5.6 Sizt' Frcqucncy. Distnbution and Colour Variation of CarCil1LlS maCI11lS
171
tlats (see above) and the better adaptation of green crabs to living conditions in the
intertidal (Reid et al. 1989, 1997). A higher proportion of the green morphotypes
of C. maenas and smaller sized crabs closer to the shore has also been recorded by
Kaiser et al. (1990); Hunter & Naylor (1993); Warman et al. (1993) and Abello et
al. (1997).
In female shore crabs, the seasonal cycle of colour forms was related to their
reproductive cycle. Ovigerous female crabs were red because they do not moult
until the larvae are released in spring. Afterwards they have to complete a moulting cycle until the mating season in autumn. In male crabs, the red colour form
was more frequent in the sub- and intertidal during the mating season in autumn.
Together with the larger individual size of male red crabs, especially in the subtidal, this supports a relation of the red colour form in males with competitive
dominance and reproductive success (Kaiser et al. 1990; Reid et al. 1997).
Wolf (1997) discusses the colour forms of the shore crabs as either growth or
reproduction strategies. Given that the crabs change between the red and green
colour form during their life cycle, they are not committed to a certain strategy
once and forever. Instead, the colour morphotypes represent rather a "life stage
strategy". The shore crabs can alternate their phenotype repeatedly according to
their life cycle or environmental conditions, adding a further dimension to their
phenotypic plasticity. For ELA W A T, it was discussed whether this phenotypic
plasticity is a relevant mechanism for stability properties of the Wadden Sea ecosystem. The individual variability within a population due to phenotypically different ecotypes (in the case of C. maenas the colour forms with their associated
life strategies) could substitute niche development and speciation. Different phenotypes allow a survival under varying environmental conditions, giving a species
with a high phenotypic plasticity an evolutionary advantage over a species with a
low phenotypic plasticity (Aldrich 1989; Hadfield & Strathmann 1996). In a constantly changing ecosystem like the Wadden Sea, phenotypic plasticity may be a
better individual strategy to achieve a maximum reproductive output (Wolf 1997,
Chap. 9). Furthermore, since every crab can change the phenotype depending on
environmental or reproductive requirements, intraspecific competition is reduced.
The generalistic character of the shore crab population is not attributable to the
ecological width of single individuals, but to the phenotypic variability within the
population (Wolf 1997).
Acknowledgements
This chapter is based on the studies in ELA WAT by Frank Wolf, Michael Turkay
and their colleagues from the Senckenberg Institute. The results were published in
the PhD thesis by Frank Wolf and as cited in the text, and further unpublished data
were kindly provided by him. We thank the reviewer and Volker Grimm for
comments and discussions on this chapter. The project was funded by the German
Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie
(BMBF) under grant number 03F0112 A and B. The responsibility for the contents
of the publication rests with the authors.
171
tlats (see above) and the better adaptation of green crabs to living conditions in the
intertidal (Reid et al. 1989, 1997). A higher proportion of the green morphotypes
of C. maenas and smaller sized crabs closer to the shore has also been recorded by
Kaiser et al. (1990); Hunter & Naylor (1993); Warman et al. (1993) and Abello et
al. (1997).
In female shore crabs, the seasonal cycle of colour forms was related to their
reproductive cycle. Ovigerous female crabs were red because they do not moult
until the larvae are released in spring. Afterwards they have to complete a moulting cycle until the mating season in autumn. In male crabs, the red colour form
was more frequent in the sub- and intertidal during the mating season in autumn.
Together with the larger individual size of male red crabs, especially in the subtidal, this supports a relation of the red colour form in males with competitive
dominance and reproductive success (Kaiser et al. 1990; Reid et al. 1997).
Wolf (1997) discusses the colour forms of the shore crabs as either growth or
reproduction strategies. Given that the crabs change between the red and green
colour form during their life cycle, they are not committed to a certain strategy
once and forever. Instead, the colour morphotypes represent rather a "life stage
strategy". The shore crabs can alternate their phenotype repeatedly according to
their life cycle or environmental conditions, adding a further dimension to their
phenotypic plasticity. For ELA W A T, it was discussed whether this phenotypic
plasticity is a relevant mechanism for stability properties of the Wadden Sea ecosystem. The individual variability within a population due to phenotypically different ecotypes (in the case of C. maenas the colour forms with their associated
life strategies) could substitute niche development and speciation. Different phenotypes allow a survival under varying environmental conditions, giving a species
with a high phenotypic plasticity an evolutionary advantage over a species with a
low phenotypic plasticity (Aldrich 1989; Hadfield & Strathmann 1996). In a constantly changing ecosystem like the Wadden Sea, phenotypic plasticity may be a
better individual strategy to achieve a maximum reproductive output (Wolf 1997,
Chap. 9). Furthermore, since every crab can change the phenotype depending on
environmental or reproductive requirements, intraspecific competition is reduced.
The generalistic character of the shore crab population is not attributable to the
ecological width of single individuals, but to the phenotypic variability within the
population (Wolf 1997).
Acknowledgements
This chapter is based on the studies in ELA WAT by Frank Wolf, Michael Turkay
and their colleagues from the Senckenberg Institute. The results were published in
the PhD thesis by Frank Wolf and as cited in the text, and further unpublished data
were kindly provided by him. We thank the reviewer and Volker Grimm for
comments and discussions on this chapter. The project was funded by the German
Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie
(BMBF) under grant number 03F0112 A and B. The responsibility for the contents
of the publication rests with the authors.