5.6 Slll~ Frequency. Dl;,tnhutl()n and Colour Variation of CarcinLl.I II1(lCIW.I
163
5.6
Size Frequency, Distribution and Colour Variation of
Carcinus maenas in the Spiekeroog Backbarrier System
Sabine Dittmann & Marlies ViII brandt
Abstract: Shore crabs (Carcinus maenas) were studied in the backbarrier system
of Spiekeroog to assess the size of the (sub- )populations and colour variations in
sub- and intertidal areas. Juvenile shore crabs occurred exclusively in the intertidal
and preferentially within mussel beds. The record of relative abundances and size
frequency distributions showed that small and medium-sized crabs migrate from
the subtidal into the intertidal during the summer months. Thus the crabs occurring
in the intertidal are not representative for the entire population. After the ice winter \995/96 very few shore crabs appeared on the tidal flats and not until late
summer. The majority of the shore crabs in the study area were of the green morphotype, especially in the intertidal. The number of male red crabs increased during the mating season. Female red crabs were frequent in spring and early summer. Implications of the colour morphotypes as a phenotypic plasticity of the shore
crabs are discussed in relation to the variable living conditions in the Wadden Sea.
5.6.1
Introduction
The shore crab Carcinus maenas occurs in the coastal seas of the entire Northeast
Atlantic and has been introduced to many areas of the world (Grosholz & Ruiz
1995; Cohen et al. 1995; Hayward & Ryland 1996). The crabs live mainly subtidally, but a part of the population occurs sporadically on tidal flats. Shore crabs
settle in the tidal flats and migrate into the subtidal late in autumn (Thiel & Dernedde 1994) or remain in the intertidal over winter (Klein-Breteler 1976a).
Scherer & Reise (1981) report a mean density of about 15 crabs 100 m
2
on sandflats of the North Frisian Wadden Sea and much higher values of juvenile crabs in
seagrass beds and mussel beds (e.g. 500 O-group crabs m'). Migrations of adult
shore crabs between the sub- and intertidal vary with crab size, but also with season, daytime and tides (Naylor 1962; Atkinsons & Parsons 1973; Klein-Breteler
1976b; Scherer & Reise 1981; Warman et al. 1993; Hunter & Naylor 1993; Thiel
& Dernedde 1994). Over winter, the crabs are inactive in deeper areas of the subtidal. They don't feed during this time and their metabolism is reduced (Crothers
1968). In spring, the first crabs appear in shallow subtidal areas and become more
active with rising water temperatures. In the course of the summer increasing
numbers of crabs occur in the shallow subtidal and intertidal (Beukema 1991).
Towards the end of the year the crabs withdraw to the subtidal. e. maenas can
theoretically reproduce all year round, but most pre- and in-copula pairs are recorded in late summer (van der Meeren 1994). The larvae hatch in the following
spnng.
e. maenas plays a central role in the food web of intertidal areas and is a
prominent mobile predator in the Wadden Sea with a wide food spectrum. Their
prey choice and predation pressure varies with the abundance and availability of
163
5.6
Size Frequency, Distribution and Colour Variation of
Carcinus maenas in the Spiekeroog Backbarrier System
Sabine Dittmann & Marlies ViII brandt
Abstract: Shore crabs (Carcinus maenas) were studied in the backbarrier system
of Spiekeroog to assess the size of the (sub- )populations and colour variations in
sub- and intertidal areas. Juvenile shore crabs occurred exclusively in the intertidal
and preferentially within mussel beds. The record of relative abundances and size
frequency distributions showed that small and medium-sized crabs migrate from
the subtidal into the intertidal during the summer months. Thus the crabs occurring
in the intertidal are not representative for the entire population. After the ice winter \995/96 very few shore crabs appeared on the tidal flats and not until late
summer. The majority of the shore crabs in the study area were of the green morphotype, especially in the intertidal. The number of male red crabs increased during the mating season. Female red crabs were frequent in spring and early summer. Implications of the colour morphotypes as a phenotypic plasticity of the shore
crabs are discussed in relation to the variable living conditions in the Wadden Sea.
5.6.1
Introduction
The shore crab Carcinus maenas occurs in the coastal seas of the entire Northeast
Atlantic and has been introduced to many areas of the world (Grosholz & Ruiz
1995; Cohen et al. 1995; Hayward & Ryland 1996). The crabs live mainly subtidally, but a part of the population occurs sporadically on tidal flats. Shore crabs
settle in the tidal flats and migrate into the subtidal late in autumn (Thiel & Dernedde 1994) or remain in the intertidal over winter (Klein-Breteler 1976a).
Scherer & Reise (1981) report a mean density of about 15 crabs 100 m
2
on sandflats of the North Frisian Wadden Sea and much higher values of juvenile crabs in
seagrass beds and mussel beds (e.g. 500 O-group crabs m'). Migrations of adult
shore crabs between the sub- and intertidal vary with crab size, but also with season, daytime and tides (Naylor 1962; Atkinsons & Parsons 1973; Klein-Breteler
1976b; Scherer & Reise 1981; Warman et al. 1993; Hunter & Naylor 1993; Thiel
& Dernedde 1994). Over winter, the crabs are inactive in deeper areas of the subtidal. They don't feed during this time and their metabolism is reduced (Crothers
1968). In spring, the first crabs appear in shallow subtidal areas and become more
active with rising water temperatures. In the course of the summer increasing
numbers of crabs occur in the shallow subtidal and intertidal (Beukema 1991).
Towards the end of the year the crabs withdraw to the subtidal. e. maenas can
theoretically reproduce all year round, but most pre- and in-copula pairs are recorded in late summer (van der Meeren 1994). The larvae hatch in the following
spnng.
e. maenas plays a central role in the food web of intertidal areas and is a
prominent mobile predator in the Wadden Sea with a wide food spectrum. Their
prey choice and predation pressure varies with the abundance and availability of