The complete known invertebrate fauna from Woods Hole numbers 1286 species, while in the open marine, somewhat more saline waters, the number is even larger (270, 85).
Brackish Water. In dealing with the brackish-water faunas many difficulties are encountered, because not very much work has been done in connection with the various brackish-water bodies and it is thus hard to obtain data. Two examples, the Baltic Sea, and the Severn Estuary will be discussed.
The Baltic Sea. One of the best known of brackish-water bodies is the Baltic, which, though it cannot be considered an estuary may yet serve admirably to demonstrate the changes in fauna which occur with changes of salinity. The Baltic lacks the tides which are characteristic in estuaries and therefore does not exhibit the pronounced changes from fresh to salt water twice a day. It is more static and shows in a large way the responses of the fauna to salinity. The North Sea has a normal marine salinity of 35.00 permille, which decreases steadily eastward in the Baltic. In the Skager Rak it is 34 permille, off Skagen, the northeasternmost point of Denmark, it is 30 permille, in the Kattegat 22, and in the Bay of Kiel 20 permille. Throughout the southern part of the Baltic, from the "Scheren," at the mouth of the Gulf of Finland, to Bornholm the salinity is from 7 to 8 permille at the surface and does not vary greatly in the depths. For instance, in the deepest part of the Baltic off the Island of Gotland the salinity is only 12 permille, and in the Bay of Danzig, which shows a yearly average of 7.22 permille at the surface, it is only 11.66 permille (average) at the depth of 105 meters. In the Bay of Riga the salinity is 6 permille, in the southern part of the Gulf of Bothnia it is 4 permille and gradually diminishes until the water is entirely fresh. Corresponding to these changes in salinity are certain very definite changes in the fauna (Fig. 1).
As the salinity decreases from that of normal sea water, 35.00 permille, the fauna changes from a typical marine one to one in which only certain groups are represented and finally to an entirely fresh-water fauna. Each phylum shows this change; Pouchet and de Guerne have reported that a truly marine crustacean fauna extends into the Baltic as far as Kalmar Sound, between Öland and Sweden, but that beyond this point the marine species are gradually replaced by certain euryhaline forms and finally by the fresh-water ones until at the head of the Gulf of Finland the planktonic crustacean fauna is made up entirely of fresh-water types. Thus Evadne nordmanni
