are formed singly. Gigartina species do not have specially formed papillae, have isomorphic gametophytes and sporophytes, and tetrasporangia are produced in branched or unbranched chains. Mastocarpus species share many features in common with the Phyllophoraceae and Gigartinaceae but it is proposed that the Petrocelidaceae Denizot be adopted for this genus on the basis of the character of the formation of tetrasporangia singly rather than in chains.