Penetration of parenteral plerocercoids of Proteocephalus ambloplitis (Leidy) into the gut of smallmouth bass.

All 877 smallmouth bass, Micropterus dolomieui Lacepede, more than 7.6 cm in fork length from Lake Opeongo, Algonquin Park, Ontario, had plerocercoids of Proteocephalus ambloplitis (Leidy) in the viscera. The incidence of this species in the gut was seasonal, being highest in the spring, declining in the summer, and disappearing in late autumn. When bass, kept over winter at 4 C, were raised to 7 C or higher, some parenteral plerocercoids left the viscera and penetrated into the gut lumen. Similar penetration, observed only in May and early June at comparable lake temperatures, apparently accounts for the seasonal incidence of P. ambloplitis in the gut. The large end organ of parenteral plerocercoids secreted a substance which apparently aided in penetration. Once the plerocercoid reached the gut, the end organ atrophied. A method for control of this species is suggested. The tapeworm Proteocephalus ambloplitis (Leidy) is one of the most common parasites of the smallmouth bass, Micropterus dolomieui Lacepede. The wandering plerocercoids of this cestode may damage the gonads to the extent that spawning of bass is inhibited (Moore, 1925, 1926; Bangham, 1927; Hunter and Hunter, 1929; Hunter and Hunninen, 1934). Because of the damage suffered by this important sportfish, considerable research concerning the life history of P. ambloplitis has already been done (for review see Hoffman, 1967). The life cycle was originally postulated in 1915 by Cooper, although it was not until 1927 that Bangham specified some of the possible first intermediate hosts. The presently accepted life cycle was established experimentally by Hunter (1928). Basically this is a three-host cycle. The definitive host, usually a smallmouth bass, harbors the mature cestode in the gut. Gravid segments voided with the feces shed eggs on contact with water. "Five (?) species of Cyclops and Hyalella knickerbockeri eat the eggs from the bottom" (Hunter and Hunter, 1929). In suitable species of intermediate hosts the oncosphere penetrates the hemocoel where a "procercoid" may Received for publication 28 February 1969. * Financial support was provided in part by the Ontario Research Foundation, The Canadian National Research Council through Grant A1969, and the Canadian National Sportsmen's Show. Based in part on a paper read at the annual meetings of The American Society of Parasitologists 16-20 June 1968, at Madison, Wisconsin. develop. If infected first intermediate hosts are consumed by a second intermediate host, usually a centrarchid or percid fish (for a more complete list see Hoffman, 1967), the "procercoid" bores through the gut of the fish into its body cavity where further growth may occur. The definitive host acquires the tapeworm by eating infected second intermediate hosts ". . . thus bringing the larval tapeworm back to the digestive tract . . ." (Hunter and Hunter, 1929). In such a cycle the second intermediate host is mandatory. Large bass usually harbor numerous plerocercoids in their viscera, which is a paradox since it is unlikely that these large bass will be eaten by other bass. Formerly such plerocercoids were presumed to be at a dead end since there appeared to be no way for them to reach the gut of the definitive host. This paper presents evidence which indicates that parenteral plerocercoids of P. ambloplitis are not at a dead end but normally leave the viscera and penetrate into the gut of the same bass following a suitable stimulus. The large granular end organ of these plerocercoids apparently is involved in the penetration. MATERIALS AND METHODS Smallmouth bass from Lake Opeongo, Algonquin Park, Ontario, were captured by angling and by gill and trap netting. Bass were autopsied, beginning in May and extending into September, during 1963 through 1968. It was difficult to catch bass at lake temperatures below 7 C, and only 4 bass, 1 in October 1967 and 3 in April 1968, were taken from colder water. Other bass captured alive during the summer and fall were,