Lepomis cyanellus were fed 10, 20, or 40 Leptorhynchoides thecatus cystacanths, and examined after 1, 3, 7, 14, 28, or 56 days. Some cystacanths remained in the stomach 1 day, and in the small intestine for 7 days. By 14 days, worms were restricted to the pyloric ceca and mesenteries. Parenteric L. thecatus, first observed at 14 days, were encapsulated and destroyed by host cells. After 8 wk, cecal males averaged 3.7 mm in length, and females, 4.8 mm. The number of worms recovered from fish fed 40 cystacanths declined between days 3 and 14, to levels similar to those observed in fish fed 20 cystacanths. No decline was detected at either of the lower dosages. Leptorhynchoides thecatus establishes and matures in the pyloric ceca of this host species. Parasites that enter the mesenteries must be capable of surviving in the alimentary canal for at least 7 days. Lepomis cyanellus is a poor transport host for L. thecatus. Parasite activation and site selection are density-independent processes, but establishment and survival are apparently density-dependent. Lepomis cyanellus provides sufficient resources for the establishment of 10 to 15 L. thecatus. Excess parasites are lost within 14 days. Surveys indicate that Leptorhynchoides thecatus is a common, widely distributed parasite of centrarchid fishes in North America (Lincicome and Van Cleave, 1949; Lang and Edson, 1976; Samuel et al., 1976). However, DeGiusti's (1949) experimental infections of Ambloplites rupestris, performed to confirm the life cycle, are the sole source of information regarding growth, maturation, site selection, and parenteric migration. Each infection was initiated with a fixed, relatively small (10) number of cystacanths, and thus little information on parasite survival and population processes was provided. Weekly recruitment of L. thecatus in Lepomis macrochirus tethered in Gull Lake, Michigan, has been studied (Esch et al., 1976), but the results are puzzling. Although pyloric ceca are considered the primary infection site (DeGiusti, 1949; Venard and Warfel, 1953), Esch et al. (1976) found only 25% of newly recruited L. thecatus in the ceca. Tethered fish recruited L. thecatus throughout the summer, but an accumulation of parasites in untethered fish from the vicinity of the cages was not documented. In the absence of concomitant loss, accumulation is a necessary consequence of a continuous recruitment process. Received 6 April 1981; revised 29 June 1981, 20 October 1981; accepted 9 December 1981. * Present address: 15 Ridgeland Circle, Wallingford, Connecticut 06492. Clearly, there are considerable gaps in our knowledge of this ubiquitous parasite of fishes. Our study, designed to examine population processes in Lepomis cyanellus, an important host in northern Nebraska (Samuel et al., 1976), provides new information on growth, maturation, site selection, and parenteric infections. Many conclusions DeGiusti (1949) derived from A. rupestris cannot be extended to L. cyanellus. MATERIALS AND METHODS Leptorhynchoides thecatus were obtained from naturally infected fish seined at Rice Creek, Oswego Co., New York. Eggs were refrigerated at 4 C within female worms until used to infect amphipods. Hyalella azteca, obtained from Lancaster Co., Nebraska, were exposed to eggs for 1 day in small, aerated plastic cups. The entire contents of the cup were then poured into 9 x 19 x 30-cm plastic culture boxes containing water and algae. In order to allow for growth and development of cystacanths, amphipods were maintained in aerated culture boxes for at least 35 days, and fed spinach. Cystacanths were dissected from amphipods and held up to 1 hr in 0.25% saline before being fed to fish. Several batches of amphipods were required to obtain sufficient parasites for this study. The relative infectivity of parasites from certain batches was quite low, and fish fed less infective cystacanths were excluded from portions of the analysis. Lepomis cyanellus were obtained in Lancaster Co., Nebraska, where Leptorhynchoides thecatus does not occur (Samuel et al., 1976). Fish were isolated in 4-liter glass jars, and fed canned Ken-L-Ration dog food at 2or 3-day intervals. Prior to the experimental period, fish were trained to accept foodfilled No. 5 gelatin capsules. Any fish larger than 55
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