Evolutionary Ecology of Parasites by Poulin Robert

Author:Poulin, Robert
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2007-06-13T16:00:00+00:00

7.3.2 The Nematode Cystidicola cristivomeri

The nematode Cystidicola cristivomeri also has a two-host life cycle. It uses a single species of mysid shrimp, Mysis relicta, as intermediate host. The life cycle is completed after an infected intermediate host is ingested by a suitable fish host (Salvelinus alpinus or S. namaycush). There the nematode migrates to the swimbladder of the host where it matures (Black and Lankester 1980). This parasite is long-lived, and may survive more than ten years in the definitive host. For this reason, the dynamics of infrapopulations do not show seasonal fluctuations but must be examined over longer time spans.

Typically, infected shrimps each harbor a single infective larva. Therefore, a single new recruit is added to an infrapopulation in a fish each time the host consumes an infected prey. In the related C. farionis in a Norwegian population of S. alpinus, the fish host increases its consumption of crustacean prey as it gets older, such that mean infrapopulation sizes increase steadily with fish age (Amundsen et al. 2003). In the Canadian study of C. cristivomeri, however, mysid shrimps are only important food items in the diet of small host fish, and tend to become secondary as fish grow and become increasingly piscivorous. As a consequence, recruitment rates are high in young infrapopulations harbored by young fish hosts, but decrease with age. The pattern observed in natural populations shows an increase in mean infrapopulation size with increasing fish age up to a certain age, which varies among populations in different lakes, after which infrapopulation size stabilizes (Black and Lankester 1981). This probably results from a parasite recruitment rate higher than the parasite mortality rate in the first few years of an infrapopulation’s existence, followed by a period in which both rates are roughly identical until the death of the host.

Whereas infrapopulations may not often reach sizes at which parasite survivorship becomes negatively affected, there is evidence of density-dependent reproductive output in this nematode. Both the proportion of female nematodes reaching sexual maturity and the average length of females are inversely related to infrapopulation size. Since fecundity is strongly correlated with body length in Cystidicola nematodes (Black and Lankester 1981; Black 1985), per capita egg production is therefore reduced in large infrapopulations. Thus both truly density-dependent effects and temporal changes in recruitment rates, which coincide with increases in infrapopulation size but are dependent only on host feeding behavior, contribute to the long-term stability of the nematode population.

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