G .A. Croll, J.B. McClintock J. Exp. Mar. Biol. Ecol. 254 2000 109 –121
113
mm x 5 grain size. The numbers of grains per square millimeter in each category were counted and the percent comprising each size fraction determined. A non-parametric
mixed model two-factor ANOVA Potvin and Roff, 1993; Zar, 1999 was used to determine if significant differences existed in the granulometry of open and vegetated
areas in the marshes at Dauphin Island and Hunting Island. Correspondingly, a Kruskal–Wallis test Potvin and Roff, 1993; Zar, 1999 was used to detect differences
across the four biogeographic regions. Differences in granulometry between open and vegetated sites could indicate a differential capacity of the substratum to support
breeding burrow construction and provide an alternate reason for male aggregation in open areas.
3. Results
Significant P , 0.05 differences in mean male female ratios were detected when sub-populations of crabs in open areas were compared to those in vegetated areas within
marshes at Dauphin Island, Hunting Island, and Wallops Island F 5 24.9, df 5 1. In all cases, male female ratios were 3 to 4 times greater in open areas compared to vegetated
sites Table 2. The marsh at Saxis was excluded from analysis because only one suitable open area was available for observation.
Mean juvenile adult ratios in vegetated areas within marshes at Dauphin Island, Hunting Island, and Wallops Island were greater than 2.16 while the same sub-
populations in open areas had no juveniles. Significant P , 0.05, F 5 4.03, df 5 2 differences in mean juvenile adult ratios occurring in vegetated areas were detected
between biogeographic regions and species. Further analysis using the Tukey method indicated that the Wallops Island marsh had particularly high densities of juveniles
Table 2 Male Female ratios of Uca spp. in open and vegetated areas of marshes at Dauphin Island, Alabama, Hunting
a
Island, South Carolina, Saxis, Virginia, and Wallops Island, Virginia Dauphin
Hunting Saxis
Wallops Island
Island Island
b
Open areas Uca pugnax
– 4.561.5
– 4.960.6
n 5 10 n 5 5
Uca longisignalis 2.5, 5.5
– –
– n 5 2
Uca minax –
– 4.3
– n 5 1
Vegetated Uca pugnax
– 1.360.2
1.160.2 2.060.3
marsh n 5 46
n 5 10 n 5 50
Uca longisignalis 1.560.0
– –
– n 5 53
Uca minax –
– 1.160.2
– n 5 22
a 2
Shown are the means61 S.E. n 5 number of independent 1-m quadrats sampled.
b
Species not present.
114 G
.A. Croll, J.B. McClintock J. Exp. Mar. Biol. Ecol. 254 2000 109 –121 Table 3
Juvenile adult ratios of Uca spp. in open and vegetated areas of marshes at Dauphin Island, Alabama, Hunting
a
Island, South Carolina, Saxis, Virginia, and Wallops Island, Virginia Dauphin
Hunting Saxis
Wallops Island
Island Island
b
Open areas Uca pugnax
– NJ
– NJ
Uca longisignalis NJ
– –
– Uca minax
– –
NJ –
Vegetated Uca pugnax
– 3.160.9
NJ 7.862.0
marsh n 5 45
n 5 50 Uca longisignalis
2.260.04 –
– –
n 5 32 Uca minax
– –
NJ –
a
Both the vegetated marsh habitat and specific open areas where large males aggregate are shown. Shown are the means61 S.E. NJ, No juveniles.
b
Species not present.
P , 0.05, Table 3 when compared to the marsh at Wallops Island. The marsh at Saxis was excluded from analysis because no juveniles were encountered in any sampled open
or vegetated area Table 3. Examination of open and vegetated areas within all four marshes indicated significant
P , 0.05, F 5 116.2, df 5 1 differences in the mean percent time devoted to reproduc- tive behavior Fig. 1. In all marshes, adult male fiddler crabs in open areas devoted a
greater percentage of their time to reproductive behaviors than adult male crabs in vegetated areas. In contrast, there were no significant differences in the mean time
Fig. 1. Mean61 S.E. percent time allocated to reproductive behavior in open versus vegetated areas derived from 1-h observations of individuals at Dauphin Island, Alabama Uca longisignalis; n 5 10, Hunting Island,
South Carolina Uca pugnax; n 5 10, Saxis, Virginia Uca minax; n 5 5, and Wallops Island, Virginia Uca pugnax; n 5 8.
G .A. Croll, J.B. McClintock J. Exp. Mar. Biol. Ecol. 254 2000 109 –121
115 Table 4
Mean percent time61 S.E. of selected reproductive behaviors exhibited by adult Uca male crabs in open
a
marsh areas Burrow
Burrow Claw
Maintenance Defense
Waving mean time
mean time mean time
Dauphin Island n 5 10 160.7
360.7 3366.0
Hunting Island n 5 10 361.7
461.2 3466.7
Saxis n 5 5 562.7
060.3 55614
Wallops Island n 5 8 764.3
160.4 462.0
a
Means are derived from pooled observations of individuals representing three species in four marshes: Dauphin Island, AL Uca longisignalis, Hunting Island, SC Uca pugnax, Saxis, VA Uca minax, and
Wallops Island, VA Uca pugnax. The mean percent times are based on 1-h observational blocks.
allocated to reproductive behaviors between the four marshes P . 0.05, F 5 1.6, df 5 3 or between P . 0.05, F 5 2.2, df 5 3 fixed vegetation type and random marsh
location effects Table 4. Comparison of substratum organic content between open and vegetated areas within
the two marshes examined indicated significant P , 0.05, F 5 16.1, df 5 1 differences in substratum organic content at both Dauphin Island and Hunting Island Fig. 2. In
both marshes the mean percent organic content was less in open areas Dauphin Island 5 15.4; Hunting Island 5 5.5 than in vegetated areas Dauphin Island 5 18.9;
Hunting Island 5 12.8. Conversely, there were no significant P . 0.05, F 5 0.7, df 5 1 interactions between fixed vegetation type and random marsh location factors within
Fig. 2. Mean61 S.E. percent organic content of the surface substratum collected in open and vegetated sites at Dauphin Island, Alabama open, n 5 16; vegetated, n 5 52 and Hunting Island, South Carolina open, n 5 25;
vegetated, n 5 23.
116 G
.A. Croll, J.B. McClintock J. Exp. Mar. Biol. Ecol. 254 2000 109 –121
Fig. 3. Mean61 S.E. percent organic content of the surface on vegetated substratum at Dauphin Island, Alabama n 5 52, Hunting Island, South Carolina n 5 23, Saxis, Virginia n 5 59, and Wallops Island,
Virginia n 5 61.
the same marshes. When vegetated site substratum organic content was compared across the four biogeographic regions, a significant H 5 120.9, df 5 3 difference was detected.
Further analysis using the Tukey method indicated that only the Dauphin Island and Wallops Island marshes had similar substratum organic content Fig. 3.
Granulometric analysis of substratum from open and vegetated areas of Dauphin Island and Hunting Island indicated no significant P . 0.05, F 5 0.0, df 5 1 differences
in the mean percent of any grain size category at either marsh Fig. 4. Similarly, there were no significant P . 0.05 interactions between the two marshes F 5 9.3, df 5 1 or
between the marshes and the type vegetation F 5 0.1, df 5 1. Examination of substratum in all sampled biogeographic areas revealed no significant P . 0.05, H 5
2.0, df 5 3 differences in the mean percent of grains in any size category Fig. 5.
4. Discussion
