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Induction of spawning and artificial incubation of eggs in the edible snail Pomacea urceus (Muller)
Aquaculture 215 (2003) 163 – 166 www.elsevier.com/locate/aqua-online
Induction of spawning and artificial incubation of eggs in the edible snail Pomacea urceus (Muller) Indar W. Ramnarine * Department of Life Sciences, The University of the West Indies, St. Augustine, Trinidad and Tobago Received 7 August 2001; received in revised form 5 July 2002; accepted 16 July 2002
Abstract The neotropical freshwater gastropod Pomacea urceus is amphibious, herbivorous and attains a size of 145 mm. The snail fetches a high price of $US 5/kg, and current demand is met by collection from the wild, leading to over-exploitation. This is a potential aquaculture species, but there is the need for hatchery production of juveniles, since collection of seedstock from the wild is difficult. Wild-collected adult P. urceus were placed in 0.4 m2 concrete troughs filled with soil and water. Mating was induced by a decrease in water level, and after mating, the snails burrowed into the substratum where egg-laying occurred. The recently laid eggs were carefully removed from the females and placed into darkened aquaria where the temperature was maintained at 27 F 3 jC and the humidity was kept above 80%. After an incubation period of 21 to 34 days, young crawling snails emerged. Mean survivorship was 96%. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Pomacea urceus; Spawning; Egg incubation
1. Introduction The freshwater gastropod Pomacea urceus (Muller) belongs to the family Ampullaridae and is found in Brazil, Colombia, Guyana, Venezuela, and Trinidad (Geijskes and Pain, 1957), inhabiting slowly flowing rivers and swamps. It is a favoured food item in Trinidad and Guyana (Pain, 1950) and fetches a price of $US 5/kg. Snails are collected from the wild, leading to over-exploitation. The snail is amphibious and herbivorous, and can attain a size of up to 145 mm (Burky, 1974). It has been suggested as a potential species for culture
*
Tel.: +1-868-645-3232/9x3093; fax: +1-868-663-9686. E-mail address: [email protected] (I.W. Ramnarine).
0044-8486/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 ( 0 2 ) 0 0 3 6 4 - 2
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I.W. Ramnarine / Aquaculture 215 (2003) 163–166
(Bacon, 1970; Bruce, 1981; Lum Kong, 1989). However, before the species can be commercially cultured, there is the need for a hatchery technology for production of juveniles, including determination of its nutritional requirements for feed formulation. In addition, the husbandry has yet to be developed. The objective of this study was to develop a method of induced spawning and an egg incubation system.
2. Materials and methods 2.1. Induction of spawning Adult P. urceus were collected from the Rio Grande River in Trinidad, West Indies, and brought to the laboratory. They were fed leaves of lettuce (Lactuca sativa), pak choi (Brassica chinensis) and dasheen (Colocasia esculenta). Twenty concrete tanks 60 60 30 cm were each filled with clay to a depth of 15 cm. Ten adult snails (the sexes are not externally distinguishable) were stocked into each tank and each tank was filled with dechlorinated freshwater to a depth of 10 cm. The alkalinity of the water was 68 mg/l (equivalent CaCO3) and the pH was 7.6 pH units. Ten tanks were allowed to dry out via evaporation (Treatment A) while the water level in the other tanks was maintained by frequent addition of water (Treatment B). The animals were carefully observed for 6 weeks. 2.2. Incubation of eggs Twenty clutches of newly laid eggs were carefully removed from the female snails and placed into 10 darkened glass aquaria, each 45 45 30 cm. The eggs were roughly spherical in shape with a mean diameter of 9.32 mm. In five of the aquaria, moistened paper was placed and humidity was maintained at 80% or greater (Treatment C). In the other aquaria, humidity was not controlled and fluctuated between 52% and 85% (Treatment D). The mean temperature was 27 F 3 jC. The eggs were observed until they hatched. The number of snails that hatched, their lengths and the incubation period were recorded. The mortality rate was calculated for each clutch. The data generated were tested with the paired t-test.
3. Results 3.1. Induction of oviposition Mating was observed in all tanks where the water level was allowed to fall (Treatment A; Table 1). Mating lasted several hours and there was little movement of the animals during mating. After about 10 days when the water level fell below 2 cm, the snails began burrowing into the substratum. All females that were known to have mated produced eggs, and there was no repeat mating. The mean number of eggs per female was 55 F 14 S.D. (range 34– 84). Regression analysis revealed that there was no significant relationship between female size and clutch size, although the larger females generally had a larger
I.W. Ramnarine / Aquaculture 215 (2003) 163–166
165
Table 1 Influence of decreasing water level on mating and egg-laying in P. urceus Treatment
Initial water level (cm) (n = 10)
Final water level (cm)
Occurrence of mating (%)a
Occurrence of egg-laying (%)
A B
10 10
0–2 10
100 0
100 0
A: Water level was allowed to decrease via natural evaporation. B: Water level was maintained at 10 cm by regular addition of water. a Occurrence of at least one pair of animals mating in a tank.
clutch size. In tanks where water level was maintained (Treatment B), no mating was observed, and therefore, no eggs were produced. 3.2. Artificial incubation Hatching success averaged 95.7 F 3.8% S.D. (Table 2), when relative humidity was maintained at above 80% (Treatment C). In Treatment D, hatching success averaged 35.2 F 11.3% S.D. The difference between hatching success was very highly significant ( P < 0.001; paired t-test). Mean incubation period was 29.2 F 2.9 days in Treatment C, and 26.3 F 3.5 days in Treatment D. This difference is not quite significant ( P = 0.056). Table 2 Artificial incubation of eggs of P. urceus Treatment
Length of female (mm)
Number of eggs
Incubation period (days)
Number of hatchlings
% Hatching success
C
68.0 75.5 79.3 85.2 85.7 90.1 90.1 90.2 93.6 94.0 85.2 F 8.5 76.1 82.3 84.5 84.6 86.5 88.0 89.7 92.8 93.5 94.0 87.2 F 5.6
53 37 79 55 55 41 63 65 43 51 54 F 13 67 34 45 59 38 51 61 48 84 76 56 F 16
27 31 29 25 32 28 26 34 28 32 29.2 F 2.9 31 29 23 31 28 25 23 27 25 21 26.3 F 3.5
51 33 78 54 53 39 63 65 39 47 52 F 14 21 15 19 11 21 19 12 14 28 31 19 F 7
96.2 89.2 98.7 98.2 96.4 95.1 100 100 90.7 92.2 95.7 F 3.8 31.3 44.1 42.2 18.6 55.3 37.3 19.7 29.2 33.3 40.8 35.2 F 11.3
Mean D
Mean
C: Relative humidity in aquaria maintained above 80%. D: Relative humidity in aquaria not maintained; ranged from 52% to 85%.
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I.W. Ramnarine / Aquaculture 215 (2003) 163–166
4. Discussion In the wild, adult P. urceus mate at the end of the rainy season and burrow into the substratum as the water levels of the rivers and swamps decrease (Lum Kong and Ramnarine, 1988). The snails therefore use a decrease in water level as the cue for spawning. Conditions that occur in the wild were simulated in this study and it was found that a decrease in water level did in fact trigger mating and spawning. In the tanks where the water level was maintained, no mating or spawning took place. This method is cheap and reliable and has important practical applications for the culture of P. urceus. Although adult P. urceus may be left to incubate their eggs, Lum Kong and Kenny (1989) found that egg mortality ranged from 16% to 100% due to predation by dipteran larvae, fungal infection and desiccation. With the incubation method developed in this study, egg mortality was reduced to 4% under humid conditions. Although there were no predation or disease problems, egg mortality in the treatment where relative humidity was left to fluctuate was still 65%. Death of embryos was most likely due to desiccation. Egg incubation in humidity chambers is inexpensive and reliable, and is well suited for use by the small farmer. The average clutch size recorded in this study was 55 eggs, less than that recorded by Burky (1974) who reported clutch size between 50 and 100 eggs in Venezuela. The lower fecundity observed here may be related to adult diet and it may be feasible to increase fecundity by simply placing the broodstock snails on a more appropriate diet for several weeks prior to induction of spawning. Such a diet has yet to be determined.
References Bacon, P.R., 1970. Studies on the biology and cultivation of the mangrove oyster in Trinidad with notes on other shellfish resources. Tropical Science 12, 265 – 278. Bruce, R.W., 1981. Local species with potential for aquaculture in Trinidad and Tobago. The Potential for an Aquaculture Industry in Trinidad and Tobago. Institute of Marine Affairs, Trinidad, pp. 43 – 49. Burky, A.J., 1974. Growth and biomass production of an amphibious snail Pomacea urceus (Muller) from the Venezuelan Savannah. Proceedings of the Malacological Society of London 41, 127 – 143. Geijskes, D.C., Pain, T., 1957. Suriname freshwater snails of the genus Pomacea. Studies on the Fauna Suriname 1 (3), 41 – 48. Lum Kong, A., 1989. The potential of Pomacea urceus as a culture species in Trinidad. BCPC Monograph. No. 41; Slugs and snails in world agriculture. Lum Kong, A., Kenny, J.S., 1989. The reproductive biology of the ampullariid snail Pomacea urceus (Muller). Journal of Molluscan Studies 55, 53 – 65. Lum Kong, A., Ramnarine, I.W., 1988. Biology of the river conch Pomacea urceus (Muller, 1774). U.W.I. Biospectrum 1, 43 – 44. Pain, T., 1950. Pomacea (Ampullariidae) of British Guiana. Proceedings of the Malacological Society of London 28, 63 – 74.
Induction of spawning and artificial incubation of eggs in the edible snail Pomacea urceus (Muller) Indar W. Ramnarine * Department of Life Sciences, The University of the West Indies, St. Augustine, Trinidad and Tobago Received 7 August 2001; received in revised form 5 July 2002; accepted 16 July 2002
Abstract The neotropical freshwater gastropod Pomacea urceus is amphibious, herbivorous and attains a size of 145 mm. The snail fetches a high price of $US 5/kg, and current demand is met by collection from the wild, leading to over-exploitation. This is a potential aquaculture species, but there is the need for hatchery production of juveniles, since collection of seedstock from the wild is difficult. Wild-collected adult P. urceus were placed in 0.4 m2 concrete troughs filled with soil and water. Mating was induced by a decrease in water level, and after mating, the snails burrowed into the substratum where egg-laying occurred. The recently laid eggs were carefully removed from the females and placed into darkened aquaria where the temperature was maintained at 27 F 3 jC and the humidity was kept above 80%. After an incubation period of 21 to 34 days, young crawling snails emerged. Mean survivorship was 96%. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Pomacea urceus; Spawning; Egg incubation
1. Introduction The freshwater gastropod Pomacea urceus (Muller) belongs to the family Ampullaridae and is found in Brazil, Colombia, Guyana, Venezuela, and Trinidad (Geijskes and Pain, 1957), inhabiting slowly flowing rivers and swamps. It is a favoured food item in Trinidad and Guyana (Pain, 1950) and fetches a price of $US 5/kg. Snails are collected from the wild, leading to over-exploitation. The snail is amphibious and herbivorous, and can attain a size of up to 145 mm (Burky, 1974). It has been suggested as a potential species for culture
*
Tel.: +1-868-645-3232/9x3093; fax: +1-868-663-9686. E-mail address: [email protected] (I.W. Ramnarine).
0044-8486/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 ( 0 2 ) 0 0 3 6 4 - 2
164
I.W. Ramnarine / Aquaculture 215 (2003) 163–166
(Bacon, 1970; Bruce, 1981; Lum Kong, 1989). However, before the species can be commercially cultured, there is the need for a hatchery technology for production of juveniles, including determination of its nutritional requirements for feed formulation. In addition, the husbandry has yet to be developed. The objective of this study was to develop a method of induced spawning and an egg incubation system.
2. Materials and methods 2.1. Induction of spawning Adult P. urceus were collected from the Rio Grande River in Trinidad, West Indies, and brought to the laboratory. They were fed leaves of lettuce (Lactuca sativa), pak choi (Brassica chinensis) and dasheen (Colocasia esculenta). Twenty concrete tanks 60 60 30 cm were each filled with clay to a depth of 15 cm. Ten adult snails (the sexes are not externally distinguishable) were stocked into each tank and each tank was filled with dechlorinated freshwater to a depth of 10 cm. The alkalinity of the water was 68 mg/l (equivalent CaCO3) and the pH was 7.6 pH units. Ten tanks were allowed to dry out via evaporation (Treatment A) while the water level in the other tanks was maintained by frequent addition of water (Treatment B). The animals were carefully observed for 6 weeks. 2.2. Incubation of eggs Twenty clutches of newly laid eggs were carefully removed from the female snails and placed into 10 darkened glass aquaria, each 45 45 30 cm. The eggs were roughly spherical in shape with a mean diameter of 9.32 mm. In five of the aquaria, moistened paper was placed and humidity was maintained at 80% or greater (Treatment C). In the other aquaria, humidity was not controlled and fluctuated between 52% and 85% (Treatment D). The mean temperature was 27 F 3 jC. The eggs were observed until they hatched. The number of snails that hatched, their lengths and the incubation period were recorded. The mortality rate was calculated for each clutch. The data generated were tested with the paired t-test.
3. Results 3.1. Induction of oviposition Mating was observed in all tanks where the water level was allowed to fall (Treatment A; Table 1). Mating lasted several hours and there was little movement of the animals during mating. After about 10 days when the water level fell below 2 cm, the snails began burrowing into the substratum. All females that were known to have mated produced eggs, and there was no repeat mating. The mean number of eggs per female was 55 F 14 S.D. (range 34– 84). Regression analysis revealed that there was no significant relationship between female size and clutch size, although the larger females generally had a larger
I.W. Ramnarine / Aquaculture 215 (2003) 163–166
165
Table 1 Influence of decreasing water level on mating and egg-laying in P. urceus Treatment
Initial water level (cm) (n = 10)
Final water level (cm)
Occurrence of mating (%)a
Occurrence of egg-laying (%)
A B
10 10
0–2 10
100 0
100 0
A: Water level was allowed to decrease via natural evaporation. B: Water level was maintained at 10 cm by regular addition of water. a Occurrence of at least one pair of animals mating in a tank.
clutch size. In tanks where water level was maintained (Treatment B), no mating was observed, and therefore, no eggs were produced. 3.2. Artificial incubation Hatching success averaged 95.7 F 3.8% S.D. (Table 2), when relative humidity was maintained at above 80% (Treatment C). In Treatment D, hatching success averaged 35.2 F 11.3% S.D. The difference between hatching success was very highly significant ( P < 0.001; paired t-test). Mean incubation period was 29.2 F 2.9 days in Treatment C, and 26.3 F 3.5 days in Treatment D. This difference is not quite significant ( P = 0.056). Table 2 Artificial incubation of eggs of P. urceus Treatment
Length of female (mm)
Number of eggs
Incubation period (days)
Number of hatchlings
% Hatching success
C
68.0 75.5 79.3 85.2 85.7 90.1 90.1 90.2 93.6 94.0 85.2 F 8.5 76.1 82.3 84.5 84.6 86.5 88.0 89.7 92.8 93.5 94.0 87.2 F 5.6
53 37 79 55 55 41 63 65 43 51 54 F 13 67 34 45 59 38 51 61 48 84 76 56 F 16
27 31 29 25 32 28 26 34 28 32 29.2 F 2.9 31 29 23 31 28 25 23 27 25 21 26.3 F 3.5
51 33 78 54 53 39 63 65 39 47 52 F 14 21 15 19 11 21 19 12 14 28 31 19 F 7
96.2 89.2 98.7 98.2 96.4 95.1 100 100 90.7 92.2 95.7 F 3.8 31.3 44.1 42.2 18.6 55.3 37.3 19.7 29.2 33.3 40.8 35.2 F 11.3
Mean D
Mean
C: Relative humidity in aquaria maintained above 80%. D: Relative humidity in aquaria not maintained; ranged from 52% to 85%.
166
I.W. Ramnarine / Aquaculture 215 (2003) 163–166
4. Discussion In the wild, adult P. urceus mate at the end of the rainy season and burrow into the substratum as the water levels of the rivers and swamps decrease (Lum Kong and Ramnarine, 1988). The snails therefore use a decrease in water level as the cue for spawning. Conditions that occur in the wild were simulated in this study and it was found that a decrease in water level did in fact trigger mating and spawning. In the tanks where the water level was maintained, no mating or spawning took place. This method is cheap and reliable and has important practical applications for the culture of P. urceus. Although adult P. urceus may be left to incubate their eggs, Lum Kong and Kenny (1989) found that egg mortality ranged from 16% to 100% due to predation by dipteran larvae, fungal infection and desiccation. With the incubation method developed in this study, egg mortality was reduced to 4% under humid conditions. Although there were no predation or disease problems, egg mortality in the treatment where relative humidity was left to fluctuate was still 65%. Death of embryos was most likely due to desiccation. Egg incubation in humidity chambers is inexpensive and reliable, and is well suited for use by the small farmer. The average clutch size recorded in this study was 55 eggs, less than that recorded by Burky (1974) who reported clutch size between 50 and 100 eggs in Venezuela. The lower fecundity observed here may be related to adult diet and it may be feasible to increase fecundity by simply placing the broodstock snails on a more appropriate diet for several weeks prior to induction of spawning. Such a diet has yet to be determined.
References Bacon, P.R., 1970. Studies on the biology and cultivation of the mangrove oyster in Trinidad with notes on other shellfish resources. Tropical Science 12, 265 – 278. Bruce, R.W., 1981. Local species with potential for aquaculture in Trinidad and Tobago. The Potential for an Aquaculture Industry in Trinidad and Tobago. Institute of Marine Affairs, Trinidad, pp. 43 – 49. Burky, A.J., 1974. Growth and biomass production of an amphibious snail Pomacea urceus (Muller) from the Venezuelan Savannah. Proceedings of the Malacological Society of London 41, 127 – 143. Geijskes, D.C., Pain, T., 1957. Suriname freshwater snails of the genus Pomacea. Studies on the Fauna Suriname 1 (3), 41 – 48. Lum Kong, A., 1989. The potential of Pomacea urceus as a culture species in Trinidad. BCPC Monograph. No. 41; Slugs and snails in world agriculture. Lum Kong, A., Kenny, J.S., 1989. The reproductive biology of the ampullariid snail Pomacea urceus (Muller). Journal of Molluscan Studies 55, 53 – 65. Lum Kong, A., Ramnarine, I.W., 1988. Biology of the river conch Pomacea urceus (Muller, 1774). U.W.I. Biospectrum 1, 43 – 44. Pain, T., 1950. Pomacea (Ampullariidae) of British Guiana. Proceedings of the Malacological Society of London 28, 63 – 74.