Photoperiodic and temperature responses in the reproduction of north-eastern Atlantic Gigartina acicularis (Rhodophyta: Gigartinales)

Abstract

Gigartina acicularis (Roth) Lamour., a predominantly intertidal red alga, has only rarely been found with reproductive structures in the British Isles and northern France. Elsewhere in the north-eastern Atlantic, reports of cystocarpic plants are largely from November to February while those of tetrasporangial plants are from July to October. Male and female plants formed gametangia only at daylengths of 12 h or less and at temperatures of 14–18°C. Photon exposures ≥ 1.5 mmol m^–2 of incandescent light, given in the middle of a 16 h dark period at 16°C, completely inhibited cystocarp formation, although some carpogonial branches were formed at up to 3.34 mmol m^–2. Five photoperiodic cycles of 8: 16 h at 16°C were the minimum necessary to induce the formation of carpogonial branches and cystocarps. Carpospores gave rise to plants which formed tetrasporangia at daylengths of 16, 12, 10 and 8 h at 16°C. The precise photoperiodic and temperature requirements for gametangial reproduction in G. acicularis result in gamete formation being limited to autumn in north-eastern Atlantic populations. It is suggested that, in the northern part of its range, populations of G. acicularis are largely maintained by vegetative propagation. As one goes further south the gametangial reproductive ‘window’ gradually enlarges due to higher ambient temperatures in the autumn. The paucity of records of tetrasporangial plants in the British Isles, however, needs further investigation. This is the first report of photoperiodic control of gametogenesis in the Rhodophyta and the response would appear to be true and absolute.