Abstract
The intertidal mudflats of Barr al Hikman (Oman) are inhabited by deposit-feeding Sentinel
crabs (Macrophthalmus spp.) and partly covered with seagrass meadows. Whereas Sentinel crab
burrow excavation and feeding can cause sediment bioturbation and erosion, seagrass beds
stabilize the substrate and promote sedimentation. These species could therefore be negatively
related: seagrass beds may be damaged through burial and erosion, whereas Sentinel crab
burrow excavation could be limited within stabilized seagrass habitat. Interactions between
Sentinel crabs and their predators, such as the Crab Plover Dromas ardeola, might therefore
eventually affect seagrass abundance. This project investigates the spatial distribution of
Sentinel crabs in relation to seagrass and habitat characteristics to explore this idea.
Sentinel crabs, seagrass and habitat characteristics were sampled during a field survey in
November-December 2011. Sentinel crab and seagrass biomass as well as sediment organic
matter content were subsequently determined in the laboratory. Further, elevation and grain
size data were obtained for each sampling station by developing a digital elevation model from
satellite imagery and geostatistical interpolation respectively. Lastly, logistic and linear
regression models were developed to relate Sentinel crab occurrence and mean weight to
seagrass and habitat variables.
A significant part of the spatial variation in Sentinel crab occurrence and mean weight
could be explained by the considered environmental variables; the developed regression models
correctly predict Sentinel crab presence or absence in approximately 70% of the cases and
explain 40% of the variation in mean crab weight. Particularly sediment grain size, elevation and
seagrass biomass are important model predictors.
Model results suggest that although Sentinel crabs can occur both in- and outside of
seagrass meadows, the weight and size of these crabs is related to seagrass abundance. Juvenile
Sentinel crabs could depend on seagrass habitat as a nursery ground whereas bigger Sentinel
crabs may damage seagrass beds through bioturbation or prefer bare patches for burrow
excavation. Total Sentinel crab biomass is therefore expected to be lower within seagrass habitat
than on bare patches. Further, Sentinel crabs occur most often within an optimal elevation
range; physical stress or predation pressure may limit survival at higher elevations. Predatorprey interactions between Crab Plovers and Sentinel crabs could thus potentially impact
seagrass abundance in Barr al Hikman: Crab Plover predation at high elevations may affect
Sentinel crab occurrence, which is in turn linked to seagrass biomass.
However, the most important predictor of both Sentinel crab occurrence and mean
weight is sediment grain size. Grain size could be a measure for deposit-feeder food availability;
alternatively, Sentinel crabs might prefer areas with less erosion and finer sediments or trap silt
and clay particles in their burrows.
Model development and reliability are restricted because of the limited number of
Sentinel crabs found in the samples. This will hopefully be alleviated through future field
surveys, which may also clarify whether the spatial distribution of Sentinels crabs and its
relation with habitat characteristics vary through time. Lastly, the suggested relationships
between Sentinel crabs, seagrass and habitat should be further supported by means of
experimental evidence.