Aspects of the life history and ecology of Porphyra linearis (Bangiales, Rhodophyceae) in nature

The life history of P. linearis Grev. in nature was studied for 2 years at Sandy Cove, Halifax County, Nova Scotia, Conchospores of this winter alga were present in the supralittoral zone as early as August, although thalli were not visually apparent until mid-October to late October. The first major deposition of conchospores coincided approximately with decreasing sea temperature at 13–14 °C. Spores were deposited throughout the winter until March, when sporulation presumably declined and few new thalli appeared on the study site. A few conchospores were retrieved from the supralittoral zone in April, when extant thalli were bleached or moribund, and as late as June, after the population had died. Conchocelis of P. linearis was not found in the supralittoral habitat of the leafy phase, but several specimens were located at a depth of 9 m in the sublittoral zone near the study site. Leafy thalli became fertile 3–5 weeks after their appearance on the site, and were largely dioecious; broader thalli in the upper littoral zone were frequently monoecious, and displayed α- and β-spores simultaneously. Development of the population was enhanced by thin films of moisture but not by standing water such as tide pools; nor did young P. linearis thrive in Ulothrix flacca – Urospora penicilliformis associations. Death of leafy thalli in spring appeared to result from an interaction of increasing temperature and photoperiod. Alternation of the leafy phase with conchoecelis is obligatory in P. linearis; vegetative reproduction by neutral spores was not observed, and thalli did not perennate via the remnants of basal portions which sometimes persisted through the summer. Intergradation of P. linearis with P. umbilicalis is discussed.