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that fluctuate widely in abundance Ieno and Bastida, 1998. The few studies of meiofauna in these environments showed also low diversity of ostracods two species,
Whatley et al., 1997 and nematodes two species, Mammoli, 1992. No previous studies of other meiofaunal organisms exist for these environments.
Due to their behavior, abundance and extensive distribution C . granulata may play a
determinant role in the intertidal mudflat communities of most SW Atlantic estuaries. Particularly, species that are sensitive to modifications of the structure, composition or
chemistry of the sedimentary environment, may suffer higher mortality rate due to crab activities e.g., Kneib, 1991; Billick and Case, 1994. Also burrows may enhance spatial
heterogeneity, in a small scale affecting meiofauna distribution e.g., Bell, 1980. Thus, the objective of this work is to determine abundance and distribution patterns of benthic
organisms associated with areas inhabited by C
. granulata, and experimentally evaluate the effect of these crabs on benthic organisms.
2. Materials and methods
2.1. Study area The study was performed between January 1996 and April 1997 in the Mar Chiquita
coastal lagoon, a body of brackish water salinity range: 6 to 33‰, affected by low amplitude 1 m tides and characterized by mudflats surrounded by large beds of
cordgrass S . densiflora, Olivier et al., 1972a. The burrowing crab C. granulata is
distributed in both, the S . densiflora areas and mudflats Spivak et al., 1994; Iribarne et
al., 1997. However, only the part of the population living in the open intertidal mudflats was studied.
2.2. Distribution patterns of benthic organisms associated with the presence of crabs 2.2.1. Sampling design
Macroinfauna samples were taken with a core 10 cm diameter and 10 cm depth, organisms were sieved through a 0.5-mm mesh screen. Organisms retained were
preserved in alcohol 70 and sorted under a 20 3 dissection microscope. Meiofaunal sub-samples core size: 2 cm diameter 10 cm depth were taken, sieved through a
0.44-mm mesh screen and then stained with bengal rose to facilitate sorting. Organisms retained were preserved with formalin 5, identified to the major taxa, and counted
under 20 3 dissection microscope.
2.2.2. Relationship between the presence of the burrowing crab C. granulata, the abundance of other benthic organisms and sediment characteristics
A preliminary study was performed to evaluate if there was any difference in the abundance of benthic macroinfauna associated with the presence of this crab. Two areas,
one inhabited by C . granulata and the other without crabs were selected. These areas
were approximately 5 ha each and were separated by 200 m. Ten macroinfauna samples were taken randomly in each area separated each other at least by 10 m at the same
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intertidal level [0.5 m over mean lower low water MLLW]. To evaluate vertical distribution of macroinfauna in each area, samples were divided in five depth layers of 2
cm in width each. To evaluate possible temporal variations over a year, these samples were taken in May, July, September, November 1996, January and March 1997.
Sediment samples 10 replicates per area; 10 cm diameter and 10 cm depth were taken to evaluate water content, organic matter content and grain size. Water content was
calculated as the difference between wet and dry weight after drying at 708C during 72 h. Organic matter content was calculated as the percentage of ash free dry weight
AFDW, ashes were obtained after incinerating sub-samples 10 g each, at 5508C during 6 h. Grain size distribution was evaluated by sieve and sedimentation analysis
following Carver 1971.
2.2.3. Small scale patterns in meiofaunal distribution associated with burrows of C. granulata
To evaluate if there was any difference in the spatial distribution of meiofauna related to the presence of burrows of C
. granulata, meiofaunal samples were taken at three distances from the burrow: edge, 5 cm and 10 cm from the burrow opening. Given that
crabs make sediment mounds on one side of the burrow entrance Iribarne et al., 1997 which may affect meiofaunal distribution, samples at each distance were repeated on the
side of the mound and on the opposite side. These samples were taken from 10 burrows of similar shape located at the same intertidal level approximately 0.5 m over MLLW.
2.3. Experiments 2.3.1. Experimental designs
All experiments described below were performed as follows: inclusion and exclusion treatment plots of adult crabs were performed with completely closed wire cages
50 3 50 cm area, with a mesh size of 5 mm. Cages had 60 cm length sides which were pushed into the sediment to a depth of 40 cm with 20 cm protruding above the surface.
This design was performed to avoid crabs passing through their burrows. In inclusion treatments six individuals of the corresponding species three males and three females
within a size range of 30 to 35 mm were included. In exclusion treatment, crabs were extracted using dead fish tied at the end of a cord as bait. In all cases treatments had 10
replicates and were randomly distributed and separated each other by 10 m.
2.3.2. Discriminating the effect of crabs on benthic organisms inside crab beds A field experiment was performed during summer 1996 to discriminate the effect of
C . granulata from the effect of other predators on benthic organisms. This experiment
lasted 80 days starting on 15 February to ensure the presence of all predators in the area. This experiment was run on the open mudflat with the following treatments: 1
exclusion of crabs, 2 inclusion of C . granulata; 3 inclusion of Cy. angulatus; 4
exclusion of big fishes and birds with ceilings of 50 3 50 cm standing 5 cm over the sediment by PVC pipes, 5 control for cage effect border without ceilings, and 6
control 50 3 50 cm previously marked areas without any manipulation. Birds excluded in this experiment were principally plovers e.g., Charadrius spp., Pluvialis squatarola,
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sandpipers Calidris spp. and yellowlegs Tringa spp.. Fishes were croaker Micro- pogonias furnieri and mullet Mugil platanus. At the end of the experiment,
macroinfauna and meiofauna samples were taken from the center of each plot.
2.3.3. Effect of C. granulata on infaunal organisms living at different depths in sediment
Given that the previous experiment showed the effect of this species on benthic fauna, an experiment was performed during summer 1997 to evaluate if this effect varies with
sediment depths. The experiment had two treatments: 1 exclusion and 2 inclusion of C
. granulata. The experiment ran from 18 January to 10 February 1997. Samples of macroinfauna and meiofauna were taken from the center of each treatment and were
divided into two layers 5 cm each to evaluate if effect differs between depth. 2.3.4. Effect of crabs on recruitment of Cy. angulatus
Given that recruitment of the crab Cy . angulatus occurs mainly at the end of the
summer–early autumn Luppi et al., 1994, an experiment similar to the previous ones was performed to evaluate the effect of C
. granulata and Cy. angulatus on crab recruitment. The experiment had a duration of 40 days starting on 2 March and included
the recruitment period and consisted of three treatments: 1 inclusion of C . granulata;
2 inclusion of Cy . angulatus; and 3 exclusion of crabs.
2.3.5. Inclusion of crabs in an area not inhabited by crabs Previous experiments were performed in areas inhabited and therefore disturbed by
the burrowing crab C . granulata. However, areas not inhabited by crabs present the
opportunity to evaluate their actual effect on the environment. Therefore, an experiment was performed in an intertidal zone close to crab beds distant 200 m, with the same
tidal regime, and similar sediment characteristics, but not disturbed by crabs. This experiment consisted of a C
. granulata inclusion treatment and control treatment similar closed cages without crabs. Macroinfauna and meiofauna samples were taken after 40
days when placement of crabs in the new habitat was evident and samples were divided into two depth layers 5 cm each.
2.3.6. Effect of juvenile crabs on abundance of meiofaunal organisms A field experiment was performed to evaluate the effect of juveniles of C
. granulata on meiofaunal organisms. The experiment consisted of three treatments: 1 inclusion of
crabs of 12 to 15 mm carapace width, 2 inclusion of crabs of 7 to 10 mm carapace width, and 3 control. All treatments consisted of PVC pipes 10 cm diameter 15 cm
depth, covered at the top with a plastic net 1 mm mesh size and inserted 12 cm inside the substrate. Four crabs were included in the two first treatments while no crabs were
added in controls. Treatments were arranged in the intertidal zone inhabited by adults of
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this species. Cores were homogeneously perforated all around 0.5 perforations cm to
allow water flow through sediment during the tidal cycle. The experiment had 10 replicates and was run during 20 days, after which samples of meiofauna were taken and
divided into two depth layers 5 cm each.
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2.4. Statistical analysis A fixed factor MANOVA was performed to evaluate differences in polychaete
abundance between areas and among months and depth layers in Section 2.2.2. Percentage of water content and AFDW were compared between areas with t-tests Zar,
1984 and differences in grain size distribution between areas was evaluated with a Kolmogorov–Smirnov two-sample test Conover, 1980.
In Section 2.2.3 differences in mean abundance of meiofaunal organisms among distances from crab burrows and in both directions were evaluated with a two-fixed
factors analysis of variance ANOVA and a posteriori Tukey multiple comparison test Zar, 1984.
In all experiments, treatment effect was evaluated independently for each species with t-tests or ANOVAs Zar, 1984 and abundances were then compared between depth
layers with paired t-tests Zar, 1984, taking each sample as a pair. When assumptions were not met, transformations were performed or, when the problem persisted the
non-parametric Mann–Whitney or Kruskall–Wallis test Conover, 1980 were used. A Tukey test Zar, 1984 or a non-parametric multiple comparison test Conover, 1980
was performed after ANOVA or Kruskall–Wallis, respectively.
In Sections 2.3.2, 2.3.3 and 2.3.5, abundance of benthic organisms in experiments were also evaluated with non-metric multi-dimensional scaling ordination MDS, using
the Bray–Curtis similarity measure Warwick and Clarke, 1991. Then, the significance of differences between treatments were evaluated with one-way ‘‘analysis of similarity’’
ANOSIM following Clarke 1993.
3. Results
