164
5 Spatial and Temporal DistrIhutlOn Patterns
prey as well as with age, size and sex of the crabs (Ropes 1968; Walne & Dean
1972; Scherer & Reise 1981; Jubb et al. 1983; Rangely & Thomas 1987). Shore
crabs are also scavengers (Moore & Howarth 1996). Juvenile crabs feed on meiofauna (nematodes) and juvenile stages or small-sized macrofauna (larvae, barnacles, molluscs or polychaetes) (Klein-Breteler 1975; Reise 1985; Rangeley &
Thomas 1987). Given a respective supply and optimal size, blue mussels (Mytilus
edulis) can constitute the greatest part of the crabs' diet (Elner & Hughes 1978;
Elner 1981; Jubb et al. 1983). C. maenas itself is preyed upon by birds (for example curlews, redshanks, and gulls; see e.g. Dernedde 1993, 1994), fish (Jensen &
Jensen 1985), and seals (Crothers 1968).
Adult shore crabs occur in different colour forms, refering to their ventral carapace colour varying from green to red. Colour morphotypes are known from many
crab species and are often, but not always, indicators for moulting stages (Reid et
al. 1997; Ti.irkay, pers. comm.). Crabs can regulate their moulting cycle to a certain degree and delay a forthcoming moult (Reid et al. 1997). This can be triggered
by external stimuli on the hormonal system via the sensory organs of the crabs or
by photo-denaturation of pigments in the carapace (Carlisle 1957; Reid et al.
1997). In the course of their life, shore crabs can undergo individually different
moulting frequencies and thus alternate between a "green" and "red" colour form
(Reid et al. 1997). These different coloured phenotypes go along with physiological differences as well as modified behaviour and activities (McGaw & Naylor
1992 a,b; McGaw et al. 1992: Warman et al. 1993; Aagaard et al. 1995). Red
crabs are in a prolonged intermoult and thus have a thicker and stronger carapace
and chelae, giving them a competitive advantage during aggressive interactions
with conspecifics (Kaiser et al. 1990). In turn, red colour forms have to cope with
physiological disadvantages (higher oxygen demand, reduced tolerance towards
temperature and salinity variations) (Aldrich 1986; Reid & Aldrich 1989; Reid et
al. 1997). Green crabs are more tolerant to salinity fluctuations and aerial exposure
and are thus better adapted to living conditions in the intertidal (Reid et al. 1989,
1997).
As part of the ecosystem research project ELA W A T, the shore crab population
in the backbarrier system of Spiekeroog Island was investigated to assess distribution, size frequency, sex distribution and colour forms of the crabs in the sub- and
intertidal. This chapter summarizes these studies that were carried out by Wolf
(1997, 1998 and unpublished).
5.6.2
Material and Methods
Several methods were used by Wolf (1997, 1998) for surveying the shore crab
population. In the subtidal, crabs were caught using a 2 m beam trawl from the
research vessel Senckenberg. Samples were taken at a water depth of 5 m in the
main tidal channel of the backbarrier area of Spiekeroog Island, on the northern
edge of the Janssand (Fig. 3.1.2), over a trawling distance of 1 nautical mile. The
net had a mesh size of 10 mm which captured crabs> 15 mm carapace width
(CW). These trawls were taken from late 1992 to 1994 with varying sampling
frequencies between the years and varying replication of the monthly trawls. In
5 Spatial and Temporal DistrIhutlOn Patterns
prey as well as with age, size and sex of the crabs (Ropes 1968; Walne & Dean
1972; Scherer & Reise 1981; Jubb et al. 1983; Rangely & Thomas 1987). Shore
crabs are also scavengers (Moore & Howarth 1996). Juvenile crabs feed on meiofauna (nematodes) and juvenile stages or small-sized macrofauna (larvae, barnacles, molluscs or polychaetes) (Klein-Breteler 1975; Reise 1985; Rangeley &
Thomas 1987). Given a respective supply and optimal size, blue mussels (Mytilus
edulis) can constitute the greatest part of the crabs' diet (Elner & Hughes 1978;
Elner 1981; Jubb et al. 1983). C. maenas itself is preyed upon by birds (for example curlews, redshanks, and gulls; see e.g. Dernedde 1993, 1994), fish (Jensen &
Jensen 1985), and seals (Crothers 1968).
Adult shore crabs occur in different colour forms, refering to their ventral carapace colour varying from green to red. Colour morphotypes are known from many
crab species and are often, but not always, indicators for moulting stages (Reid et
al. 1997; Ti.irkay, pers. comm.). Crabs can regulate their moulting cycle to a certain degree and delay a forthcoming moult (Reid et al. 1997). This can be triggered
by external stimuli on the hormonal system via the sensory organs of the crabs or
by photo-denaturation of pigments in the carapace (Carlisle 1957; Reid et al.
1997). In the course of their life, shore crabs can undergo individually different
moulting frequencies and thus alternate between a "green" and "red" colour form
(Reid et al. 1997). These different coloured phenotypes go along with physiological differences as well as modified behaviour and activities (McGaw & Naylor
1992 a,b; McGaw et al. 1992: Warman et al. 1993; Aagaard et al. 1995). Red
crabs are in a prolonged intermoult and thus have a thicker and stronger carapace
and chelae, giving them a competitive advantage during aggressive interactions
with conspecifics (Kaiser et al. 1990). In turn, red colour forms have to cope with
physiological disadvantages (higher oxygen demand, reduced tolerance towards
temperature and salinity variations) (Aldrich 1986; Reid & Aldrich 1989; Reid et
al. 1997). Green crabs are more tolerant to salinity fluctuations and aerial exposure
and are thus better adapted to living conditions in the intertidal (Reid et al. 1989,
1997).
As part of the ecosystem research project ELA W A T, the shore crab population
in the backbarrier system of Spiekeroog Island was investigated to assess distribution, size frequency, sex distribution and colour forms of the crabs in the sub- and
intertidal. This chapter summarizes these studies that were carried out by Wolf
(1997, 1998 and unpublished).
5.6.2
Material and Methods
Several methods were used by Wolf (1997, 1998) for surveying the shore crab
population. In the subtidal, crabs were caught using a 2 m beam trawl from the
research vessel Senckenberg. Samples were taken at a water depth of 5 m in the
main tidal channel of the backbarrier area of Spiekeroog Island, on the northern
edge of the Janssand (Fig. 3.1.2), over a trawling distance of 1 nautical mile. The
net had a mesh size of 10 mm which captured crabs> 15 mm carapace width
(CW). These trawls were taken from late 1992 to 1994 with varying sampling
frequencies between the years and varying replication of the monthly trawls. In