J .E. Dugan et al. J. Exp. Mar. Biol. Ecol. 255 2000 229 –245
233
marine laboratory of the University of California at Santa Barbara on July 28, 29, and 30, 1997. The study site was a modally reflective beach Dean’s parameter
,1.0 with narrow surf and swash zones, small breaker heights, a steep beach face, and prevailing
longshore currents to the NE. Mean grain size was 0.28 mm, a value falling between the fine and medium size grades used in the laboratory trials. Sediments were not well sorted
sorting, I.G.S.D. 51.28 compared to the sand grades used in the laboratory burrowing
trials Table 1. Swash zone dynamics and the slope of the beach at the effluent line during the experiments were typical of conditions measured monthly at the site in the
previous year swash zone width, 5 to 10 m; period, 4 to 8 s; slope, 3 to 5 8 Dugan and
Hubbard, unpublished data. To observe the behavior of crabs in active swash, we marked the upper and lower
limits of the swash zone and the release point for the crabs with PVC stakes. Release points were located 2 to 3 m above the lower limit of swashes. We adjusted the positions
of the stakes approximately every 20 min to account for changes in tidal level. Crabs were released individually into slack water of 5 to 30 cm depth at the time of swash
maximum.
We measured several components of the responses of individuals released in the swash zone including: time elapsed between release and the initiation of burrowing,
distance moved to a burrowing location, and burrowing times. The time between release and the start of burrowing, and the time to completion of burrowing was measured with
a stopwatch. We measured the distance between release point and the location of burrowing in two components: perpendicular and parallel to shore. We also recorded
behavior of each crab from the release point to the burrowing position as: 1 swimming in the water column, 2 drifting passively in the water column, 3 sliding in contact
with the substrate, 4 tumbling across the substrate, and 5 orienting, pivoting to a head down current position in contact with the sediments.
3. Results
The sizes CL of individuals used in burrowing trials formed a continuous size distribution with some overlap in size between the three species: Blepharipoda
occidentalis ranged from 25.3 to 65.5 mm CL, Lepidopa californica from 10.4 to 19.5 mm CL, and Emerita analoga from 5.7 to 33.9 mm CL Table 2. The smallest animals
tested in the laboratory were recently recruited E . analoga which were approximately
the size of the coarsest sediment used in the laboratory burrowing trials. In the very coarse sediments, those small individuals burrowed successfully only when they initiated
burrowing in the gaps between sediment particles. The fastest burrowing time observed in the laboratory was 0.3 s for Lepidopa
californica in very fine sediments Table 2. The slowest burrowing time observed in the laboratory was 21.5 s for Blepharipoda occidentalis in very coarse sediments.
Burrowing time increased significantly with carapace length and mass in each of the 15 treatments three species in five sediment sizes Table 3. No pattern of decreasing
or increasing slope of that relationship was evident with sediment grade in Emerita analoga or Blepharipoda occidentalis. The slope of the relationships between burrowing
234 J
.E. Dugan et al. J. Exp. Mar. Biol. Ecol. 255 2000 229 –245 Table 2
Range of crab sizes and burrowing times in five sediment grades in the laboratory and in the field trials Species
Size range Burrowing time range s
CL mm Very fine
Fine Medium
Coarse Very coarse
Emerita analoga Laboratory
5.7–33.9 1.7–7.2
1.1–8.2 1.0–4.5
1.2–5.6 0.7–7.5
Field 7.3–30.5
1.3–13.4 Lepidopa californica
Laboratory 10.4–19.5
0.3–2.7 0.6–2.9
0.6–2.5 1.7–5.7
2.8–15.3 Field
11.3–19.7 1.4–3.6
Blepharipoda occidentalis Laboratory
25.3–65.5 1.4–11.4
1.0–9.9 2.1–8.8
3.4–13.5 9.2–21.5
Field 37.9–62.2
3.5–36.0
time, and carapace length and mass increased with increasing grain size for Lepidopa californica.
The burrowing rate index BRI did not consistently remove the effect of size for use in interspecific comparisons of burrowing ability of crabs. For Emerita analoga and
Blepharipoda occidentalis, BRI varied significantly with crab size in regressions for eight of the 10 treatments Table 3. BRI increased significantly with crab size for five
of those eight treatments, suggesting that larger crabs may be relatively slower burrowers. For Lepidopa californica, the regressions of BRI and carapace length were
negative and not significant in the five sediment grades Table 3.
Sediment size affected burrowing times for all three species. For each crab species, the slope of regressions of burrowing time and size mass or CL differed significantly
among the five sediment grades ANCOVA for CL: Emerita analoga, F 5 8.69, n 5 175,
P , 0.001; Lepidopa californica, F 5 17.08, n 5 109, P , 0.001; Blepharipoda occiden-
talis, F 5 2.97, n 5 186, P , 0.001. Where those differences occurred varied among the
species. Post hoc tests indicated no difference in slopes among the very fine, fine, and very coarse sediment grades for E
. analoga. For L. californica, there were no differences in slopes among the very fine, fine and medium sediment grades. For B
. occidentalis, only the fine and medium sediment grades did not have different slopes.
The interaction between sediment size and species was significant in the two-way ANOVA with mass as a covariate P
, 0.001, Table 4. Therefore, although the main effects of both species and sediment grade on burrowing time were also significant
P , 0.001, Table 4, those effects on burrowing time must be interpreted with caution.
The patterns of the response of burrowing times to increasing sediment size differed among the three crab species Table 2, Fig. 1. A much greater range of actual
burrowing times was observed for Blepharipoda occidentalis and Lepidopa californica across the five sediment grades |15-fold and |41-fold, respectively than for Emerita
analoga about eight-fold Table 2. For standard sizes, the responses of the burrowing times of the two albuneid crab species B
. occidentalis, L. californica to increasing grain size differed markedly from that of the hippid crab E
. analoga Fig. 1. In medium grade sand, the burrowing rates of the three species were similar. In fine sands,
J .E
. Dugan
et al
. J
. Exp
. Mar
. Biol
. Ecol
. 255
2000 229
– 245
235 Table 3
Ordinary least squares regressions of burrowing time and 1 carapace length, 2 mass, and of BRI burrowing rate index and carapace length for each species in five sediment grades significance levels for r values: ns, not significant, P
, 0.05, P , 0.01, P , 0.001
Species Very fine
Fine Medium
Coarse Very coarse
Slope Inter.
r n
Slope Inter.
r n
Slope Inter.
r n
Slope Inter.
r n
Slope Inter.
r n
Emerita analoga Length
0.14 1.32
0.73 35
0.18 0.08
0.83 35
0.08 0.57
0.79 35
0.07 1.67
0.66 35
0.16 0.89
0.83 35
Mass 0.26
3.05 0.69
35 0.38
2.06 0.80
35 0.17
1.32 0.83
35 0.14
2.47 0.59
35 0.33
2.73 0.73
35 BRI
0.74 18.94
0.54 35
0.39 30.22
0.28 ns 35
1.52 35.04
0.58 35
1.56 11.86
0.81 35
0.43 25.08
0.40 35
Lepidopa californica Length
0.14 20.92
0.66 22
0.15 20.70
0.62 22
0.17 20.73
0.66 22
0.29 20.72
0.60 22
0.95 205.40
0.81 21
Mass 0.26
0.50 0.60
22 0.33
0.69 0.63
22 0.36
0.85 0.65
22 0.60
1.98 0.63
22 2.08
3.54 0.83
21 BRI
210.7 288.74
0.39 ns 22
24.59 163.85
20.34 ns 22
24.44 148.41
20.30 ns 22
20.07 42.97
20.01 ns 22
20.74 26.75
20.31 ns 21
Blepharipoda occidentalis Length
0.20 24.40
0.80 29
0.14 23.33
0.73 45
0.08 20.19
0.61 38
0.21 23.30
0.74 37
0.12 10.42
0.38 37
Mass 0.09
1.35 0.80
– 0.75
0.06 0.75
– 0.04
2.31 0.62
– 0.09
3.48 0.74
– 0.04
14.25 0.34
– BRI
22.02 177.07
0.63 –
22.06 212.67
0.52 –
0.22 81.42
0.09 ns –
20.44 72.29
20.34 –
0.30 7.01
0.56 –
236 J
.E. Dugan et al. J. Exp. Mar. Biol. Ecol. 255 2000 229 –245 Table 4
Results of two-way ANOVA on crab species and sediment grade grain size with mass as a covariate Source
SS DF
MS F-ratio
P Species
86.00 2
43.00 16.71
,0.001 Sediment grade
2493.96 4
623.49 242.29
,0.001 SpeciesSediment
grade 1840.94
8 230.12
89.43 ,0.001
Mass 470.34
1 470.34
182.77 ,0.001
Error 1168.29
454
E . analoga burrowed in longer times than L. californica and B. occidentalis. However,
in coarse sediments, E . analoga burrowed in shorter times than the two albuneid species.
Burrowing times for B . occidentalis and L. californica of standard lengths increased by
340 to 700 from fine to very coarse sediments, while burrowing times for E . analoga
of standard sizes remained relatively constant across all sediment grades Fig. 1. BRI values followed a similar pattern among the three species. Mean values of BRI
for Emerita analoga varied over a narrower range, and were lower than those of the other two species in fine sand and higher in coarse sand. Mean BRI varied from 18 to
150 range 11 to 301 across the five sediment sizes tested for Lepidopa californica, 22 to 109 range 12 to 214 for Blepharipoda occidentalis and 34 to 66 range 16 to 124
for Emerita analoga.
The net movement of all crabs released in the swash zone was generally seaward and to the NE with the longshore current Fig. 2. In general, Emerita analoga moved the
shortest distance m from the release point prior to burrowing Fig. 2. Blepharipoda occidentalis generally moved the greatest distance from the release point before
burrowing successfully, while Lepidopa californica moved an intermediate distance Fig. 2.
The time before burrowing in the swash zone varied among the three species. Pre-burrowing time was not correlated with carapace length for any of the species. Mean
pre-burrowing times were shorter for Emerita analoga than for the other two species, averaging
,5 s and ranging from 1.5 to 13.2 s Fig. 3a. Pre-burrowing times were generally shorter than the average swash period mean swash period 6.2 s
10.38 s, n
5 3 for all but two individual E. analoga and three Lepidopa californica. For L. californica, pre-burrowing times averaged 25 s and ranged from 1.0 to 41.1 s. For
Blepharipoda occidentalis, pre-burrowing times were longer than the average swash period for more than half of the animals observed, averaging
.21 s and ranging from 6.0 to 83.0 s.
Once individual crabs made contact with the sediments, burrowing times in the swash zone were somewhat longer but generally similar to those recorded in the laboratory in
medium grade sediments for Lepidopa californica 1.4 to 3.6 s and Emerita analoga 1.3 to 13.4 s, note: all but one individual burrowed in
,5.5 s Table 2. Burrowing times in field conditions were longer than in the laboratory for Blepharipoda occiden-
talis, ranging from 3.5 to 36.0 s Table 2. Burrowing times in the field increased significantly with carapace length for B
. occidentalis and L. californica but not for E. analoga. However, we used a narrower size range of E
. analoga in the field trials due to
J .E. Dugan et al. J. Exp. Mar. Biol. Ecol. 255 2000 229 –245
237
Fig. 1. Response of burrowing times to sediment grain size for standard sizes of each species of crab: a Emerita analoga, b Lepidopa californica and c Blepharipoda occidentalis.
238 J
.E. Dugan et al. J. Exp. Mar. Biol. Ecol. 255 2000 229 –245
Fig. 2. Estimated distance moved before burrowing, shown as means and standard deviations of the positions of individuals of each species at the initiation of successful burrowing, following standardized release in the
swash zone for j Emerita analoga, d Lepidopa californica and m Blepharipoda occidentalis.
the difficulty of tracking the smaller animals in the swash zone data was collected on only one individual
,20 mm CL. When burrowing and pre-burrowing times were added together, differences between
the three species remained evident in the field trials Fig. 3b. The proportion of individuals that reached the substrate and burrowed successfully in times less than or
equal to the average swash period 6.2 s varied among species. The majority of Emerita analoga individuals reached the substrate and burrowed in times less than or equal to the
swash period. For Lepidopa californica, 50 of the individuals reached the substrate and burrowed in that time. For Blepharipoda occidentalis, the time to reach the substrate
and burrow was greater than the swash period for all the individuals observed.
The behavioral response to swash conditions also varied noticably between the three species Fig. 4. Individual Emerita analoga were not observed tumbling, swimming at
the surface, or sliding prior to orienting and burrowing and were not dislodged from the substrate by the swash once burrowing had been initiated. Lepidopa californica
individuals typically swam or drifted at the water surface with the abdomen closest to the water surface prior to orienting and burrowing and were not observed tumbling or
sliding Fig. 4. This species was not dislodged by swash once burrowing was initiated.
J .E. Dugan et al. J. Exp. Mar. Biol. Ecol. 255 2000 229 –245
239
Fig. 3. a Pre-burrowing times, measured as the time between release and the initiation of successful burrowing, for individuals of the three species released in the swash zone. b The sum of pre-burrowing and
burrowing times for individuals of the three species released in the swash zone. For both figures: s Emerita analoga, n Lepidopa californica and
1 Blepharipoda occidentalis.
240 J
.E. Dugan et al. J. Exp. Mar. Biol. Ecol. 255 2000 229 –245
Fig. 4. Pre-burrowing behaviors of individuals of each species prior to the initiation of successful burrowing following standardized release in the swash zone.
Blepharipoda occidentalis individuals tumbled and or drifted in the swash, and one individual slid prior to burrowing. Thirty percent of the individuals of this species
became dislodged by swash after burrowing had been initiated.
4. Discussion
