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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
3.1. Distribution patterns of benthic organisms associated with the presence of crabs 3.1.1. Relationship between the presence of the burrowing crab C. granulata, the
abundance of polychaetes and sediment characteristics The only species found in all sampling periods was the polychaete L
. acuta Treadwell. The MANOVA showed effects of months and areas but did not show
effects of depth Table 1; Fig. 1. Also, interactions were found between month and depth and area and month and all three factors Table 1. A posteriori Tukey test showed
that polychaetes were more abundant in May, July, September and November than in the other months inside the crab bed Fig. 1. However, outside crab bed, L
. acuta density remained constant all year around Fig. 1. In May and July abundance of polychaetes
were greater in the upper layers inside crab beds Fig. 2. In September samples were taken after a period of low rainfall and sediment was much dryer than in the other
sampling periods. In this month, abundance increased in the deepest layers inside the crab bed Fig. 2. Outside crab bed no differences among depth layers were found in any
month sampled.
The analysis of sediment characteristics showed no differences in organic matter content between areas t 5 0.31, df 5 12, P . 0.05; Fig. 3A. However, sediment water
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Fig. 1. Density ind m of the polychaete L
. acuta in different months of the year inside IN and outside OUT crab bed. Lines connect no significant differences P , 0.05; Tukey test among months at each area
lines outside graph and between areas for each month lines inside graph. Here and thereafter box plots are constructed with limits of boxes being the 75th and 25th percentile, lines represent 10th and 90th percentiles,
lines or points inside boxes are medians, circles are outliers StatSoft, 1998.
content was significantly greater inside the area inhabited by crabs than outside t 5 5.1, df 5 8, P , 0.05; Fig. 3B. Grain size distribution differed between areas T 5 0.42,
n 5 26, n 5 33; P , 0.05; Fig. 3C. Sediment in the area inhabited by crabs was
1 2
mainly silt 80 silt, 16 sand and outside crab beds, it was mainly sand 58 sand and 40 silt.
3.1.2. Small scale patterns in meiofaunal distribution associated with burrows of C. granulata
In this sampling site, the most abundant organisms recorded were nematodes. The two-way fixed factor ANOVA showed only interaction effects data were log trans-
formed to meet the assumptions of homocedasticity; direction: F 5 4.5, df 5 1, 42,
Table 1 Results of MANOVA analysis 5 P , 0.05
Source of variation df
MS F
Effect Month
5 96.028
13.478 Area
1 498.719
69.989 Depth
4 12.291
1.7255 Month–area
5 84.738
11.892 Month–depth
20 21.110
2.962 Area–depth
4 14.369
2.016 Month–area–depth
20 19.484
2.734 Error
380 7.126
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Fig. 2. Density ind m of the polychaete L
. acuta in each depth layer inside IN and outside OUT crab bed and in each sampling month: May A, July B, September C, November D, January E and March
F.
P . 0.05; distance: F 5 3.51, df 5 2, 42, P . 0.05; interaction: F 5 14.4, df 5 2, 42, P , 0.05; Fig. 4. Interaction was important indicating that distance has different effects
depending on if we consider mound or no mound direction. The multiple mean comparison Tukey test performed taking both factors jointly Neter et al., 1990, showed
that in the mound direction the lower abundance was at the edge of the burrow and the maximum was at 10 cm. However, in the opposite direction, the lower abundance was at
5 cm, and it increased at the edge of the burrow Fig. 4. The abundance of ostracods was lower, and no effect was found direction: F 5 2.44, df 5 1, 42, P . 0.05; distance:
F 5 2.08, df 5 2, 42, P . 0.05; interaction: F 5 0.8, df 5 2, 42, P . 0.05; Fig. 4. Copepods and amphipods were not present in this sampling.
3.2. Experiments 3.2.1. Discriminating the effect of crabs on benthic organisms inside crab beds
The benthic organisms found in the experimental sites were: nematodes, ostracods, copepods, the polychaetes L
. acuta, Nephtys fluviatilis Monro and H. similis Soath- ern, the amphipod Corophium insidiosus and a nemertean non-identified species.
When each species was evaluated independently, treatment effects were found for L .
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Fig. 3. Sediment characteristics INSIDE and OUTSIDE crab beds. A Percentage of AFDW mean 1 1 SD, B percentage of water content mean 1 1 SD; and C grain size distribution. Lines connect no significant
differences P . 0.05.
acuta Kruskall–Wallis test; H 5 11.22, df 5 5, 60; P , 0.05, increasing the density in the total exclusion treatment compared with the control and the exclusion of fishes and
birds Fig. 5A. Differences among treatments were also found for nematodes Kruskall– Wallis test; H 5 12.25, df 5 5, 60; P , 0.05; Fig. 6A, where differences were due to an
increase of this group in the exclusion treatment in comparison with the treatment with enhanced density of C
. granulata. No differences existed among treatments for H. similis Kruskall–Wallis test; H 5 2.45, df 5 5, 60, P . 0.05; Fig. 5B, copepods
Kruskall–Wallis test; H 5 4.7, df 5 5, 60, P . 0.05; Fig. 6B ostracods Kruskall–
22
Wallis test; H 5 4.9, df 5 5, 60, P . 0.05; Fig. 6C, N . fluviatilis x 5 6.4 ind m
, SD 5 27.9, Kruskall–Wallis test: H 5 3.1, df 5 5, 60, P . 0.05, Co
. insidiosus x 5 18
22
ind m , SD 5 45, Kruskall–Wallis test: H 5 2.11, df 5 5, 60, P . 0.05 or the
22
nemertean non-identified species x 5 22.7 ind m , SD 5 48.9, Kruskall–Wallis test:
H 5 8.6, df 5 5, 60, P . 0.05.
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Fig. 4. Density of nematodes and ostracods ind cm at different distances from the burrow, in the mound
direction mound and in the opposite one no mound. Equal letters indicate no significant differences P . 0.05.
MDS ordination of samples showed no treatment effect on the benthic community structure Fig. 7 and this was confirmed by ANOSIM R 5 0.09; P . 0.01.
3.2.2. Effect of C. granulata on infaunal organisms living at different depths in sediment
The most abundant organisms found during this experiment were nematodes and the polychaete L
. acuta. When each species was evaluated, no effect was found on the abundance of L
. acuta at any depth layer [upper layer: data were transformed; x9 5 log x 1 1, t 5 1.6, df 5 16; lower layer: Mann–Whitney U 5 44.5, n 5 9, n 5 10; P .
1 2
0.05; Fig. 8A. No difference was found in abundance of this polychaete between both depth layers [transformed data x9 5 log x 1 1, t 5 1.98, df 5 18, P . 0.05; Fig. 8A.
However, nematodes significantly increased in abundance in the upper layer data were log transformed, t 5 2.4, df 5 18, P , 0.05, while no difference were found between
treatments in the lower layer log transformed data, t 5 2 0.75, df 5 17, P . 0.05; Fig. 8B. Abundance of nematodes was greater in the upper layer than in the lower layer
t 5 8.42, df 5 21, P , 0.05; Fig. 8B. Other organisms found in the upper layer without
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Fig. 5. Results on macroinfauna of the experiment to discriminate the effect of crabs. Box plots show the
22
abundance ind m of the polychaetes A L
. acuta and B H. similis in each treatment: control CON, birds and fishes exclusion BF EXC, C
. granulata inclusion CG INC, Cy. angulatus inclusion CA INC, cage control CAGE CON, and total exclusion EXC. Lines connect no significant differences P . 0.05.
22
differences in abundance between treatments were: ostracods x 5 0.7 ind cm , SD 5
22
0.8, t 5 1.67, df 5 17; P . 0.05; copepods x 5 1.5 ind cm , SD 5 1.34, t 5 0.75,
22
df 5 17; P . 0.05; N . fluviatilis x 5 6.36 ind m
, SD 5 28.48, t 5 1, df 5 9; P .
c 22
0.05 and Co . insidiosus x 5 25.4 ind m
, SD 5 66.6, t 5 0.8, df 5 12.7; P . 0.05.
c 22
Also, no treatment effect was found for H . similis x 5 95.5 ind m
, SD 5 48.3,
22
t 5 1.3, df 5 18; P . 0.05 and the nemertean non-identified species x 5 9.1 ind m ,
c
SD 5 6.6, t 5 1.9, df 5 9; P . 0.05, which were both present in the lower layers.
c
MDS and ANOSIM showed significant grouping of treatments stress 5 0.001; R 5 0.39 P , 0.01; Fig. 9, indicating differences between treatments in the community
structure. 3.2.3. Effect of crabs on recruitment of Cy. angulatus
Juveniles of the crab Cy . angulatus carapace width: x 5 5.82 mm, SD 5 1.35, n 5 18
22
were found only in the control treatment density: x 5 165.5 ind m , SD 5 46.7,
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Fig. 6. Results on meiofauna of the experiment to discriminate the effect of crabs. Density ind cm of A
nematodes, B copepods and C ostracods in each experimental treatment: C . granulata inclusion CG INC,
birds and fishes exclusion BF EXC, control CON, cage control CAGE CON, Cy . angulatus inclusion
CA INC, and total exclusion EXC. Lines connect no significant differences P . 0.05.
n 5 10. These crabs made superficial burrows that covered the entire control areas. The most abundant infaunal organisms in this period were H
. similis, copepods and nematodes. No differences among treatments were found for H
. similis at any depth layer ANOVA: superficial layer: F 5 2.56, df 5 2, 17; deep layer: F 5 4.36, df 5 2, 17;
P . 0.05; Fig. 10A. Abundance of this polychaete was the same in both depth layers t 5 0.08, df 5 29; P . 0.05. However, copepods were found only in the upper layer,
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275
Fig. 7. MDS ordination of replicates of the different treatments in the experiment to discriminate the effect of crabs from other predators. C
. granulata inclusion CG INC, birds and fishes exclusion BF EXC, control CON, cage control CAGE CON, Cy
. angulatus inclusion CA INC, and total exclusion EXC.
and no significant differences in abundance among treatments were found log transformed data: ANOVA F 5 2.59, df 5 2, 24, P . 0.05; Fig. 10B. Nematodes
decreased significantly in the upper layer when Cy . angulatus crabs were included log
transformed data, ANOVA: F 5 4.01, df 5 2, 20, P , 0.05, Fig. 10C. However, no treatment effect was found at the lower layer ANOVA: F 5 0.75, df 5 2, 20, P . 0.05.
Nematodes were more abundant in the superficial layer t 5 2.76, df 5 24, P , 0.05; Fig. 10C. Other organisms found in this experiment in low abundance with no differences
22
between treatments were the polychaete Neanthes succinea x 5 46.7 ind m , SD 5
22
78.3; Kruskall–Wallis test: H 5 0.75 ind m , df 5 2, 30, P . 0.05, L
. acuta x 5 25.47
22
ind m , SD 5 77.7; Kruskall–Wallis: H 5 0.67, df 5 2, 30, P . 0.05, Co
. insidiosus
22 22
x 5 11 ind cm , SD 5 22; Kruskall–Wallis: H 5 4 ind m
, df 5 2, 30, P . 0.05 and
22
the nemertean non-identified species x 5 21.2 ind m , SD 5 48.2; Kruskall–Wallis:
H 5 4.5, df 5 2, 30, P . 0.05. 3.2.4. Inclusion of crabs in an area not inhabited by crabs
The most abundant infauna in this experiment were the polychaetes H . similis and L.
acuta, nematodes and copepods. Both polychaete species were found only in the superficial layer and significantly decreased in abundance in the inclusion treatment H
. similis: log transformed data t 5 2 2.46, df 5 12; Fig. 11A; L
. acuta: t 5 2 2.34, df 5 14; P , 0.05; Fig. 11B. Copepods were also found only in the upper layer, but no
significant differences between treatments were found log transformed data: t 5 0.86, df 5 12, P . 0.05; Fig. 11C. Nematodes were found in both depth layers but were more
abundant in the superficial layer t 5 5.88, df 5 13, P , 0.05. These organisms showed
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Fig. 8. Results of the experiment to evaluate crab effect on organisms living at two depths in sediment.
22 22
Density of A the polychaete L . acuta ind m
and B nematodes ind cm , in exclusion and inclusion
treatments and at both depth layers. Lines connect no significant differences P . 0.05.
Fig. 9. MDS of experiment samples of EXCLUSION and INCLUSION of C . granulata.
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277
22 22
Fig. 10. Density of A the polychaete H . similis ind m
, B copepods ind cm and C nematodes ind
22
cm , in the three treatments, and at both depth layers. Lines connect no significant differences.
no differences in abundance between treatments at any layer upper layer: t 5 2 0.35, df 5 12; lower layer: t 5 0.84, df 5 12; P . 0.05, Fig. 11D. Other organisms found in
22
low density without differences between treatments were ostracods x 5 0.13 ind cm ,
22
SD 5 22; t 5 0.6, df 5 12, P . 0.05 Co . insidiosus x 5 27.2 ind m
, SD 5 54.2;
22
t 5 0.61, df 5 12, P . 0.05 and the nemertean non-identified species x 5 27.2 ind m ,
SD 5 54.2; t 5 0.6, df 5 12, P . 0.05. MDS ordination did not show grouping of treatments stress 5 0.00001; ANOSIM:
R 5 0.09, P . 0.01; Fig. 12. 3.2.5. Effect of juvenile crabs on abundance of meiofaunal organisms
During this experiment, the meiofauna was composed only by nematodes. These organisms were significantly more abundant in the upper layer than in the lower layer
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22
Fig. 11. Density of A H . similis ind m
, B L . acuta ind m
, C copepods ind cm and D
22
nematodes ind cm , in INCLUSION and CONTROL treatments outside crab bed, and at both depth layers.
Lines connect no significant differences.
[t 5 5.04, df 5 9, P , 0.05; data were transformed by x9 5 log x 1 1 to meet normali- ty]. However, no effect of juvenile crabs was found at any layer upper layer: ANOVA:
F 5 2.96, df 5 2, 27; lower layer: log transformed data, ANOVA: F 5 0.44, df 5 2, 27; P . 0.05; Fig. 13. Copepods and ostracods were also found in this experiment, but due
to their low density no statistical analysis was performed.
4. Discussion
