Annals of Tropical Medicine & Parasitology
ISSN: 0003-4983 (Print) 1364-8594 (Online) Journal homepage: http://www.tandfonline.com/loi/ypgh19
The susceptibility of Marisa cornuarietis, a predator of schistosome bearing snails, to Ntritylmorpholine Claus Meier-Brook & Kuang Yung Tan Tjhen To cite this article: Claus Meier-Brook & Kuang Yung Tan Tjhen (1977) The susceptibility of Marisa cornuarietis, a predator of schistosome bearing snails, to N-tritylmorpholine, Annals of Tropical Medicine & Parasitology, 71:1, 95-100, DOI: 10.1080/00034983.1977.11687165 To link to this article: https://doi.org/10.1080/00034983.1977.11687165
Published online: 24 Mar 2016.
Submit your article to this journal
View related articles
Citing articles: 1 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ypgh19
Annals of Tropical Medicine and Parasitology, Vol. 71, No. 1 (1977)
The susceptibility of Marisa cornuarietis, a predator of schistosome bearing snails, to N-tritylmorpholine BY CLAUS MEIER-BROOK
AND
KUANG YUNG TAN TJHEN*
Tropenmedi;:.inisches lnstitut der Universitiit Tubingen, Federal Republic of GermaTI)I Received 3 September 1975
The ampullariid snail, Marisa cornuarietis (L.), has during the last decade proved an efficient agent for the biological control of the intermediate hosts of Schistosoma haematobium and S. mansoni. Its use, however, cannot fully replace chemical control of schistosome intermediate hosts. After the application of molluscicides the treated water is usually repopulated by the snail hosts within a few months, necessitating repeated treatments. Hence Ferguson and Palmer (1961) recommended combined chemical and biological control. They suggested the use of M. cornuarietis to prevent repopulation following the application of chemical molluscicides. This raised two questions: (I) what is the maximum residual concentration of a given molluscicide which can be tolerated by M. cornuarietis and (2) which concentration would eventually be needed to eliminate the biological control agent in case it spread to cultivated areas and endangered crops? For laboratory trials, N-tritylmorpholine (trade name: Frescon, Shell) was selected as currently one of the most promising molluscicides. The experiments were performed between autumn 1968 and autumn 1973.
MATERIALS AND METHODS The snails used in the experiments were reared from ( 1) strain MCFL (isolated in Florida in 1963), (2) a strain MCPR (from Puerto Rico) and (3) a hybrid stock of these, viz. various generations following a crossing of MCFL-females with MCPR-males. Before the experiments began, the snails were kept in aerated dechlorinated tap water at 25 o± 0 ·5 oc, with a 10-14 hours light-dark-rhythm. The aquaria were illuminated by Osram L40W/15 fluorescent tubes; the light intensity in the aquaria at water surface level was 100-240 Lux. The total hardness of the water was around 26-27 degrees German hardness ( = 9-45 mvalfl ), composed of approximately 130 ppm Ca 2 + and 30 ppm Mg 2 +. These conditions were maintained during the molluscicide exposures. The snails were fed fresh lettuce ad libitum except for the first two weeks of their lives when they were fed standard food (after Standen, 1951). Prior to each experiment the water was aerated for 48 hours. Immediately before exposure an appropiate amount of an emulsion containing 10 ppm of Frescon was added and the experimental medium was stirred. At intervals of 24 hours the medium was replaced by freshly prepared medium and during this procedure the aquaria were brushed and rinsed with clear water. At each change a fresh emulsion of the molluscicide was prepared by adding 60 microlitres of a 16·5% solution of Frescon in tetrachloroethylene (Code-no. FX 28) to tap water and making up to one litre. Any old food was also removed *Present address: Universitatskinderklinik, Ttibingen.
N-TRITYLMORPHOLINE AND SNAIL MAR/SA
96
and replaced by fresh lettuce. Frescon soon becomes unstable due to hydrolysis at pH values below 7·5 (Beynon, Crossland and Wright, 1967), and so pH values were checked every 24 hours (pH-Meter E-444 Metrohm, Herisau, Switzerland). At the end of each exposure time equal numbers of individuals were randomly taken from each jar and, after a brief rinse, transferred to recovery aquaria, where they were observed for 48 hours. The recovery aquaria were aerated 48 hours before use. After 24 hours, the water and food were changed. After 48 hours the mortality was observed. Individuals which were motionless and did not respond to a needle prick were regarded as dead. Eggs were exposed by dividing egg masses into groups of 20, using scissors and forceps. These were transferred to 30 x 30 mm glass tubes, closed at each end with nylon gauze, maximum mesh size 1·5 mm. For exposure the tubes were suspended by nylon threads in the upper parts of the aerated media. At the end of an exposure period they were rinsed and transferred to fresh aerated tap water; this was renewed after 24 hours in order to remove any residues of the molluscicide which would diffuse from the egg capsules into the water. Control experiments were performed in the same way except that no Frescon was added. The hatching rate was determined after 14 days. A molluscicide concentration was termed lethal when hatching was inhibited, regardless of the developmental stages at which the embryos were killed. LT 50 and LC 50 values were estimated according to the method of Spaerman-Karber (after Cavalli-Sforza, 1969).
RESULTS
Eft'ect of 0.03 ppm. Frescon on Snalls In a series of preliminary experiments Tjhen (1972) found that a 24-hour exposure to 0·02 ppm Frescon failed to kill adult snails (52 weeks old) while 0·03 ppm applied for 12-96 hours killed up to 100% (27% after 24 hours); 0·03 ppm was therefore used for all subsequent molluscicide tests. Age-dependent susceptibility is demonstrated in the Fig. The three younger age groups represent postembryonal and immature stages; at 18 weeks
------------:::::;·
100
·'.r-~---o--
/
>.....
:::;
~
-
~,·'
·'"' /
/
.~.
~50
,.,,.
,.,·' /
..................
et:
/
/
/
..... z
0
w (.)
/
•-"
w a..
.-A."'"'
,..,
_,,.A
..,..,.""'
/~
......
/ //
_,.,"
.... ·····
......
,...,"' ...·· .··
...·· ...··.·
,.·" •
MARISA C. 003 PPM FRESCON
...•••
..···········
.//
et:
0
·--
--·p-•--·--·--.1'--·-·--·-·-·::·:::::;:1?-=~.:::.~::"'--.......... -- ,-' .. ··
...···
.a····
-tt·"!:·· ... ·················~· ~~6~--1~2------~2~4------~3~6------7 4~8------~------~7~2------~------~9~6---+
·----c o--o
4---
2WEEKSOL6 5 7
EXPOSURE
TIME
(HOURS)
18
............ 52
Fig. Age-dependent susceptibility to N-tritylmorpholine of a Floridan strain of Marisa comuarietis (MCFL) at 25° C.
MEIER-BROOK AND TJHEN
97
snails of our strains reach sexual maturity, i.e. they begin to spawn; one-year-old snails represent older adults: further growth during the second and third year of life is insignificant. For comparison mature or adult snails of the Puerto Rican strain (MCPR) and of the hybrid strain (MCFL x MCPR) were treated in the same way. The Table, which includes some LT 60 estimates, shows that considerable variation in susceptibility between different strains must be expected. Effect of Varying Concentrations of Frescon on Eggs Three batches each of 20 eggs of the hybrid strain were exposed for 24 hours to doubling concentrations of Frescon, viz. 0·03, 0·06, 0·12, 0·25, 0·50, and 1·00 ppm. As controls 10 batches each of 20 eggs were exposed to a medium without Frescon. The average hatching rate of the latter (94·5 ± 10·4%) was taken as 100 and the mortality of eggs exposed to Frescon related to this figure. A 100% mortality was attained with 0·50 ppm and higher concentrations; 77·0% died in 0·25 ppm, 19·6% in 0·12 ppm, 4·8% in 0·06 ppm, and 3·0% in 0·03 ppm. The latter two results lie within the range of natural variation of hatching rates. The LC 60 value estimated from these figures is 0·14 ppm Frescon for a 24-hour exposure.
DISCUSSION The results show that Marisa and schistosome intermediate hosts have similar susceptibilities to N-tritylmorpholine. According to Boyce, Jones and van Tongeren (1967) the LC 50 for 24-hour exposures is 0·025 ppm for Biomphalariaglabrata, 0·014 ppm for B. pfeifferi and around 0·05 for Bulinus truncatus and B. globosus. Using mortality data for adult Marisa (MCFL) for 24-hour exposure (Tables 4 and 5, Tjhen, 1972) one can estimate an LC 60 of approx. 0·035 ppm. Crossland (personal communication, 1968) suggested that in Marisa "the LC 50 for 24-hour exposures varies from 0·01 ppm for small, young snails up to 1 ppm or more for large adults"; he gave no information on the size of his sample, and an LC 60 of up to 1 ppm appears unusually high. Strain differences alone, as shown in the present paper (Table), are not expected to account for these discrepancies. Moreover Crossland's (1965) figures, obtained from a few experiments with another ampullariid, Lanistes ovum, are much higher than ours for Marisa. Crossland indicated that Lanistes is unaffected by 24hour exposure to 3 ppm of N-tritylmorpholine but shows a mortality of one out of two in 5 ppm. An experiment with two individuals may not be significant; it is nevertheless surprising that a snail could survive such a concentration. One explanation is suggested from our observations. Higher concentrations affect a snail as soon as it makes a first attempt to detach its operculum from the apertural margin of its shell. This leads to an immediate reclosure before much molluscicide can enter the shell, and in this state the snail can obviously survive for 24 hours with virtually no uptake of oxygen. Lower concentrations, e.g. 0·03 ppm, fail to stimulate the snails to withdraw into their shells after first contact. Snails generally moved about for more than two hours before they retracted their head-footmasses. Snails exposed to low concentrations of Frescon began to creep even earlier than control snails. Differences in susceptibility to Frescon of Biomphalaria and Bulinus and an increase in the time-concentration-product with prolonged exposures found for Biomphalaria glabrata (Boyce and Williams, 1967), would render short term applications more economical, but in practice the control of these snails with low concentrations over long periods is preferable. For example, in Southern Rhodesia control of B. pfeifferi and Bulinus globosus with a dose of 0·03 ppm, applied for 10 days, was highly effective without affecting the fish population (Crossland, 1967). B.pfeifferi was controlled with a dose of0·025 ppm applied for 30 days
TABLE
20/3 10/3
5-10
11-19
30-45
MCFL
MCFL
MCFL
MCFL
Hybrid MCFL xMCPR
MCPR
5
7
18
52
18
52 4/3
10/3
8·1±0·1 7·9±0·0
8·2±0·1 7·7±0·1
7·9±0·1 8·0±0·0
8·1 ±0·1 8·0±0·1
8·0±0·1 8·0±0·0
8·0±0·1 8·0±0·0
8·1 ±0·1 8·0±0·0
pH x ±s.d.
0·03 nil
0·03 nil
0·03 nil
0·03 nil
0·03 nil
0·03 nil
0·03 nil
Concentratwn (ppm)
0/20 n.d.
OJ30b n.d.
OJIO"
0/20"
0/30" n.d.
n.d. n.d.
n.d. n.d.
n.d. n.d.
6h
2/20 n.d.
2/30 n.d.
1/20 n.d.
3/40 0/10
n.d. n.d.
25/50 n.d.
34/50 n.d.
12h
2/20 n.d.
2/30 n.d.
1/20 n.d.
OJIO
10/40
OjlO
20/50
32/50 n.d.
47/50 n.d.
24h
30/30 n.d. 19/20 n.d.
17/20d n.d.
9/20 n.d.
28/40 0/20
40/50 0/10
42/50 0/40
49/50 0/50
48h
20/30° n.d.
4/20 n.d.
23/40 0/10
39/50 0/10
41/50 n.d.
49/50 n.d.
36h
30/30 0/30
30/30 0/30
20/20 0/20
20/20 0/10
29/30 0/17
20/20 0/20
30/308 0/30
49/50 0/20
n.d. n.d.
n.d. n.d.
96h
38/40 0/20
46/50 0/20
49/50 0/40
OJ50
50/50
72h
Mortality (dead/total) after exposure of
Notes: n.d.- not done a -exposure added later with progeny of the same stock b-an additional experimental period of 3 hours resulted in 0/30 mortality c-an additional experimental period of 30 hours resulted in 13/30 mortality d- an additional experimental period of 30 hours resulted in 12/30 mortality
32·5±1·5
19·8±2·2
42/2·4
4-8
MCFL
2
4/3
50/3
2-5
Strain
Density (individuals/ L water volume)
Snail diam. (mm), range orx±s.d.
Age (n ±0·5 weeks)
Mortality q[Marisa cornuarietis after exposure for 6-96 h to 0·03 ppm Frescon at 25°C
~ ~ 27·3
~
t""
~......
rJl
t::i
~
z1:'1
t"" ......
:I: 0
"1:1
:;cl
0
~
>
ISSN: 0003-4983 (Print) 1364-8594 (Online) Journal homepage: http://www.tandfonline.com/loi/ypgh19
The susceptibility of Marisa cornuarietis, a predator of schistosome bearing snails, to Ntritylmorpholine Claus Meier-Brook & Kuang Yung Tan Tjhen To cite this article: Claus Meier-Brook & Kuang Yung Tan Tjhen (1977) The susceptibility of Marisa cornuarietis, a predator of schistosome bearing snails, to N-tritylmorpholine, Annals of Tropical Medicine & Parasitology, 71:1, 95-100, DOI: 10.1080/00034983.1977.11687165 To link to this article: https://doi.org/10.1080/00034983.1977.11687165
Published online: 24 Mar 2016.
Submit your article to this journal
View related articles
Citing articles: 1 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ypgh19
Annals of Tropical Medicine and Parasitology, Vol. 71, No. 1 (1977)
The susceptibility of Marisa cornuarietis, a predator of schistosome bearing snails, to N-tritylmorpholine BY CLAUS MEIER-BROOK
AND
KUANG YUNG TAN TJHEN*
Tropenmedi;:.inisches lnstitut der Universitiit Tubingen, Federal Republic of GermaTI)I Received 3 September 1975
The ampullariid snail, Marisa cornuarietis (L.), has during the last decade proved an efficient agent for the biological control of the intermediate hosts of Schistosoma haematobium and S. mansoni. Its use, however, cannot fully replace chemical control of schistosome intermediate hosts. After the application of molluscicides the treated water is usually repopulated by the snail hosts within a few months, necessitating repeated treatments. Hence Ferguson and Palmer (1961) recommended combined chemical and biological control. They suggested the use of M. cornuarietis to prevent repopulation following the application of chemical molluscicides. This raised two questions: (I) what is the maximum residual concentration of a given molluscicide which can be tolerated by M. cornuarietis and (2) which concentration would eventually be needed to eliminate the biological control agent in case it spread to cultivated areas and endangered crops? For laboratory trials, N-tritylmorpholine (trade name: Frescon, Shell) was selected as currently one of the most promising molluscicides. The experiments were performed between autumn 1968 and autumn 1973.
MATERIALS AND METHODS The snails used in the experiments were reared from ( 1) strain MCFL (isolated in Florida in 1963), (2) a strain MCPR (from Puerto Rico) and (3) a hybrid stock of these, viz. various generations following a crossing of MCFL-females with MCPR-males. Before the experiments began, the snails were kept in aerated dechlorinated tap water at 25 o± 0 ·5 oc, with a 10-14 hours light-dark-rhythm. The aquaria were illuminated by Osram L40W/15 fluorescent tubes; the light intensity in the aquaria at water surface level was 100-240 Lux. The total hardness of the water was around 26-27 degrees German hardness ( = 9-45 mvalfl ), composed of approximately 130 ppm Ca 2 + and 30 ppm Mg 2 +. These conditions were maintained during the molluscicide exposures. The snails were fed fresh lettuce ad libitum except for the first two weeks of their lives when they were fed standard food (after Standen, 1951). Prior to each experiment the water was aerated for 48 hours. Immediately before exposure an appropiate amount of an emulsion containing 10 ppm of Frescon was added and the experimental medium was stirred. At intervals of 24 hours the medium was replaced by freshly prepared medium and during this procedure the aquaria were brushed and rinsed with clear water. At each change a fresh emulsion of the molluscicide was prepared by adding 60 microlitres of a 16·5% solution of Frescon in tetrachloroethylene (Code-no. FX 28) to tap water and making up to one litre. Any old food was also removed *Present address: Universitatskinderklinik, Ttibingen.
N-TRITYLMORPHOLINE AND SNAIL MAR/SA
96
and replaced by fresh lettuce. Frescon soon becomes unstable due to hydrolysis at pH values below 7·5 (Beynon, Crossland and Wright, 1967), and so pH values were checked every 24 hours (pH-Meter E-444 Metrohm, Herisau, Switzerland). At the end of each exposure time equal numbers of individuals were randomly taken from each jar and, after a brief rinse, transferred to recovery aquaria, where they were observed for 48 hours. The recovery aquaria were aerated 48 hours before use. After 24 hours, the water and food were changed. After 48 hours the mortality was observed. Individuals which were motionless and did not respond to a needle prick were regarded as dead. Eggs were exposed by dividing egg masses into groups of 20, using scissors and forceps. These were transferred to 30 x 30 mm glass tubes, closed at each end with nylon gauze, maximum mesh size 1·5 mm. For exposure the tubes were suspended by nylon threads in the upper parts of the aerated media. At the end of an exposure period they were rinsed and transferred to fresh aerated tap water; this was renewed after 24 hours in order to remove any residues of the molluscicide which would diffuse from the egg capsules into the water. Control experiments were performed in the same way except that no Frescon was added. The hatching rate was determined after 14 days. A molluscicide concentration was termed lethal when hatching was inhibited, regardless of the developmental stages at which the embryos were killed. LT 50 and LC 50 values were estimated according to the method of Spaerman-Karber (after Cavalli-Sforza, 1969).
RESULTS
Eft'ect of 0.03 ppm. Frescon on Snalls In a series of preliminary experiments Tjhen (1972) found that a 24-hour exposure to 0·02 ppm Frescon failed to kill adult snails (52 weeks old) while 0·03 ppm applied for 12-96 hours killed up to 100% (27% after 24 hours); 0·03 ppm was therefore used for all subsequent molluscicide tests. Age-dependent susceptibility is demonstrated in the Fig. The three younger age groups represent postembryonal and immature stages; at 18 weeks
------------:::::;·
100
·'.r-~---o--
/
>.....
:::;
~
-
~,·'
·'"' /
/
.~.
~50
,.,,.
,.,·' /
..................
et:
/
/
/
..... z
0
w (.)
/
•-"
w a..
.-A."'"'
,..,
_,,.A
..,..,.""'
/~
......
/ //
_,.,"
.... ·····
......
,...,"' ...·· .··
...·· ...··.·
,.·" •
MARISA C. 003 PPM FRESCON
...•••
..···········
.//
et:
0
·--
--·p-•--·--·--.1'--·-·--·-·-·::·:::::;:1?-=~.:::.~::"'--.......... -- ,-' .. ··
...···
.a····
-tt·"!:·· ... ·················~· ~~6~--1~2------~2~4------~3~6------7 4~8------~------~7~2------~------~9~6---+
·----c o--o
4---
2WEEKSOL6 5 7
EXPOSURE
TIME
(HOURS)
18
............ 52
Fig. Age-dependent susceptibility to N-tritylmorpholine of a Floridan strain of Marisa comuarietis (MCFL) at 25° C.
MEIER-BROOK AND TJHEN
97
snails of our strains reach sexual maturity, i.e. they begin to spawn; one-year-old snails represent older adults: further growth during the second and third year of life is insignificant. For comparison mature or adult snails of the Puerto Rican strain (MCPR) and of the hybrid strain (MCFL x MCPR) were treated in the same way. The Table, which includes some LT 60 estimates, shows that considerable variation in susceptibility between different strains must be expected. Effect of Varying Concentrations of Frescon on Eggs Three batches each of 20 eggs of the hybrid strain were exposed for 24 hours to doubling concentrations of Frescon, viz. 0·03, 0·06, 0·12, 0·25, 0·50, and 1·00 ppm. As controls 10 batches each of 20 eggs were exposed to a medium without Frescon. The average hatching rate of the latter (94·5 ± 10·4%) was taken as 100 and the mortality of eggs exposed to Frescon related to this figure. A 100% mortality was attained with 0·50 ppm and higher concentrations; 77·0% died in 0·25 ppm, 19·6% in 0·12 ppm, 4·8% in 0·06 ppm, and 3·0% in 0·03 ppm. The latter two results lie within the range of natural variation of hatching rates. The LC 60 value estimated from these figures is 0·14 ppm Frescon for a 24-hour exposure.
DISCUSSION The results show that Marisa and schistosome intermediate hosts have similar susceptibilities to N-tritylmorpholine. According to Boyce, Jones and van Tongeren (1967) the LC 50 for 24-hour exposures is 0·025 ppm for Biomphalariaglabrata, 0·014 ppm for B. pfeifferi and around 0·05 for Bulinus truncatus and B. globosus. Using mortality data for adult Marisa (MCFL) for 24-hour exposure (Tables 4 and 5, Tjhen, 1972) one can estimate an LC 60 of approx. 0·035 ppm. Crossland (personal communication, 1968) suggested that in Marisa "the LC 50 for 24-hour exposures varies from 0·01 ppm for small, young snails up to 1 ppm or more for large adults"; he gave no information on the size of his sample, and an LC 60 of up to 1 ppm appears unusually high. Strain differences alone, as shown in the present paper (Table), are not expected to account for these discrepancies. Moreover Crossland's (1965) figures, obtained from a few experiments with another ampullariid, Lanistes ovum, are much higher than ours for Marisa. Crossland indicated that Lanistes is unaffected by 24hour exposure to 3 ppm of N-tritylmorpholine but shows a mortality of one out of two in 5 ppm. An experiment with two individuals may not be significant; it is nevertheless surprising that a snail could survive such a concentration. One explanation is suggested from our observations. Higher concentrations affect a snail as soon as it makes a first attempt to detach its operculum from the apertural margin of its shell. This leads to an immediate reclosure before much molluscicide can enter the shell, and in this state the snail can obviously survive for 24 hours with virtually no uptake of oxygen. Lower concentrations, e.g. 0·03 ppm, fail to stimulate the snails to withdraw into their shells after first contact. Snails generally moved about for more than two hours before they retracted their head-footmasses. Snails exposed to low concentrations of Frescon began to creep even earlier than control snails. Differences in susceptibility to Frescon of Biomphalaria and Bulinus and an increase in the time-concentration-product with prolonged exposures found for Biomphalaria glabrata (Boyce and Williams, 1967), would render short term applications more economical, but in practice the control of these snails with low concentrations over long periods is preferable. For example, in Southern Rhodesia control of B. pfeifferi and Bulinus globosus with a dose of 0·03 ppm, applied for 10 days, was highly effective without affecting the fish population (Crossland, 1967). B.pfeifferi was controlled with a dose of0·025 ppm applied for 30 days
TABLE
20/3 10/3
5-10
11-19
30-45
MCFL
MCFL
MCFL
MCFL
Hybrid MCFL xMCPR
MCPR
5
7
18
52
18
52 4/3
10/3
8·1±0·1 7·9±0·0
8·2±0·1 7·7±0·1
7·9±0·1 8·0±0·0
8·1 ±0·1 8·0±0·1
8·0±0·1 8·0±0·0
8·0±0·1 8·0±0·0
8·1 ±0·1 8·0±0·0
pH x ±s.d.
0·03 nil
0·03 nil
0·03 nil
0·03 nil
0·03 nil
0·03 nil
0·03 nil
Concentratwn (ppm)
0/20 n.d.
OJ30b n.d.
OJIO"
0/20"
0/30" n.d.
n.d. n.d.
n.d. n.d.
n.d. n.d.
6h
2/20 n.d.
2/30 n.d.
1/20 n.d.
3/40 0/10
n.d. n.d.
25/50 n.d.
34/50 n.d.
12h
2/20 n.d.
2/30 n.d.
1/20 n.d.
OJIO
10/40
OjlO
20/50
32/50 n.d.
47/50 n.d.
24h
30/30 n.d. 19/20 n.d.
17/20d n.d.
9/20 n.d.
28/40 0/20
40/50 0/10
42/50 0/40
49/50 0/50
48h
20/30° n.d.
4/20 n.d.
23/40 0/10
39/50 0/10
41/50 n.d.
49/50 n.d.
36h
30/30 0/30
30/30 0/30
20/20 0/20
20/20 0/10
29/30 0/17
20/20 0/20
30/308 0/30
49/50 0/20
n.d. n.d.
n.d. n.d.
96h
38/40 0/20
46/50 0/20
49/50 0/40
OJ50
50/50
72h
Mortality (dead/total) after exposure of
Notes: n.d.- not done a -exposure added later with progeny of the same stock b-an additional experimental period of 3 hours resulted in 0/30 mortality c-an additional experimental period of 30 hours resulted in 13/30 mortality d- an additional experimental period of 30 hours resulted in 12/30 mortality
32·5±1·5
19·8±2·2
42/2·4
4-8
MCFL
2
4/3
50/3
2-5
Strain
Density (individuals/ L water volume)
Snail diam. (mm), range orx±s.d.
Age (n ±0·5 weeks)
Mortality q[Marisa cornuarietis after exposure for 6-96 h to 0·03 ppm Frescon at 25°C
~ ~ 27·3
~
t""
~......
rJl
t::i
~
z1:'1
t"" ......
:I: 0
"1:1
:;cl
0
~
>
