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<br />1122 THE JOURNAL OF PARASITOLOGY, VOL. 69, NO, 6, DECEMBER 1983
<br />
<br />under natural conditions (except see Wassom et
<br />aI., 1974) and especially in piscine hosts.
<br />Third, it could be argued that there is tem-
<br />perature-dependent selective mortality which af-
<br />fects heavily infected hosts more quickly than
<br />less heavily infected individuals, Such mortality
<br />would be different from the rejection response
<br />described by Kennedy (1969) in that it would be
<br />directed at the host rather than the parasite, but
<br />would have the same impact on parasite infra-
<br />populations, A mechanism for causing the tem-
<br />perature-dependent selective mortality response
<br />cannot, at present, be considered any further.
<br />A fourth mechanism which may operate to
<br />reduce the number of B, acheilognathi in mos-
<br />quitofish is related to the so-called "crowding
<br />effect" (Read, 1951; Roberts, 1961), According
<br />to Holmes et aI. (1977), the crowding effect, or,
<br />in ecological jargon, "intraspecific exploitative
<br />competition," may affect (1) the number of par-
<br />asites which either establish or survive to a re-
<br />productive state, (2) the proportion of surviving
<br />parasites which reproduce, or (3) the number of
<br />eggs produced per reproducing parasite.
<br />While temperature-dependent rejection, im-
<br />munity, or selective host mortality could be a
<br />factor in causing mean infrapopulation densities
<br />of B, acheilognathi to decline, we favor the no-
<br />tion of intraspecific exploitative competition,
<br />Based on both laboratory and field observations,
<br />seasonal changes in growth and maturation of B,
<br />acheilognathi could easily promote intraspecific
<br />competition during certain times ofthe year. Our
<br />studies show that B, acheilognathi infrapopula-
<br />tion densities remained relatively constant dur-
<br />ing the winter months when no development of
<br />the cestode took place and when most of the
<br />individuals present were immature. However,
<br />with increasing water temperatures in the spring,
<br />and with concomitant growth and development
<br />of individual cestodes, infrapopulation densities
<br />decreased. This clearly suggests that competition
<br />for space or nutrients may have occurred. Another
<br />factor indicating the possibility of competitive
<br />interaction is the extreme size difference between
<br />nonsegmented and segmented or gravid worms,
<br />and mosquitofish. Nonsegmented worms are
<br />generally lttss than 1.0 mm in total length while
<br />gravid worms in mosquitofish may range up to
<br />4.0 cm; Davydov (1978) has shown an exponen-
<br />tial relationship between weight (= biomass) and
<br />length of B, acheilognathi, The average length of
<br />mosquitofish in Belews Lake varies seasonally,
<br />ranging from about 2,0 to nearly 4,0 cm, The
<br />highest monthly average was 3.72 ::t 4,5 (SE) cm
<br />
<br />in June. In July, the average length of females
<br />had dropped to 1.96 ::t 2,6 (SE) cm, The sharp
<br />decline was the result of a combination of new
<br />recruitment into the host population and to nat-
<br />ural mortality of senescent females, (The poten-
<br />tial role of this turnover in the host population
<br />is discussed in Granath and Esch, 1983,) The
<br />disparity in size between the host and the mature
<br />parasite during these periods strongly suggests
<br />that G, affinis is incapable of supporting large
<br />infrapopulations of segmented or gravid worms
<br />and that competitive interactions could well serve
<br />to reduce and maintain smaller densities when
<br />water temperatures are rising in the spring or at
<br />their highest during summer.
<br />Experimental laboratory studies indicate that
<br />water temperature controls maturation and
<br />growth of B, acheilognathi. When mosquitofish
<br />harboring only nonsegmented worms were ex-
<br />posed to three different temperature regimes, de-
<br />velopment was shown to be temperature-depen-
<br />dent, No apparent maturation of the cestode
<br />occurred at 20 C, while nearly all the parasites
<br />in mosquitofish held at 30 C became segmented,
<br />Furthermore, the mean density of B, acheilo-
<br />gnathi remained constant throughout the exper-
<br />iment at 20 C, whereas density decreased as ces-
<br />tode development was stimulated at higher
<br />temperatures. These observations also indicated
<br />that competition would be encouraged when
<br />growth and development are stimulated by rising
<br />temperatures.
<br />Water temperature plays a crucial role in the
<br />development and activity of other life cycle stages
<br />of B, acheilognathi, Laboratory studies show that
<br />maturation and hatching of eggs, and the motility
<br />of coracidia, are low at 20 C, rise at 25 C, peak
<br />at 30 C, and decline at 35 C. Results of these
<br />laboratory investigations are also reflected in the
<br />seasonal dynamics of B, acheilognathi in Belews
<br />Lake, When water temperatures are below 25 C,
<br />egg hatching and coracidium motility are re-
<br />duced to such an extent that successful recruit-
<br />ment is stopped or is certainly curtailed, When
<br />water temperatures rise above 25 C, growth and
<br />development of segmented worms are stimulat-
<br />ed, egg maturation and hatching are stimulated,
<br />and recruitment of coracidia by copepods takes
<br />place, Temperatures of 35 C and above, such as
<br />occur at the thermally altered station during cer-
<br />tain periods each summer, reduce egg hatching;
<br />recruitment of the parasite by the intermediate
<br />or definitive host is interrupted during most of
<br />the same period.
<br />Kennedy (1977) argued that parasite popula-
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