2.2. THE CASE STUDIES
Individual species, of a given general morphology, may express subtle
or extreme morphological variations either within or between genotypes.
There are four different kinds of variation within species; two of which
involve drastic switches between morphologies so different that they look
like they should be classified in different genera or even orders (1 and 2,
below) and two of which represent continua of plasticity within a bodyplan
(3 and 4, below). These are: (1) heteromorphic alternation of generations, (2)
environmentally driven morphological switches, (3) ecotypic or geographic
variation, and (4) phenotypic plasticity.
Most seaweeds have separate, free-living, macroscopic haploid and
diploid phases of their life cycles. The structures of the two phases may
be similar or strikingly different. The latter case is called the heteromorphic
alternation of generations. One example is Mastocarpus stellatus (Fig. 2.7C)
which has a flat, encrusting phase formerly classified as the genus Petrocelis
and a leafy, branched upright phase. The giant kelps, Macrocystis sp. which
have complex internal and external morphology in the diploid stage with
individuals up to 100 m long, alternate with a microscopic, branched, haploid filament . Unless the spores produced by each phase are grown through
the complete life cycle, there is nothing in the gross morphology to indicate
that these incredibly different shapes are produced by a single genotype. Although heteromorphic alternation of generations occurs in many species of
red and brown seaweeds, the biological basis of the biphasic morphogenesis
is unknown.
Switches in morphology within a genotype can also be elicited by environmental conditions in some species (Luning 1990). For example, the brown
seaweed, Scytosiphon lomentaria, grows as a clump of elongated tubes or as
a felty encrusting sheet, formerly known as Ralfsiasp.,depending on the temperature and photoperiod during its development (Dring and Luning 1975).
Similarly, there are seaweeds which do or do not grow branches, whorls, or
hairs depending on the color of light they are exposed to. The complex red
seaweed species Bonnemaisoniahamiferadevelops into an alternate, filamentous form when the water temperature is below 13°C. The two morphotypes
of this species were originally thought to be different species with overlapping geographic distributions. The green alga Urospora wormskioldiiexhibits
three different morphologies - a filament, a hollow ball, or a globular multicellular shape - depending only on the temperature during its development
from zoospores (Bachman et al. 1976; Luning 1990) .
As is the case for most species of both modular and unitary organisms,
many seaweeds vary over their geographic range with "stress tolerant" morphologies occurring at the limits of its distribution. Geographic variation is
probably due to differential selection of morphologies in local habitats. For
example, species of the brown, fucoid algae have distinct miniature morphs
in the Baltic Sea, at the limit of their salinity tolerance. Ascophyllum nodosum
has long, straight fronds attached firmly to the substrate over most of its
range in the Northern Atlantic but exhibits a tightly curled, free living form
in the lochs and fjords of Northwest Europe. Some examples of geographic
variation in seaweed morphology can be found in the work of Hanisak and
Samuel 1987 and Chopin et al. 1996.
The most intensively studied type of morphological variation in seaweeds is phenotypic plasticity in response to environmental conditions
including light color and intensity, temperature, water flow and nutrient
33
Individual species, of a given general morphology, may express subtle
or extreme morphological variations either within or between genotypes.
There are four different kinds of variation within species; two of which
involve drastic switches between morphologies so different that they look
like they should be classified in different genera or even orders (1 and 2,
below) and two of which represent continua of plasticity within a bodyplan
(3 and 4, below). These are: (1) heteromorphic alternation of generations, (2)
environmentally driven morphological switches, (3) ecotypic or geographic
variation, and (4) phenotypic plasticity.
Most seaweeds have separate, free-living, macroscopic haploid and
diploid phases of their life cycles. The structures of the two phases may
be similar or strikingly different. The latter case is called the heteromorphic
alternation of generations. One example is Mastocarpus stellatus (Fig. 2.7C)
which has a flat, encrusting phase formerly classified as the genus Petrocelis
and a leafy, branched upright phase. The giant kelps, Macrocystis sp. which
have complex internal and external morphology in the diploid stage with
individuals up to 100 m long, alternate with a microscopic, branched, haploid filament . Unless the spores produced by each phase are grown through
the complete life cycle, there is nothing in the gross morphology to indicate
that these incredibly different shapes are produced by a single genotype. Although heteromorphic alternation of generations occurs in many species of
red and brown seaweeds, the biological basis of the biphasic morphogenesis
is unknown.
Switches in morphology within a genotype can also be elicited by environmental conditions in some species (Luning 1990). For example, the brown
seaweed, Scytosiphon lomentaria, grows as a clump of elongated tubes or as
a felty encrusting sheet, formerly known as Ralfsiasp.,depending on the temperature and photoperiod during its development (Dring and Luning 1975).
Similarly, there are seaweeds which do or do not grow branches, whorls, or
hairs depending on the color of light they are exposed to. The complex red
seaweed species Bonnemaisoniahamiferadevelops into an alternate, filamentous form when the water temperature is below 13°C. The two morphotypes
of this species were originally thought to be different species with overlapping geographic distributions. The green alga Urospora wormskioldiiexhibits
three different morphologies - a filament, a hollow ball, or a globular multicellular shape - depending only on the temperature during its development
from zoospores (Bachman et al. 1976; Luning 1990) .
As is the case for most species of both modular and unitary organisms,
many seaweeds vary over their geographic range with "stress tolerant" morphologies occurring at the limits of its distribution. Geographic variation is
probably due to differential selection of morphologies in local habitats. For
example, species of the brown, fucoid algae have distinct miniature morphs
in the Baltic Sea, at the limit of their salinity tolerance. Ascophyllum nodosum
has long, straight fronds attached firmly to the substrate over most of its
range in the Northern Atlantic but exhibits a tightly curled, free living form
in the lochs and fjords of Northwest Europe. Some examples of geographic
variation in seaweed morphology can be found in the work of Hanisak and
Samuel 1987 and Chopin et al. 1996.
The most intensively studied type of morphological variation in seaweeds is phenotypic plasticity in response to environmental conditions
including light color and intensity, temperature, water flow and nutrient
33