230 J
.E. Dugan et al. J. Exp. Mar. Biol. Ecol. 255 2000 229 –245
exposed sandy beaches in California and other regions.
2000 Elsevier Science B.V. All rights reserved.
Keywords : Albuneidae; Hippidae; Beach morphodynamics; Sandy beach; Swash zone
1. Introduction
Exposed sandy beaches are composed of unconsolidated sediments subject to constant movement by waves and represent a challenging and physically unstable habitat for
intertidal organisms. Macrofauna species which inhabit exposed sandy beaches generally exhibit behaviorial and morphological adaptations which allow them to withstand the
harsh physical conditions. The ability to burrow rapidly in disturbed sediments of different grain sizes and to orient in surging swash are central to the success of
individual species and ultimately to the composition of intertidal macrofauna com- munities on exposed sandy beaches.
The structure of intertidal macrofauna communities of exposed sandy beaches is believed to be controlled largely by physical processes such as wave and sediment
dynamics McLachlan, 1990. In general, individual physical factors, such as beach slope or sand particle size, have not been broadly successful in predicting macrofaunal
community structure. However, a dimensionless index Dean’s parameter, e.g. Short, 1996 which incorporates wave height, wave period and sediment fall velocity to
describe beach morphodynamic state has been useful in predicting community structure e.g., McLachlan, 1990, 1996. A number of studies of intertidal macrofauna com-
munities have found that species richness increases linearly and abundance and biomass increase exponentially across a continuum from reflective to dissipative beaches as
values of Dean’s increase McLachlan, 1990; Jaramillo and McLachlan, 1993; McLach- lan et al., 1993, 1995, 1996, 1998; Hacking, 1998.
Intertidal macrofauna experience the wave regime of a particular beach type most directly as swash. The swash zone is the intertidal area landward of the surf zone that is
alternately submerged and exposed by wave wash or swash. Swash climates are generally closely related to the morphodynamic state of the beach McArdle and
McLachlan, 1991, 1992. Long period swashes characteristically occur on flat, dissipa- tive beaches while short period swashes occur on steep, reflective beaches McArdle and
McLachlan, 1991, 1992. McLachlan et al. 1993 proposed the ‘swash exclusion’ hypothesis as a possible explanation for the observed patterns of decreasing richness,
biomass and abundance of macrofauna from dissipative to reflective beaches. That hypothesis states that swash climate is a key factor influencing the intertidal macrofauna
McLachlan et al., 1993, 1995. Across the continuum from dissipative to reflective beaches, swash climate changes from a relatively benign regime which can support
almost all available macrofauna species to an increasingly inhospitable regime which excludes less robust species, until all but the supralittoral forms may be excluded on
reflective beaches. This dynamic is proposed to result in decreasing values of species richness, abundance and biomass of macrofauna communities with increasingly reflec-
tive beach state McLachlan et al., 1993, 1995.
J .E. Dugan et al. J. Exp. Mar. Biol. Ecol. 255 2000 229 –245
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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
