Effects of density and substrate type on recruitment and growth of Pyropia torta (Rhodophyta) gametophytes

Does density affect recruitment and growth in the annual, blade phase of Pyropia sp., does self-thinning occur, and does substrate affect recruitment? These ques- tions were investigated in laboratory-cultured Pyropia torta, a naturally occurring species in Alaska with mari- culture potential. Three density levels were produced from conchospores. Measurements were made, initially at set- tlement and germination and, subsequently, at approxi- mately 3-week intervals, in 12 randomly selected cultures from each density level. Settled spores, germlings, or growing blades were counted microscopically and stand- ardized to unit area. Blade surface area was measured microscopically using image analysis software. Three density levels were still distinct at germination, but the high and medium levels were not significantly different. The germination rate of conchospores was highest at the medium density level, suggesting facilitation at moderate densities but inhibition at higher densities. Significant self-thinning occurred at each density level but differed among levels, while overall blade growth was about 10-fold greater at low density than at the other two levels. In a separate experiment, counts of attached spores per unit area on artificial substrate materials were greatest on materials with interstitial spaces large enough to trap spores until they become firmly attached.