Info

NR NR NE NE CM CM CF CF *PC% fPPC% PC% PPC% PC% PPC% PC% PPC%

*PC - presumably pyloric "PRC - post-pyloric

Figure 26 The distribution patterns in percentages of the major parasites (key as in Figure 19) in the pyloric (PC) and post-pyloric (PPC) region of the trout gut.

NR NR NE NE CM CM CF CF *PC% fPPC% PC% PPC% PC% PPC% PC% PPC%

*PC - presumably pyloric "PRC - post-pyloric

Figure 26 The distribution patterns in percentages of the major parasites (key as in Figure 19) in the pyloric (PC) and post-pyloric (PPC) region of the trout gut.

invertebrates, such as those in the River Wye (500-22 000 per m2 according to Edwards and Brooker, 1982).

According to Read (1956) and Smyth (1962) the pyloric caeca region immediately posterior to the pyloric sphincter is the most favourable microhabitat for the gut parasites of salmonid fish. It is relatively calm and as the pancreas opens into it it is rich in digestive enzymes and highly nutritive food. In contrast the contents of the post-pyloric caeca region are subject to greater movement and have a lower nutrient content. These changes along the length of the intestine resemble those described for other teleost fish with pyloric caecae such as the green sunfish (Lepomis cyanellus) (Richardson and Nickol, 1999). These authors found that protein, free amino acid, lipid and carbohydrate concentrations, pH and aminopeptidase activity were all higher in the caecae than in the intestinal region and suggest that this was the reason for the caecae of L. cyanellus being the preferred habitat of the acanthocephalan Leptorhynchoid.es thecatus. According to Kennedy (1996) the pyloric caeca is also the preferred site for Eubothrium crassum in the brown trout and rapid growth of this parasite only occurs in this region of the gut.

In the present investigation it is mainly C. metoecus and the nematode C. truttae which occupy what appears to be the optimal region in the gut for parasites, namely the pyloric caeca region (Figure 26). As these species are not congeneric the extent of niche overlap will be less than for congeneric species such as C. metoecus and C. farionis. Thus, C. metoecus and C. truttae occupy different microhabitats with the former species moving freely in the pyloric lumen whereas the latter is generally attached to the mucosa by its peribuccal teeth. It is therefore possible that both C. metoecus and C. truttae will have a competitive advantage over N. rutili and C. farionis, as the latter tend to occupy the less favourable post-pyloric region of the gut. To what extent these distributional patterns are due to innate behavioural patterns or to short-term behavioural responses due to the presence of potential competitors remains to be elucidated experimentally. However, Chappell (1969) has demonstrated active site selection in the cases of Proteocephalus filicollis and N. rutili in the stickleback. It appears that the former species has a competitive advantage over the latter as when the two species coexist the favoured anterior location in the gut is occupied by P. fillicolis whereas N. rutili attaches more posteriorly. In contrast, in single-species infections the distribution becomes much more widespread in the gut.

10.2.4. Associations between different pairs of helminth species in infracommunities

This aspect of parasite community structure has received a great deal of attention (Anderson and Valtonen, 1990; Lotz and Font, 1991, 1994; Poulin, 2001a).

X2 tests were carried out on the infrapopulations of the trout collected in the present investigation to ascertain whether there were statistically significant tendencies for parasite species to coexist or not. The results (Table 31), show that in nearly all cases the associations are significantly positive (P < 0.05). The only exceptions to this rule are provided by N. rutili/C. metoecus and N. rutili/C. farionis as in these cases the associations are negative although the %2 value is only statistically significant (P < 0.05) in the former case. Rohde (1991) also found when investigating monogenetic parasites in marine fish that 35 of the statistically significant associations found were positive compared with two negatives. However, care is necessary in evaluating such data as a high proportion of rare parasite species, with a low prevalence in the component community, can produce an excess of spurious negative associations, whereas a large proportion of common species can lead to an excess of positive associations (Lotz and Font, 1994).

When the trout were segregated into age groups the N. rutili/C. farionis associations were found to be significantly negative (P < 0.05) in the case of 4-year-old trout. The associations between C. metoecus/C. truttae were also found to be significantly negative (P < 0.05) in the case of 3-year-old trout (Thomas, 1964a).

Regression analyses, using transformed numbers of trout parasites, also indicate that with the exception of N. rutili/C. farionis and N. rutili/C. metoecus, the pairs of parasite species tested exhibit statistically significant positive associations (Table 32). Neither of the above negative associations was statistically significant (P > 0.05).

Results obtained from %2 tests, using 2x2 contingency tables, on infrapopulations of parasites in salmon parr of all ages resemble those for trout as with the exception of D. sagittata/C. farionis all the associations were positive (Table 33). However, only the positive associations involving the nematodes and the other parasite species and N. rutili/C. farionis are statistically significant

Table 31 The %2 results and P values of 2 x 2 contingency tests for the parasites of the brown trout

D. sagittata C. farionis C. metoecus P. simile N. rutili Parasites %2 P x2 P %2 P %2 P y? P

D. sagittata

N. rutili +23.47 0.000 -0.913 0.339 -5.451 0.020 +47.8 0.000-

Nematodes +40.18 0.000 +5.76 0.016 +0.400 0.529 +47.63 0.000 +263.39 0.000

Table 32 The regression coefficients (C), F and P values, based on transformed values, as measures of positive (+) or negative (-) associations between the parasites of brown trout

D. sagittata C. farionis C. metoecus P. simile N. rutili

D. sagittata

0 0

Post a comment