Distribution related to salinity, and seasonal population changes of certain marine invertebrates of Texas.

Invertebrates were taken in nets over an area extending from fresh water to pure sea water from March, 1941 to November, 1942. Shrimp of the family Penaeidae are the predominant macro-invertebrates of the Texas Coast. Seasonal variations in total abundance are dominated chiefly by changes in the populations of the penaeid shrimp, swimming crabs, and palaemonid shrimp, the latter being confined to bay shores. Small shrimp and crabs tend to remain in the shallows. Invertebrates in waters of low salinity average smaller than those of the same species in saltier water. Most species can withstand high salinities but many cannot withstand low salinities. Therefore, as the salinity decreases along the salinity gradient toward fresh water, certain species disappear. There is no increase in number of species by invasion from fresh water. Therefore, the number of species in waters of low salinity are low, but those present are marine and the number of species in the Gulf is greater than the bays. Certain Gulf species enter the bays and many species grow up there in the warm months. There is a movement of crustacea away from the shallows and a vast seaward movement in the fall, that of the commercial shrimp being most striking. Several species leave the bays entirely during the winter. Similarly certain Gulf species go to deeper water and disappear. In the spring most species spawn and the young start growing up in the bays again.