Ecological differences between the isomorphic reproductive phases of two species of Indaea (Rhodophyta. Gigartinales)

Abstract

This study tests the hypothesis that species traditionally characterized as isomorphic exhibit similar physiological and ecological responses to different environmental factors, by comparing the ecological and ecophysiological responses of reproductive phases of 2 isomorphic species of Iridaea. Results indicate that although the tetrasporic and cystocarpic phases of I. laminarioides and I. ciliata have similar morphology, they do not have identical responses to ecological variables. In the field, the reproductive stages differ in their relative abundances and in their patterns of spatial and temporal distribution. In laboratory experiments, the reproductive phases showed intraspecific differences in growth rates under conditions of varying temperature, light intensity, water movement and salinity. Differences in susceptibility to grazing vary from juvenile to adult frond and from one grazer to another. The degree of ecological divergence between phases found was greater in the upper intertidal I. laminarioides and smaller in the low intertidal-shallow subtidal I. ciliata. Cystocarpic thalli of I. laminarioides dominated the upper margin of species distribution and grew faster than tetrasporic thalli under several laboratory conditions. In both species the cystocarpic phase was more abundant i the field than the tetrasporic phase. These results call into question the general belief of increased adaptivity of diploidy over haploidy in the algae.