J .E. Dugan et al. J. Exp. Mar. Biol. Ecol. 255 2000 229 –245
231
Direct tests of the swash exclusion hypothesis are problematic but some predictions of the hypothesis can be examined directly. One prediction is that burrowing and
locomotory ability could determine which species may inhabit the swash zone in sandy beaches of different morphodynamic types thus contributing to the community patterns
described above e.g., McLachlan et al., 1995. For example, species which are rapid burrowers may be able to successfully inhabit a wider range of beach morphodynamic
types than species which burrow slowly. Slow burrowing times relative to the swash period likely expose animals to higher swash velocities and additional turbulence from
multiple swashes. Exposure to multiple swashes could physically dislodge and disorient animals leading to stranding, transport to the surf or impact zone, and lateral transport by
longshore currents.
Sediment grain size may also directly limit burrowing for some macrofauna species Alexander et al., 1993; Nel et al., 1999. In general, reflective beaches tend to have
coarser sediments and steeper slopes Short, 1996 along with a harsher swash climate. Unfavorable sediment grain sizes in combination with harsh swash climates could
increase such limitations McLachlan, 1996.
The anomuran crabs of the super family Hippoidea are important components of the macrofauna communities of exposed tropical and temperate sandy beaches Efford,
1976; Trueman, 1970; Trueman and Ansell, 1969; Haley, 1982; Dugan et al., 1995. Three species of hippoid crabs occur on exposed sandy beaches along the California
coast. The common sand crab, Emerita analoga Hippidae is a suspension-feeding tidal migrant which often dominates the abundance and biomass of the intertidal macrofauna
of a wide range of types of exposed sandy beaches Dugan et al., 1995, 2000. Highest densities of this species occur in the active swash zone. Two albuneid crabs, the spiny
sand crab, Blepharipoda occidentalis, and the porcelain sand crab, Lepidopa californica, are less abundant and less widely distributed, occurring in the low intertidal zone and in
sandy sublittoral habitats Fager, 1968; Morris et al., 1980; Morin et al.,1985. The three species co-occur intertidally on a number of intermediate to dissipative type beaches in
California Dugan et al., 2000
We hypothesized that the burrowing abilities and swash behavior of the three species of hippoid crabs could vary, and that differences in distribution of the species among
beaches of different morphodynamic types and, ultimately, macrofaunal community structure may be related to that variation. To test some of the predictions of the swash
exclusion hypothesis, we compared burrowing rates of the three species in five sediment sizes in the laboratory, and investigated pre-burrowing behavior and burrowing of the
three species in the swash zone of a fine sand, reflective beach.
2. Methods
We collected a range of sizes of three species of hippoid crabs Emerita analoga, Blepharipoda occidentalis and Lepidopa californica from the intertidal zone of Pismo
Beach, a dissipative sandy beach in central California, and at Santa Claus Lane Beach, an intermediate beach in southern California Dean’s parameter: 6.1 and 3.2, respective-
ly, during July 1997. Recently molted and late pre-molt stage crabs were not used in the
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.E. Dugan et al. J. Exp. Mar. Biol. Ecol. 255 2000 229 –245
experiments. Crabs were maintained in clean sand and flowing, filtered seawater in the marine laboratory at the University of California, Santa Barbara.
We used five grades of intertidal beach sand in the burrowing trials: very fine, fine, medium, coarse and very coarse Table 1. Those sand grades consisted of 1 sand from
Pismo Beach and 2 four grades made up of sieved fractions of sand from Goleta Beach an intermediate beach, Dean’s parameter: 2.5. Samples of sediments 50 ml were
rinsed with distilled water, dried, sieved 0.5 Phi interval, and weighed to determine mean grain size and sorting.
Burrowing times of crabs were measured in the laboratory in a 4 l bucket 205 mm in diameter with 100 mm of sand and 50 mm of water column. Seawater temperatures
during the burrowing trials were between 18 and 20 8C. Individual crabs were timed with
a stopwatch from the initiation of burrowing abdomen in contact with sediment, uropods active, penetration of the substrate by fourth pereopods to the disappearance of
the carapace under the sediment surface or the end of digging behavior respiratory tube formed, antennae still. Crabs were removed from the sand immediately after burrowing.
We measured the carapace length CL, 0.1 mm, and blotted wet mass 0.01 g of each crab. We also recorded the sex of each crab and reproductive state of female crabs
ovigerous or non-ovigerous.
We calculated OLS regressions for burrowing time s, and 1 CL mm and 2 wet mass g for each species. For each species, we compared the slopes and elevations of
those regressions among the five sediment sizes with ANCOVA. We compared the burrowing times of the three species in the five sediment sizes using a two-way ANOVA
with mass as a covariate.
We calculated the burrowing rate index BRI, see Stanley, 1970; Brown and Trueman, 1994; McLachlan et al., 1995 for each crab in each sediment size as
0.33
BRI 5 Wet mass g
Burrowing time s100 We examined the ability of BRI to remove the effect of crab size for interspecific
comparisons of burrowing ability by determining the significance of correlations of BRI with CL.
We investigated the pre-burrowing and burrowing behavior of Emerita analoga, Blepharipoda occidentalis and Lepidopa californica in active swash in the intertidal
zone of an east-facing beach protected from prevailing NW wind and swell near the
Table 1 Characteristics of the five sediment grades used in the burrowing trials
Sediment grade Mean grain size
Sorting phi
mm phi
Very fine 2.73
0.15 0.27
Fine 2.46
0.18 0.38
Medium 0.87
0.55 0.48
Coarse 20.39
1.31 0.65
Very coarse 21.69
3.24 0.26
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
