Marine Botany

Porphyra Heteromorphic Life History - Why?

Porphyra title

Have you ever wondered why some plants or algae have such complicated life histories? Why do they undergo two or three different phases in their life time?

In 1967, Istock described heteromorphic life cycles as inherently unstable, as selection pressures should act independently on the separate stages. Eventually one stage should be reduced or eliminated. This is present in many life histories of animals, such as in insects, amphibians, and cnidarians. Another example would be the loss of the free-living, haploid, gametophyte stage from the life cycles of many plants and animals.

However, if Istock's hypothesis is correct, then why are there still heteromorphic life styles in nature? There still may be some sort of advantage to the biphasic life style. Heteromorphic algae occur in three major divisions of macroalgae: Chlorophyta, Phaeophyta, and Rhodophyta.

Jane Lubchenco and John Cubit conducted numerous experiments in an attempt to determine the advantage of heteromorphic life histories. They arrived at two hypotheses:

  • The non-upright morphs (usually boring stages of algae) can survive through physically harsh seasons while upright morphs are killed off by stresses such as dessication and high temperatures.
  • The two stages are in an attempt to survive the effects of herbivores, who like to dine on marine algae. Several of the heteromorphic upright morphs can survive through harsh seasons if they are protected by herbivores. The non-upright morphs are graze-resistant, and therefore are more likely to survive over the upright morphs.

The upright and non-upright stages of algae can be mutually exclusive adaptations to flunctuations in grazing pressure. The upright stages have higher rates of growth and reproduction when there is very little grazing pressure while the non-upright stages (boring, crustose phases) are adopted for times of high grazing pressure.

Reference: Lubchenco, J. & Cubit, J., Heteromorphic Life Histories of Certain Marine Algae as Adaptations to Variations in Herbivory. Ecology 61(3): 676-687.

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