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Mar Ecol Prog Ser 338; 159-168, 2007 
affect settlement, recruitment or other population pro 
cesses (Hawkins & Hartnoll 1982, Gaines & Rough- 
garden 1985, Connolly & Roughgarden 2003). 
The situation differs for species with mobile benthic 
stages, such as crabs. While transport processes affect 
the arrival of larvae in coastal waters (Forward & 
Tankersley 2001, Queiroga & Blanton 2004), mobility 
of settling stages complicates the definition of settle 
ment. For instance, in the shore crab Cardans maenas, 
settlement does not seem to be an irreversible process: 
the degree of development seems to determine 
whether megalopae settle definitively or emigrate 
(Moksnes et al. 2003). Thus, at small scales, the distri 
bution patterns of early life stages of mobile crus 
taceans are determined by the behaviour of mega 
lopae (Hedvall et al. 1998, Moksnes et al. 2003). 
In coastal species, regardless of whether these have 
mobile or sessile benthic stages, transport of larvae 
from the pelagic to the benthic habitat is seen as a 
2-step process consisting of (1) onshore transport 
towards the coastal zone by (e.g.) wind-driven currents 
(Wing et al. 2003, Queiroga et al. 2006), and (2) trans 
port to the benthic habitat as a consequence of inter 
actions between shoreline configuration and near 
shore circulation (Pineda 1999, Shanks et al. 2003). For 
mobile species, transport processes may explain pat 
terns of spatial distribution at fairly large spatial scales. 
However, it is not clear whether or not temporal pat 
terns of settlement reflect pre-settlement processes. 
We studied the temporal variability in colonisation of 
megalopae of the shore crab Cardnus maenas. Our 
main objective was to determine if physical processes 
responsible for the transport of larvae affected their 
rate of settlement. All information about the role of 
physical processes on settlement rates and larval sup 
ply of shore crab are from studies within estuaries. For 
instance, on the Portuguese coast, larval supply to 
estuaries has been related to alongshore winds and 
tidal currents (Queiroga & Blanton 2004, Paula et al. 
2006, Queiroga et al. 2006). In the present study, we 
evaluated settlement on an open coast, in the intertidal 
zone of Helgoland (German Bight, North Sea, see 
Fig. 1). The settlement season of C. maenas varies 
regionally; in the North Sea it occurs between June 
and November (Scherer & Reise 1981, Thiel & 
Dernedde 1994). Both the Portuguese coast and the 
German Bight are mesotidal systems, but the latter is 
much shallower than the former. 
In Portuguese estuaries, the colonisation rate of arti 
ficial substrata was uncoupled from larval supply 
(Paula et al. 2006, Queiroga et al. 2006). According to 
Paula et al, (2006), collectors based on artificial sub 
stratum strongly interfere with the surrounding natural 
substratum, affecting the interpretation of results on 
settlement rates. However, in our study we estimated 
Fig. 1. Location of sampling site (arrow) off Helgoland, 
North Sea, German Bight (54° 10' 67.0" N, 7° 53' 54.3" E) 
settlement on natural substratum while avoiding any 
interference in the evaluation of associations between 
settlement and physical processes related to larval 
transport. Our data suggest that there is indeed a cou 
pling between larval supply and colonisation rate. 
MATERIALS AND METHODS 
Study site and data collection. The study site, off the 
island of Helgoland, North Sea, German Bight (Fig. 1), 
is a shallow semidiurnal mesotidal system. Colonisa 
tion was estimated in an intertidal sector characterised 
by boulders and coarse sediments (mainly gravel and 
cobble). At this site Cardnus maenas juveniles reach 
densities of about 300 to 600 ind. nr 2 in gravel-cobble 
sediment (L. Giménez unpubl. data). 
Colonisation rate was estimated using 3 replicate 
traps deployed every day for 24 to 26 h, in the intertidal 
at diurnal low tide. The traps were randomly placed 
each day in a different place, between mean high 
water and mean low water; distance among traps was 
usually in the order of tens of metres. Each trap con 
sisted of a PVC ring of 4 cm height and 314 cm 2 surface 
area with a nylon mesh bottom of 500 pm. The traps 
were filled with natural defaunated sediment consist 
ing of coarse gravel and cobble; traps were buried in 
order to avoid edge effects on bottom currents. Recov 
ered traps were immediately taken to the laboratory 
where megalopae were immediately identified and 
counted. 
Wind data were kindly provided by the German 
Weather Service (Deutscher Wetter Dienst, DWD) station 
on Helgoland as wind direction (32-sector Rosette) and 
wind force (Beaufort scale converted to m s” 1 , whereby