Substrates as a Factor in Shrimp Distribution

Three species of Western Atlantic penaeid shrimps-Penaeus setijerus, P. axtecus, and P. duorarum-were allowed a free choice among five types of substrates in partitioned experimental troughs. The substrates used were beach sand, shell-sand, muddy sand, sandy mud, and loose peat. Each species was tested separately, and each experiment was replicated once. Results of the experiments showed that distribution of the shrimps on the different substrates was not random. In each experiment departures from the expected distribution were statistically significant. I’. duorarum occurred most often on shell-sand. P. aztecus and P. setijerus were found most frequently on the softer, muddier substrates-loose peat, sandy mud, and muddy sand. Food content in the bottom materials may have been a confounding factor, but the results indicate attraction to substrate aside from the possible attraction to food. Water volume, or pore space, and the compaction of bottom materials, may influence the degree of burrowing. The geographic and bathymetric distribution of Penaeus setif erus (Linn .) , P. aztecus Ives, and P. duorarum Burkenroad in the Western Atlantic is rather well known (Burkenroad 1939, Broad 1950, Springer and Bullis 1954, Hildebrand 1954, 1955, and others). Local patterns of distribution are known in detail for certain well-studied areas, but the influence of environmental factors on these patterns is still not understood. The role of salinity has been discussed as a limiting factor in estuarine nursery areas (Burkcnroad 1939, Gunter 1950, Hildebrand and Gunter 1953, Gunter and Hildebrand 1954, Williams 1955a, b), but the role of such factors as cover, temperature, food, turbidity, and substrate has been treated with less thoroughness. Williams (1955a) discussed the influence of cover on estuarine distribution, and others have considered the subject briefly. Temperature has been treated chiefly in relation to winter kill or low temperature tolerance (Gunter and Hildebrand 1951, Williams 1955a, Lindner and Anderson 1956). Food habits have been considered in general terms which are l I wish to thank Dr. Bernard Pasternack, Dept. of Biostatistics, University of Nort,h Carolina for counsel in planning the statistical approach to this problem, and my colIeagues for counsel and criticism of the manuscript. reviewed in the discussion below. Effects of turbidity have received casual mention by a number of writers, and records concerning relationships between the species and substrates have chiefly dealt with abundance of shrimp and trawlability of bottoms. Among animals so closely associated with the bottom as are littoral penaeids it appears that nature of the substrate may have an important influence on distribution. Many observations attest to this, for it is common knowledge among shrimp fishermen and others that these species are often found on certain types of bottom. However, at other times these mobile animals may search widely for food or cover and be in a measure independent of bottom type, especially during the migration from estuaries to the sea. Factual knowledge of these relationships is not well established. It is not known whether distribution may largely be governed by the nature of the substrate independent of the food supply, or whether favored habitats are those which harbor food regardless of substrate composition. The possibility of species attraction to bottom type has not been