Contagious Diseases.—As compared with other animals the fishes of the sea are subject to but few specific diseases. Those in fresh waters, being more isolated, are more frequently attacked by contagious maladies. Often these diseases are very destructive. In an "epidemic" in Lake Mendota, near Madison, Wis., Professor Stephen A. Forbes reports a death of 300 tons of fishes in the lake. I have seen similar conditions among the land-locked alewife in Cayuga and Seneca Lakes, the dead fishes being piled on the beaches so as to fill the air with the stench of their decay.
Fig. 225.—Menhaden, Brevoortia tyrannus (Latrobe). Woods Hole, Mass.
Crustacean Parasites.—The external parasites of fishes are of little injury. These are mainly lernæans and other crustaceans (fish-lice) in the sea, and in the rivers different species of leeches. These may suck the blood of the fish, or in the case of certain crustaceans which lie under the tongue, steal the food as it passes along, as is done by Cymothoa prægustator, the "bug" of the mouth of the menhaden (Brevoortia tyrannus).
Fig. 226.—Australian Flying-fish, Exonautes unicolor (Valenciennes). Specimen from Tasman Sea, having parasitic lernæan crustaceans, to which parasitic barnacles are attached. (After Kellogg.)
The relation of this crustacean to its host suggested to Latrobe, its discoverer, the relation of the "foretaster" in Roman times to the tyrant whom he served. A similar commensation exists in the mouth of a mullet (Mugil hospes) at Panama. The writer has received, through the courtesy of Mr. A. P. Lundin, a specimen of a flying-fish (Exonautes unicolor) taken off Sydney, Australia. To this are attached three large copepod crustaceans of the genus Penella, the largest over two inches long, and to the copepods in turn are attached a number of barnacles (Conchoderma virgatum) so joined to the copepods as to suggest strange flowers, like orchids, growing out of the fish.
