Natural History of Two Beach Hoppers of the Genus Orchestoidea (Crustacea: Amphipoda) with Reference to their Complemental Distribution

Beach hoppers of the genus Orchestoidea, abundant amphipods of the sandy beaches of central California, all live in similar ecological positions on these shores. In this study, differences mainly between the two large species O. californiana and O. corniculata are examined. Competition for burrows between hoppers of the same species is commonly observed. In the early morning hours, large males may be seen fighting for possession of holes left open the night before. Fighting is presumably less energy—consuming than digging a burrow, but since most pugnacity is shown by mature males, possession of a burrow already occupied by a female is also of prime importance. Skirmishes for food items are likewise to be seen. Beach hoppers are eaten by an array of avian predators, mostly diurnal birds, and there is evidence that raccoons, moles, humans, beetles, and other animals take a toll of the hopper populations. Several species of Orchestoidea occur on each of many beaches, but interspecific strife has not been seen often. O. corniculata sometimes occurs at the ends of beaches otherwise dominated by O. californiana, but the reverse situation has never been found. O. californiana is a hopper of long, exposed, flat beaches which are made up of quite fine, well—sorted sands. These shores are commonly backed by dunes or possess wide berms into which the animals can retreat. O. corniculata, on the other hand, is a hopper of short, protected, steep beaches which are made up of fine to coarse, commonly poorly sorted sands. These shores are backed by cliffs or boulder piles preventing escape to the backshore. In the laboratory, individuals of the two species tested for sand size preferences showed O. californiana on the average burrowing in finer sand than did the average O. corniculata. O. californiana has relatively longer appendages than O. corniculata. An analysis, however, indicates no appreciable difference between the two species in the relation of weight and length of the main body mass. Measurements of gill volumes indicate that O. californiana possesses relatively larger gills. Large O. californiana are commonly found in burrows high up on the beach in dry zones relatively far from the reach of the surf, and there they dig holes elliptical in cross section with the entire burrow an open shaft. The sand is actively kicked out onto the surface in two directions. This open burrow, plugged at the top, is used by O. californiana perhaps because oxygen does not percolate so well in the fine sand in which these burrows are constructed, and because these hoppers may need a rapid escape to the surface to avoid cutting surf on their exposed beaches. O. corniculata is never found in the high and dry beach zones; its holes are more nearly circular in cross section; the burrow consists only of an air chamber immediately surrounding the hopper; and the sand is merely pushed onto the surface in a rounded heap. This type of burrow can be dug in fine or coarse sands. In the laboratory, O. corniculata dug faster than O. californiana in both coarse and fine sand, and also faster in coarse than in fine sand. O. californiana dug faster in fine than in coarse sand. O. californiana digs relatively isolated burrows; O. corniculata, closely packed ones. This closeness produces a kind of cooperation in making air spaces and in softening the sand. This lessens the energy expenditure in burrowing in conjunction with the simpler burrowing method, and O. corniculata fights less over used openings in the sand than does O. californiana. In the laboratory, O. californiana tended to dig in wetter sand than did O. corniculata. Wet sands packs more firmly than drier sand and the choice of wet, firm sand by O. californiana mirrors their fine sand beach habitat in which capillarity maintains surface moisture. O. corniculata in nature burrows in softer sand, softer by virtue of its coarser texture, lower water content, and the conditioning by members of the concentrated population. These considerations suggest that the hoppers may locate optimal beach zones by sample digging to estimate the moisture content rather than by using moisture—sensing organs. Test of the relationship between sand particle size, moisture content, and penetrability of sand show that there is an inverse relationship between particle size and total water capacity of sand; there is an inverse relationship between moisture content and penetrability; and there is a direct relationship between sand particle size and penetrability. Osmotic influx represents the lethal factor for these animals in tap water, but sea water has no lethal effects provided enough oxygen is available. O. californiana is more tolerant of fresh water. O. corniculata qualifies for higher success on many beaches because of its high population densities. In contrast, O. californiana may have a larger total population because of the much longer stretched of exposed beach available to it in spite of its much lower concentration at any one point. Although there is some overlap of distribution, the requirements for survival are sufficiently different in two species that Gause's principal is not contradicted.