Journal of Experimental Marine Biology and Ecology 255 (2000) 229±245 www.elsevier.nl/locate/jembe Burrowing abilities and swash behavior of three crabs, Emerita analoga Stimpson, Blepharipoda occidentalis Randall, and Lepidopa californica Efford (Anomura, Hippoidea), of exposed sandy beaches J.E. Dugana,* , D.M. Hubbard a , M. Lastra b aMarine Science Institute, University of California, Santa Barbara, CA 93106, USA bDepartamento de Ecologia y Biologia Animal, Universidad de Vigo, Vigo, Spain Received 15 May 2000; received in revised form 12 September 2000; accepted 19 September 2000 Abstract To investigate factors related to the distribution of intertidal species, and speci®c predictions of the swash exclusion hypothesis for exposed sandy beaches, we compared the burrowing abilities and swash behavior of three species of anomuran crabs in the superfamily Hippoidea (Emerita analoga, Blepharipoda occidentalis and Lepidopa californica) which commonly inhabit the intertidal and shallow subtidal zones of beaches along the California coast. Burrowing times in the laboratory increased signi®cantly with crab size for all species in ®ve sediment grain sizes ranging from ®ne sand to gravel (0.15 to 3.24 mm). For each species, burrowing times differed signi®cantly among sand grain sizes, ranging from 0.3 to 21.5 s. Burrowing times for the hippid crab, E. analoga, were relatively constant across sediment types, while those of the albuneid crabs, B. occidentalis and L. californica, were rapid in ®ne to medium sands, and much slower in coarser sediments. Our results indicate that E. analoga is a substrate generalist while L. californica and B. occidentalis are substrate sensitive. Pre-burrowing times and behavior, distance moved, and burrowing times differed among the species in the swash zone. Combined times of preburrowing and burrowing were shorter than the swash period (6 s) for most E. analoga individuals. Fifty percent of the individuals of L. californica reached the substrate and burrowed in the swash period, while no individuals of B. occidentalis burrowed in that time. Pre-burrowing behavior and time may be valuable in explaining spatial and temporal patterns in the distribution of hippoid crabs on California beaches. Our results support predictions of the swash exclusion hypothesis concerning the burrowing and locomotory abilities of sandy beach macrofauna. The substrate generalist characteristics, and unique orientation and swimming abilities of the hippid crab, E. analoga, in intertidal swash may help explain the success of this species and its congeners, and have important implications for understanding patterns of macrofauna community structure on *Corresponding author. Tel.: 11-805-893-2675; fax: 11-805-893-8062. E-mail address: j [email protected] (J.E. Dugan). ] 0022-0981/00/$ ± see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0022-0981(00)00294-X 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 re¯ective 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 ¯at, dissipa- tive beaches while short period swashes occur on steep, re¯ective 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 re¯ective beaches. That hypothesis states that swash climate is a key factor in¯uencing the intertidal macrofauna (McLachlan et al., 1993, 1995). Across the continuum from dissipative to re¯ective 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 re¯ective beaches. This dynamic is proposed to result in decreasing values of species richness, abundance and biomass of macrofauna communities with increasingly re¯ec- tive beach state (McLachlan et al., 1993, 1995). 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, re¯ective 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 ®ve sediment sizes in the laboratory, and investigated pre-burrowing behavior and burrowing of the three species in the swash zone of a ®ne sand, re¯ective 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 232 J.E. Dugan