Gelidiella calcicolasp. nov. (Rhodophyta) from the British Isles and Northern France

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Abstract
Gelidiella calcicola sp. nov. (Gelidiales, Gelidiaceae) is described from plants growing on loose-lying subtidal coralline algae and shells on southern, western and northern shores of the British Isles, and from Roscoff in Brittany, France. The species forms narrow, creeping axes attached at intervals by peg-like rhizoidal holdfasts. Irregularly pinnate branches arise at the points of attachment, arc downwards, and become attached and pinnately branched. No erect axes are formed; in all other described species of the genus Gelidiella at least some erect axes are formed from the creeping base. Internal rhizines, considered to be characteristic of all the genera of the Gelidiales except Gelidiella, are present in the medulla, but only at the attachment points. Surface cortical cells are large and usually arranged in chevron-like transverse rows. Tetrasporangia, formed only in the winter months, occur in lateral stichidia in chevron-like rows of 8–10 per row; they are spherical and apparently tetrahedrally or irregularly divided. Gametangial and carposporangial plants have not been found; these are known in only one species of the genus. Attempts to induce reproduction in vegetatively isolated plants in culture by subjecting them to a wide range of temperature and photoperiodic regimes in various enriched sea-water media strengths were unsuccessful. The new species is compared with Gelidium pusillum from the British Isles, and with the 21 previously described species of Gelidiella. The status of the family Gelidiellaceae is assessed and it is concluded that it should be merged with the Gelidiaceae. Some doubts are expressed as to the validity of the criteria used to delimit the genus Gelidiella.
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