Dispersion of the sandy-beach amphipod Eohaustorius brevicuspis Bosworth Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rj430669v

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  • The microdistribution of the amphipod Eohaustorius brevicuspis was examined over a 19 month period. Field sampling was conducted principally at Lost Creek State Park, Oregon, and on one occasion at Driftwood State Park, Oregon. Both are high-energy, fine sand beaches which appear to be uniform. Stratified random samples were taken with each of a series of corers of varying diameters. For the samples obtained for each size of corer, indices of dispersion were calculated and evaluated to obtain information on the size of patches, the distribution of individuals within patches, and the distribution of the patches. E. brevicuspis from Lost Creek at natural densities were placed in a box of thoroughly sieved, well-mixed sand in the laboratory, together with natural densities of other macrofauna, or with other macrofauna excluded. The positions of individuals in the box were determined by partitioning the Sand into 192 blocks (2 cm by 2 cm) horizontally, and into 3 layers (6.6 cm deep) vertically. In two of five experiments, the length and sex of every individual were also recorded Additional experiments were conducted to examine the predation rate of the isopod Cirolana harfordi on E. brevicuspis, to test for endogenous tidal periodicity in the depth in the sand at which E. brevicuspis is found, and to determine the direction of burrowing during downward migration. Patches of higher density were variable in size, but occurred most frequently with diameters near 15 cm. In the field, larger-scale patches with diameters of two meters were also found In laboratory experiments, the number of individuals per patch and the number of patches varied with the overall density. Patches were surrounded by low-density areas containing from 1/4 to 1/10 as many individuals per unit area. Individuals within patches tended to be spaced uniformly. It was not possible to determine the distribution of patches. Patches were formed in the laboratory in the absence of predators, other macrofauna, and observable environmental heterogeneity. They appear to be present at all times, although downward migration at low tide and upward migration at high tide was indicated. Over six hours, an artifically created patch of dyed amphipods migrated downward but did not spread horizontally, suggesting that individual patches could remain intact through one or more tidal cycles. There was no segregation of different sexes or sizes into different patches, although smaller individuals tended to be located nearer the surface of the sand. Patchiness is this species is probably not caused by responses to physical environmental heterogeneity, or to other macrofauna (including both predators and competitors), nor by behavior associated with reproduction. Some possible consequences of the observed distribution of this species were discussed.
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