Spatial distribution of the intertidal snail Ilyanassa obsoleta in relation to parasitism by two species of trematodes (original) (raw)
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Distribution of trematodes in snails in ponds at integrated small-scale aquaculture farms
Acta Tropica, 2013
In integrated small-scale aquaculture farming, animal and human excreta maybe used as fish feed and pond fertilizer, thereby enhancing transmission of fish-borne zoonotic trematodes (FZTs) from final hosts, like humans, pigs and chickens, to snails. Areas within a pond could vary in trematode egg-load due to the immediate bordering land, and this might provide implications for control of these trematodes or sampling in field studies measuring FZT prevalence in snails. We therefore estimated the effect of bordering land use on prevalence and FZT burden in snails in different areas within small-scale aquaculture ponds. Nine sampling areas within a pond were assigned in six ponds. For each sampling area, about 120 Melanoides tuberculata snails were collected. Based on land use bordering a sampling area, these were categorized in 5 risk-categories: low-risk (road, rice planted in pond, agriculture, or middle of pond), human access point to pond, livestock sty (pigs or poultry), both human access point and livestock sty, and water connection to canal. In total, 5392 snails were collected. Percentages of snails with parapleurolophocercous cercariae varied between 6% in areas categorized as low-risk and areas with livestock sty only to 15% in areas with both human access point and livestock sty; only this 15% was significantly different from the prevalence in the low-risk category. Percentages of snails with xiphidio cercariae did not differ between risk-categories and varied between 5% and 10%. Mean snail size was 15.2 mm, and was significantly associated with both the probability of infection as well as parasite burden. Very small differences in parasite burden were found at different land use areas; the maximum difference was about 11 cercariae. This study demonstrated only small differences between areas surrounding a pond on risk of snails to be infected with fish-borne trematodes within different pond areas. In field studies on FZTs in M. tuberculata snails in ponds, sampling from ponds can therefore be done without considering areas within ponds.
Controls of Spatial Variation in the Prevalence of Trematode Parasites Infecting a Marine Snail
Ecology, 2008
Geographic variability in abundance can be driven by multiple physical and biological factors operating at multiple scales. To understand the determinants of larval trematode prevalence within populations of the marine snail host Littorina littorea, we quantified many physical and biological variables at 28 New England intertidal sites. A hierarchical, mixed-effects model identified the abundance of gulls (the final hosts and dispersive agents of infective trematode stages) and snail size (a proxy for time of exposure) as the primary factors associated with trematode prevalence. The predominant influence of these variables coupled with routinely low infection rates (21 of the 28 populations exhibited prevalence ,12%) suggest broad-scale recruitment limitation of trematodes. Although infection rates were spatially variable, formal analyses detected no regional spatial gradients in either trematode prevalence or independent environmental variables. Trematode prevalence appears to be predominantly determined by local site characteristics favoring high gull abundance.
The Biological Bulletin, 1985
The estuarine neogastropod Ilyunassa obsoleta, was studied to determine the influence of sex and trematode parasites on the well-known tendency of this snail to aggregate on carrion. Fifteen experimental arenas (1 X 5 m) were delimited on the Cape Henlopen sandflat in Delaware Bay between July 1982 and November 1983. Snails (n == 2111) were examined by dissection. Frequencies of snails in specific categories of sex and parasitism in the arenas were determined before carrion was made available (expected frequencies), and these frequencies were compared with frequencies of the same categories among snails responding to carrion (observed frequencies). Experiments were categorized for analyses into breeding and nonbreeding temporal groups based upon the presence/absence of females with egg cases. Sexual condition alone affected carrion response. Uninfected females in reproductive condition tended to respond more frequently than predicted by controls (positive response) both during and after reproductive season, or else responded as predicted (neutral response). During breeding season males tended to show a neutral response and afterward to respond less frequently than expected (negative response). Parasite influence on carrion response was not detected until after the breeding season, at which time it was pronounced. Both sexes showed a positive response when infected with Lepocreadium setiferoides. Zoogonus rubellus and Gynaecotyla adunca infections produced negative responses in females but neutral responses in males. Austrobilharzia variglandis was inhibitory to females and probably to males as well. Himasthla quissetensis infections seemed not to affect response of either sex. Multiple infections were relatively frequent (9% of snails examined) and also influenced carrion response.
1999
We found that in natural conditions, the freshwater snail Semisulcospira libertina was distributed in deeper sites when infected with trematode larvae than when uninfected, and examined whether water-depth selection was different between infected and uninfected snails in the laboratory. We compared the mean size of infected and uninfected snails, and the correlation between mean water depth and shell size in uninfected snails. Small uninfected snails (up to 11 mm in shell width) showed a correlation between shell width and water depth, while large snails (larger than 11 mm) did not, but used deeper sites on average than smaller snails. In the small-size class, snails infected with mature cercariae used significantly deeper sites than uninfected snails. On the other hand, in the large-size class, snails infected with immature cercariae used significantly shallower sites than uninfected snails. In each size class, snails infected with mature cercariae used deeper sites than snails infected with immature cercariae or parthenitae. Thus, the influence of trematode infection to host behavior varies according to the development stage of trematodes. In the field, snails infected with mature and immature cercariae had a tendency to occur in deeper sites than uninfected snails of both small and large size classes. These results suggest that some factor other than active selection of water depth by snails determined the distribution pattern of infected snails in natural conditions.
The Biological Bulletin, 1985
The estuarine neogastropod Ilyunassa obsoleta, was studied to determine the influence of sex and trematode parasites on the well-known tendency of this snail to aggregate on carrion. Fifteen experimental arenas (1 X 5 m) were delimited on the Cape Henlopen sandflat in Delaware Bay between July 1982 and November 1983. Snails (n == 2111) were examined by dissection. Frequencies of snails in specific categories of sex and parasitism in the arenas were determined before carrion was made available (expected frequencies), and these frequencies were compared with frequencies of the same categories among snails responding to carrion (observed frequencies). Experiments were categorized for analyses into breeding and nonbreeding temporal groups based upon the presence/absence of females with egg cases. Sexual condition alone affected carrion response. Uninfected females in reproductive condition tended to respond more frequently than predicted by controls (positive response) both during and after reproductive season, or else responded as predicted (neutral response). During breeding season males tended to show a neutral response and afterward to respond less frequently than expected (negative response). Parasite influence on carrion response was not detected until after the breeding season, at which time it was pronounced. Both sexes showed a positive response when infected with Lepocreadium setiferoides. Zoogonus rubellus and Gynaecotyla adunca infections produced negative responses in females but neutral responses in males. Austrobilharzia variglandis was inhibitory to females and probably to males as well. Himasthla quissetensis infections seemed not to affect response of either sex. Multiple infections were relatively frequent (9% of snails examined) and also influenced carrion response.