ROLE OF SETTLEMENT DENSITY ON GAMETOPHYTE GROWTH AND REPRODUCTION IN THE KELPS PTERYGOPHORA CALIFORNICA AND MACROCYSTIS PYRIFERA (PHAEOPHYCEAE)¹

Abstract

Laboratory studies were used to examine how variation in the density of spore settlement influences gametophyte growth, reproduction, and subsequent sporophyte production in the kelps Pterygophora californica Ruprecht and Macrocystis pyrifera (L.) C. Ag. In still (non‐aerated) cultures, egg maturation in both species was delayed when spores were seeded at densities 300 · mm−2. Although the density at which this inhibition was first observed was similar for both species, the age at which their eggs matured was not. P. californica females reached sexual maturity an average of 4 days (or ∼ 30%) sooner than did M, pyrifera. As observed previously in field experiments, per capita sporophyte production was negatively density dependent for both species when seeded at spore densities of 10 · mm−2. Total sporophyte production (i.e. number · cm−2) for both species, however, was greatest at intermediate densities of spore settlement (∼ 50 spores · mm−2). In contrast, total sporophyte production by P. californica steadily increased with increasing spore density in aerated cultures; highest sporophyte density was observed on slides seeded at a density of 1000 spores · mm−2. Preliminary experiments with P. californica involving manipulation of aeration and nutrients indicate that inhibition of gametophyte growth and reproduction at higher densities of spore settlement in non‐aerated cultures was probably caused by nutrient limitation.

Are neighbours harmful or helpful in Fucus vesiculosus populations?

“…However, in this experiment, no significant mean plant weight-density relationship was found, whereas the density-standing crop relationship was strong, so the measure seemed to be a valid one. We found no significant evidence of density dependent mortality in our laboratory populations of Fucus vesiculo.su.s, a response which has been found in other young fucoids (Ang & De Wreede 1992) and kelps (Black 1974, Reed 1990a, Reed et al 1991. Probably, the earliest signs of negative density dependence are reduced growth rates, and only at a later stage does mortality start to take a toll.…”

“…specific seaweed populations. For ~nstance Reed et al (1991) found that kelp spores needed a minimum density for subsequent fertilization and sporophyte pro-MATERIALS AND METHODS duction. 'Safety-in-numbers' factors may be responsible for other reports of higher survival at higher Laboratory populations.…”

The microclimate inhabited by macroalgal propagules

“…In species such as the kelps that have a microscopic life history phase, intense intraspecific competition almost certainly can occur. In a series of field (Reed, 1990) and laboratory (Reed, Neushul & Ebeling, 1991) experiments, a minimum settlement density of at least 1 zoospore mm 2 was needed for successful sporophyte recruitment in Macrocystis pyrifera and Pterogophora californica (presumably to insure successful fertilization). Above this critical density, however, strong density-dependent mortality occurred whenever there was sporophyte recruitment.…”

“…Consequently, Pterygophora gemetophytes mature at a faster rate than do Macrocystis gametophytes (Reed, Neushul & Ebeling, 1991 (Silvertown, 1982). Over-exploitation of a shared resource may not have been the sole mechanism of competition between the two kelps since inhibition of Macrocystis by Pterygophora was observed at low as well as high densities of spore settlement (Reed, 1990).…”

Examining the bank of microscopic stages in kelps using culturing and barcoding

“…In addition, it has been shown to help retain moisture and reduce heat stress during low tides among organisms living in the group (Scrosati & DeWreede, 1998). In the particular case of kelps, Reed et al (1991) showed that a minimum spore density was necessary for sporophyte production in Pterygophora californica and Macrocystis pyrifera, and more recently, Muth (2012) demonstrated that sporophyte production significantly increased as spores became more aggregated in the species M. pyrifera. However, our study was not designed to discriminate between these two hypotheses; thus, it would be interesting to carry out a separate study to specifically address spore settlement.…”