Plant size and reproductive phenology of six species of rhodophyta in subtidal isle of man

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Abstract
Following a quantitative study of the reproductive phenology of some subtidal red algae, the aim of the investigation was to separate the influence of plant size from the direct effect of season on their reproduction. Quarterly samples were taken from about 2 m below lowest astronomical tide (LAT) on Port Erin breakwater and, in each species, thallus length was measured before noting fertility. In most of the species there were fewer plants near their maximum length in the spring than in the summer or autumn. In epilithic Plocamium cartilagineum, Cryptopleura ramosa and Callophyllis laciniata fertility in classes of shorter plants increased from a spring minimum to an autumn maximum. High fertility in autumn was therefore only partly due to the greater abundance of large plants at that time. In Plocamium the largest plants were always fertile. In epiphytic Palmaria palmata, Membranoptera alata and Phycodrys rubens peak reproduction occurred in spring and all length classes showed reduced fertility at other times. Quarterly samples of Plocamium and Callophyllis were taken from 1–17 m below LAT at Spanish Head. Below the edge of the Laminaria forest Plocamium showed highest fertility in all length classes early in the year, the reverse of the shallower water pattern. In deeper water, the proportion of gametophytes was about three times that in shallower water. Callophyllis showed lower fertility in deeper water. Gametophytes of Plocamium, Membranoptera and Phycodrys were significantly shorter than tetrasporophytes, but the size overlap would normally obscure the difference.