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Influence of nutrition on fecundity and survival in Tigriopus fulvus Fischer (Copepoda: Harpacticoida)
Aquaculture ELSEVIER
Aquaculture 134 (1995) 113-119
Influence of nutrition on fecundity and survival in Tigriopus jdvus Fischer (Copepoda: Harpacticoida) A. Carli *, G.L. Mariottini, L. Pane Istituto di Scienze Ambientali Marine, Universid
di Genova, Viale Benedetto XV 5, 16132 Genova, Italy
Accepted 9 March 1995
Abstract In aquaculture, the availability of highly nutritive and inexpensive live food is a subject of major concern; from this point of view, some harpacticoid copepods (gen. Tigriopus) are suitable, owing to both the ease of finding in nature and their constant reproduction in culture. In order to optimize the production of nauplii in Tigriopusfulvus Fischer (a species cultured for several years in our laboratory and considered because of its high occurrence in rockpools along the Ligurian coast), the influence of diet composition on some reproductive and developmental parameters was analyzed under laboratory controlled conditions. Two series of cultured females born in culture and acclimatized in the laboratory for several weeks ( > 10) were fed with algae (Monochrisis Mm-i) and yeast (Succharomyces cerevisiue), respectively; another series of females born in culture and acclimatized in the laboratory for only one week was fed with yeast. Results show that the diet affected survival and fecundity. Yeast-fed copepods showed better survival (median 71.5 and 75 days) than those fed algae (median 35 days) (P < 0.01) . A highly significant difference (P < 0.01) was observed for the reproductive period with a high incidence of infertile females (28.15%) in the series fed M. lutheri. Lower production rate was recorded in yeast-fed (generally less than 4 nauplii per day) than in algaefed (over 5 nauplii per day) specimens; nevertheless, yeast-fed copepods showed a more prolonged production of nauplii in time with a consequent higher nauplii per day global amount. No difference was observed between the differently acclimatized yeast-fed series. Overall, a short reproductive period along with fast production of nauplii was observed in algae-fed T.fiZvus females; on the other hand, a slower production, although more durable and abundant, was recorded in yeast-fed copepods. Both results should be of interest in aquaculture in order to obtain live food for commercial fish and crustaceans culture. Keywords:
Tigriopusfiluus; Live food; Feeding and nutrition -invertebrates
* Corresponding author. Tel. ( +39-10)
3538070; Fax ( + 39-10) 3538072.
0044-8486/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDZOO44-8486(95)00046-l
114
A. Carli et al. /Aquaculture
134 (1995) 113-119
1. Introduction The availability of live food of high nutritional value is an important factor supporting growth of fish and crustacean larvae (Fagetti and Campodonico, 1971; Wickins, 1972; Perkins, 1973; Wear, 1976; Bale&a et al., 1977-78; Bale&a et al., ,198l; Feller, 1980; Lassus and Maggi, 1980; Millamena et al., 1988). Furthermore, the administration of diets having a suitable fatty acid composition is extremely important in order to support growth of early larval stages (New, 1976; Bottino et al., 1980; Martin, 1980; Teshima et al., 1983). In this connection, the utilization of the harpacticoid copepod Tigriopus japonicus in feeding studies (Nagasawa, 1985) and in aquaculture (Kahan et al., 1988) in some eastern countries is well known. Similar applications have been suggested for the Atlantic copepod Tigriopus brevicornis (Vilela, 1984). In the rockpools of the Ligurian Sea lives the Harpacticoid Copepod Tigriopusfulvus, which because of its eurythermal and euryhaline characteristics is easily adaptable to artificial culture (Ranade, 1957; Saraiva, 1973). Fatty acid composition (Carli et al., 1983; Carli et al., 1989a) as well as life and reproductive cycles (Carli and Fiori, 1977; Carli et al., 1989b) of T.fuZvus under laboratory controlled conditions are well-known. Therefore, the eventual utilization of this copepod as live food in aquaculture is a matter for consideration. The aim of this study was to investigate the life cycle and the possible variations in reproduction and survival of T. jidvus induced by diet composition.
2. Materials and methods Specimens of T. fulvus were collected in rockpools of Genova-Nervi (Ligurian Coast) and acclimated in the laboratory in rockpool water for 1 (AF,,) or several ( > 10) weeks (BF, and CF,) . Some females carrying egg sacs were isolated from the origin cultures and maintained in 20 ml of seawater, previously filtered with Gelman GN/6 (0.45 pm) membrane filters, at a temperature of 25 + 1°C water density of 1.029 and pH 7.9 +O.l. Culture medium was renewed every second day. Copepods of cultures AFe and BF,, were fed with 0.5 ml of a suspension (0.1 mg/ml) of commercial yeast (Saccharomyces cerevisiae) . Those of culture CF,, were fed a constant cell number (70 000 cells/ml) of the marine alga Monochrysis Zutheri cultured in our laboratory in Walne medium at a temperature of 22°C with a light-dark period of 12-12 h. Algal cells were counted by a hemocytometer Thoma. Nauplii born from isolated females were pooled according to the date of birth and to the culture of origin (cultures AF,, BF,, CFi ) . They were then maintained at the above described conditions and observed during the whole developmental period. The observations were carried out each day between 09.00 and 12.00. At the beginning of mating, coupled specimens were isolated. After mating, the males were discarded, whereas the females ( 17 belonging to the AFr culture, 15 to the BF, and 32 to the CF,) were maintained isolated and observed, in order to evaluate the production of nauplii during the whole life cycle. For each female, the time intervals between birth and mating start, mating end, and emission of the first egg sac were recorded. In addition, the
A. Carli et al. /Aquaculture
134 (1995) 113-119
115
lengths of the reproductive period, survival time, fecundity and incidence of non-fertile females were also recorded. Statistical evaluation was performed descriptively, by position parameters, since specimen numbers were not high enough to permit any inference on data distribution. Statistical significance of the recorded experimental results was analyzed by free-distribution tests (U-test of Mann-Whitney and variance analysis of Kruskal-Wallis). For proportion comparison the X*-test was utilized, assuming that the expected differences were not lower than 5. The graphic representation of total nauplii produced by each female was expressed as cumulative number of nauplii considering the age of the female when hatching occurred.
3. Results Table 1 shows the length of the life cycle phases evaluated in copepods as influenced by cultures AF,, BFt,CF,. No difference was recorded between the two series fed S. cereuisiae. The difference recorded between copepods fed M. lutheri and S. cerevisiae with respect to Table 1 Minimum, median and maximum (in days) of some parameters of the life cycle in T. fulvus Fischer. AF, and BFr fed with S. cerevisiae; CF, fed with M. lutheri Series
BF,
Ah
Hatching-mating start Hatching-mating end Hatching-emission 1st egg sac Reproductive period Survival
(3
Minimum
Medium
Maximum
Minimum
Medium
Maximum
Minimum
Medium
Maximum
8 10 11 16 41
16.5 18.5 22 51.5 75
25 27 30 85 112
10 12 13 13 34
15 17 20 50 71.5
22 25 32 83 128
11 13 15 2 23
19 18.5 21.5 22 35
22 24 28 45 79
,
10
20
30
ro
50
60 Days
70
80
90
Fig. 1. Percent survival of female T. fulous.AF,: -. -. -. -; BF,: --;
100
110
CFr : ----.
120
,
116
A. Carli
et al. /Aquaculture
134 (1995) 113-119
a
b
2
20
30
40
50
60
70
60
90
1CJ-l
age of female (days)
Fig. 2. Fecundity of each female T. fuluus. Cumulative number (nauplii per day) in relation to the age of females. (a) AFL (b) BF,; (c) CF,.
the period included between birth and mating start, was statistically significant (0.01
A. Carli et al. /Aquaculture
134 (1995) 113-119
117
significant (P > 0.05). Survival of algae-fed copepods (median = 35 days) was lower in comparison with yeast-fed ones (median 7 1.5 and 75 days) and the difference was highly significant (P < 0.01). Yeast-fed copepods also showed a median reproductive period of 50 and 5 1.5 days, whereas the period for the algae-fed copepods was 22 days. The occurrence of infertile females was significantly (x2 = 6.05 with 1 degree of freedom; 0.01
4. Discussion T.fulvus fed S. cerevisiae, even if acclimated in the laboratory for different periods, gave similar results. On the contrary, differences between yeast-and algae-fed copepods were noted, probably due to the different nutritional value of diet. Overall, the diet seemed to affect both survival time and fertility of female T.fuZvus. It was noticed that, as in Tigriopus califomicus (Burton, 1985), T. jkEuus males often clasp immature females and spawning occurred immediately after the last molting. In particular, yeast-fed copepods had a lower daily production of nauplii distributed over a longer time period. The algae-fed specimens produced less egg sacs and nauplii, and exhibited lower survival and a higher incidence of infertile females. Therefore, both types of feed were beneficial: the algal food provided a larger number of nauplii which could be used as food for larval organisms of concern in aquaculture; whereas, the yeast-fed specimens had a higher overall production over a longer period of time. In addition, satisfactory production of nauplii and survival during successive generations were evident when yeast was used as food for T.fulvus in culture (Carli et al., 1989b). The difficulties in preparing mass culture of T. fuluus is another matter of concern. The experimental evidence of a maternal hatching inhibition at high population densities in the related species T. juponicus (Kahan et al., 1988) could also be a problem in T. $dvus. In our experiments, this aspect was not considered because females were maintained isolated. On the other hand, it is also possible that as in barnacles (Crisp, 1956; Crisp, 1969) some substances secreted by female can promote hatching of mature eggs. The evidence of chemical signalling has already been shown in T. j&us (Lazzaretto et al., 1990). In conclusion, the production of nauplii in culture by female T. fulvus seems to be promising in order to allow the utilization of this copepod as live food in aquaculture. Despite the evident differences observed by using different feeds, T. fulvus showed satisfactory production when fed either diet. Therefore, these results provide an improvement to the knowledge of the influence of diet on reproductive and life cycles of T. jidvus. Further research on the energetic content
118
A. Carli etal. /Aquaculwe
134 (1995) 113-119
of copepods fed with different diets will be necessary, in order to give an adequate caloric supply to the organisms of concern in aquaculture, such as larval fish and crustaceans. The improvement of mass culture techniques of T. filuus in order to obtain a suitable amount of available nauplii will also be necessary. Furthermore, the survival of T. jXuus in culture can be estimated proportional to costs, with the total number of nauplii obtained from each female representing rearing yield. Therefore, the relationship between cumulative number of nauplii and the days lived by each female could represent an efficient method to plan rearing strategy towards optimizing cost-benefits. These methods will be utilized in further studies concerning the preparation of T. jiduus cultures.
Acknowledgements Special thanks are due to Prof. Vincenzo giche e Metaboliche, Universid di Genova)
Balestra (Dipartimento Scienze Endocrinolofor the statistical evaluation of data.
References Bale&a, V., Carli, A. and Truffi, L., 1977-78. Note sull’allevamento di Palnemon elegans Rathke (Crustacea, Decapoda). Ann. 1st. Mus. Zool. Univ. Napoli, XXII: 77-92. Balestra, V., Carli, A. and Vignola, S., 1981. Laboratory culture of Palaemon elegans (Rathke, 1837) (Crustacea Decapoda) (Third contribution). Boll. Sot. Ital. Biol. Sper., LVII: 225 l-2256. Bottino, N.R., Gennity, J., Lilly, M.L., Simmons, E. and Finne, G., 1980. Seasonal and nutritional effects on the fatty acids of three species of shrimp, Penaeus setiferus, P. aztecus, and P. duorarum. Aquaculture, 19: 139148. Burton, R.S., 1985. Mating system of the intertidal copepod Tigriopus californicus. Mar. Biol., 86: 247-252. Carli, A. and Fiori, M.A., 1977. Sviluppo larvale de1 Tigriopusjihus Fischer. Atti IX Congr. S.I.B.M., Lacco Ameno d’Ischia, 19-22/5/77, pp. 181-190. Chili, A., Colacello, F. and Valente, T., 1983. Variazioni stagionali degli acidi grassi in popolamenti di Tigriopus jidous delle pozze di scogliera della costa ligure (Copepoda Harpacticoida). Atti Sot. Ital. Sci. Nat. Mus. Civ. Stor. Nat. Milano, 124: 1 l-20. Carli, A., Balestra, V., Pane, L. and Valente, T., 1989a. Rapport0 di composizione percentuale degli acidi grassi nel Tigriopus jklvus delle pozze di scogliera della costa ligure (Copepoda Harpacticoida). Boll. Sot. Ital. Biol. Sper., LXV; 421427. Carli, A., Mariottini, G.L. and Pane, L., 1989b. Reproduction of the rockpools harpacticoid copepod Tigriopus filuus (Fischer 1860). suitable for aquaculture. Deuxi&me Con&s international d’Aquariologie (1988) Monaco, Bull. Inst. O&nogr. Monaco, Sp&., 5: 295-300. Crisp, D.J., 1956. A substance promoting hatching and liberation of young in Cirripedes. Nature, 178: 263. Crisp, D.J., 1969. Studies of barnacle hatching substance. Comp. Biochem. Physiol., 30: 1037-1048. Fagetti, E. and Campodonico, I., 1971. Larval development of the red crab Pleurocondes monodon (Decapoda Anomura: Galatheidae) under laboratory conditions. Mar. Biol., 8: 70-81. Feller, R.J., 1980. Development of the sand-dwelling meiobenthic Harpacticoid Copepod Huntemannia jadensis Poppe in the laboratory. J. Exp. Mar. Biol. Ecol., 46: l-15. Kahan, D., Berman, Y. and Bar-El, T., 1988. Maternal inhibition of hatching at high population densities in Tigriopus japonicus (Copepoda, Crustacea). Biol. Bull., 174: 139-144. Lassus, P. and Maggi, P., 1980. Etude de la sensibilite thermique des larves du Crustace Decapode: Palaemon serralus (Pennant). Rev. Trav. Inst. P&hes Marit., 44: 257-268.
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Lazzaretto, I., Salvato, B. and Libettini, A., 1990. Evidence of chemical signalling in TigriopusfuZuus (Copepoda, Harpacticoida), Crustaceana, 59: 171-179. Martin, B.J., 1980. Croissance et acides gras de la crevette Palaemon serratus (Crustacea, Decapoda) nourrie avec des aliments composes contenant differentes proportions d’acide linoleique et linolenique. Aquaculture, 19: 325-337. Millamena, O.M., Bombeo, R.F., Jumalon, N.A. and Simpson, K.L., 1988. Effects of various diets on the nutritional value of Artemia sp. as food for the prawn Penaeus monodon. Mar. Biol., 98: 217-221. Nagasawa, S., 1985. The digestive efficiency of the Chaetognath Sugitta crassa Tokioka, with observations on the feeding process. J. Exp. Mar. Biol. Ecol., 87: 271-282. New, M.B., 1976. A review of dietary studies with shrimp and prawns. Aquaculture, 9: 101-144. Perkins, H.C., 1973. The larval stages of the deep sea red crab, Geryon quinquedens Smith, reared under laboratory conditions (Decapoda: Brachyrhyncha). Fish. Bull., 71: 69-82. Ranade, M.R., 1957. Observations on the resistance of Tigriopusfulous (Fischer) to changes in temperature and salinity. J. Mar. Biol. Assoc. UK, 36: 115-l 19. Saraiva, M.C., 1973. Etude sur la radiosensibilite d’un Copepode benthique, Tigriopusfuluus (Fischer). J. Cons. Int. Explor. Mer., 35: 7-12. Teshima, S., Kanazawa, A. and Sasada, H., 1983. Nutritional value of dietary cholesterol and other sterols to larval prawn, Penaeus japonicus (Bate). Aquaculture, 31: 159-167. Vilela, M.H., 1984. Production experiments of the marine harpacticoid copepod Tigriopus breuicomis Mueller, reared on various feeding regimes. Bol. Inst. Nat. Invest. Pescas Lisboa, 11: 83-l 15. Wear, R.G., 1976. Studies on the larval development of Metanephrops challengeri (Balss, 1914) (Decapoda, Nephropidae). Crustaceana, 30: 113-122. Wickins, J.F., 1972. The food value of brine shrimp, Artemia suknn L., to larvae of the prawn Pulaemon serrutus Pennant. J. Exp. Mar. Biol. Ecol., 10: 151-170.
Aquaculture 134 (1995) 113-119
Influence of nutrition on fecundity and survival in Tigriopus jdvus Fischer (Copepoda: Harpacticoida) A. Carli *, G.L. Mariottini, L. Pane Istituto di Scienze Ambientali Marine, Universid
di Genova, Viale Benedetto XV 5, 16132 Genova, Italy
Accepted 9 March 1995
Abstract In aquaculture, the availability of highly nutritive and inexpensive live food is a subject of major concern; from this point of view, some harpacticoid copepods (gen. Tigriopus) are suitable, owing to both the ease of finding in nature and their constant reproduction in culture. In order to optimize the production of nauplii in Tigriopusfulvus Fischer (a species cultured for several years in our laboratory and considered because of its high occurrence in rockpools along the Ligurian coast), the influence of diet composition on some reproductive and developmental parameters was analyzed under laboratory controlled conditions. Two series of cultured females born in culture and acclimatized in the laboratory for several weeks ( > 10) were fed with algae (Monochrisis Mm-i) and yeast (Succharomyces cerevisiue), respectively; another series of females born in culture and acclimatized in the laboratory for only one week was fed with yeast. Results show that the diet affected survival and fecundity. Yeast-fed copepods showed better survival (median 71.5 and 75 days) than those fed algae (median 35 days) (P < 0.01) . A highly significant difference (P < 0.01) was observed for the reproductive period with a high incidence of infertile females (28.15%) in the series fed M. lutheri. Lower production rate was recorded in yeast-fed (generally less than 4 nauplii per day) than in algaefed (over 5 nauplii per day) specimens; nevertheless, yeast-fed copepods showed a more prolonged production of nauplii in time with a consequent higher nauplii per day global amount. No difference was observed between the differently acclimatized yeast-fed series. Overall, a short reproductive period along with fast production of nauplii was observed in algae-fed T.fiZvus females; on the other hand, a slower production, although more durable and abundant, was recorded in yeast-fed copepods. Both results should be of interest in aquaculture in order to obtain live food for commercial fish and crustaceans culture. Keywords:
Tigriopusfiluus; Live food; Feeding and nutrition -invertebrates
* Corresponding author. Tel. ( +39-10)
3538070; Fax ( + 39-10) 3538072.
0044-8486/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDZOO44-8486(95)00046-l
114
A. Carli et al. /Aquaculture
134 (1995) 113-119
1. Introduction The availability of live food of high nutritional value is an important factor supporting growth of fish and crustacean larvae (Fagetti and Campodonico, 1971; Wickins, 1972; Perkins, 1973; Wear, 1976; Bale&a et al., 1977-78; Bale&a et al., ,198l; Feller, 1980; Lassus and Maggi, 1980; Millamena et al., 1988). Furthermore, the administration of diets having a suitable fatty acid composition is extremely important in order to support growth of early larval stages (New, 1976; Bottino et al., 1980; Martin, 1980; Teshima et al., 1983). In this connection, the utilization of the harpacticoid copepod Tigriopus japonicus in feeding studies (Nagasawa, 1985) and in aquaculture (Kahan et al., 1988) in some eastern countries is well known. Similar applications have been suggested for the Atlantic copepod Tigriopus brevicornis (Vilela, 1984). In the rockpools of the Ligurian Sea lives the Harpacticoid Copepod Tigriopusfulvus, which because of its eurythermal and euryhaline characteristics is easily adaptable to artificial culture (Ranade, 1957; Saraiva, 1973). Fatty acid composition (Carli et al., 1983; Carli et al., 1989a) as well as life and reproductive cycles (Carli and Fiori, 1977; Carli et al., 1989b) of T.fuZvus under laboratory controlled conditions are well-known. Therefore, the eventual utilization of this copepod as live food in aquaculture is a matter for consideration. The aim of this study was to investigate the life cycle and the possible variations in reproduction and survival of T. jidvus induced by diet composition.
2. Materials and methods Specimens of T. fulvus were collected in rockpools of Genova-Nervi (Ligurian Coast) and acclimated in the laboratory in rockpool water for 1 (AF,,) or several ( > 10) weeks (BF, and CF,) . Some females carrying egg sacs were isolated from the origin cultures and maintained in 20 ml of seawater, previously filtered with Gelman GN/6 (0.45 pm) membrane filters, at a temperature of 25 + 1°C water density of 1.029 and pH 7.9 +O.l. Culture medium was renewed every second day. Copepods of cultures AFe and BF,, were fed with 0.5 ml of a suspension (0.1 mg/ml) of commercial yeast (Saccharomyces cerevisiae) . Those of culture CF,, were fed a constant cell number (70 000 cells/ml) of the marine alga Monochrysis Zutheri cultured in our laboratory in Walne medium at a temperature of 22°C with a light-dark period of 12-12 h. Algal cells were counted by a hemocytometer Thoma. Nauplii born from isolated females were pooled according to the date of birth and to the culture of origin (cultures AF,, BF,, CFi ) . They were then maintained at the above described conditions and observed during the whole developmental period. The observations were carried out each day between 09.00 and 12.00. At the beginning of mating, coupled specimens were isolated. After mating, the males were discarded, whereas the females ( 17 belonging to the AFr culture, 15 to the BF, and 32 to the CF,) were maintained isolated and observed, in order to evaluate the production of nauplii during the whole life cycle. For each female, the time intervals between birth and mating start, mating end, and emission of the first egg sac were recorded. In addition, the
A. Carli et al. /Aquaculture
134 (1995) 113-119
115
lengths of the reproductive period, survival time, fecundity and incidence of non-fertile females were also recorded. Statistical evaluation was performed descriptively, by position parameters, since specimen numbers were not high enough to permit any inference on data distribution. Statistical significance of the recorded experimental results was analyzed by free-distribution tests (U-test of Mann-Whitney and variance analysis of Kruskal-Wallis). For proportion comparison the X*-test was utilized, assuming that the expected differences were not lower than 5. The graphic representation of total nauplii produced by each female was expressed as cumulative number of nauplii considering the age of the female when hatching occurred.
3. Results Table 1 shows the length of the life cycle phases evaluated in copepods as influenced by cultures AF,, BFt,CF,. No difference was recorded between the two series fed S. cereuisiae. The difference recorded between copepods fed M. lutheri and S. cerevisiae with respect to Table 1 Minimum, median and maximum (in days) of some parameters of the life cycle in T. fulvus Fischer. AF, and BFr fed with S. cerevisiae; CF, fed with M. lutheri Series
BF,
Ah
Hatching-mating start Hatching-mating end Hatching-emission 1st egg sac Reproductive period Survival
(3
Minimum
Medium
Maximum
Minimum
Medium
Maximum
Minimum
Medium
Maximum
8 10 11 16 41
16.5 18.5 22 51.5 75
25 27 30 85 112
10 12 13 13 34
15 17 20 50 71.5
22 25 32 83 128
11 13 15 2 23
19 18.5 21.5 22 35
22 24 28 45 79
,
10
20
30
ro
50
60 Days
70
80
90
Fig. 1. Percent survival of female T. fulous.AF,: -. -. -. -; BF,: --;
100
110
CFr : ----.
120
,
116
A. Carli
et al. /Aquaculture
134 (1995) 113-119
a
b
2
20
30
40
50
60
70
60
90
1CJ-l
age of female (days)
Fig. 2. Fecundity of each female T. fuluus. Cumulative number (nauplii per day) in relation to the age of females. (a) AFL (b) BF,; (c) CF,.
the period included between birth and mating start, was statistically significant (0.01
A. Carli et al. /Aquaculture
134 (1995) 113-119
117
significant (P > 0.05). Survival of algae-fed copepods (median = 35 days) was lower in comparison with yeast-fed ones (median 7 1.5 and 75 days) and the difference was highly significant (P < 0.01). Yeast-fed copepods also showed a median reproductive period of 50 and 5 1.5 days, whereas the period for the algae-fed copepods was 22 days. The occurrence of infertile females was significantly (x2 = 6.05 with 1 degree of freedom; 0.01
4. Discussion T.fulvus fed S. cerevisiae, even if acclimated in the laboratory for different periods, gave similar results. On the contrary, differences between yeast-and algae-fed copepods were noted, probably due to the different nutritional value of diet. Overall, the diet seemed to affect both survival time and fertility of female T.fuZvus. It was noticed that, as in Tigriopus califomicus (Burton, 1985), T. jkEuus males often clasp immature females and spawning occurred immediately after the last molting. In particular, yeast-fed copepods had a lower daily production of nauplii distributed over a longer time period. The algae-fed specimens produced less egg sacs and nauplii, and exhibited lower survival and a higher incidence of infertile females. Therefore, both types of feed were beneficial: the algal food provided a larger number of nauplii which could be used as food for larval organisms of concern in aquaculture; whereas, the yeast-fed specimens had a higher overall production over a longer period of time. In addition, satisfactory production of nauplii and survival during successive generations were evident when yeast was used as food for T.fulvus in culture (Carli et al., 1989b). The difficulties in preparing mass culture of T. fuluus is another matter of concern. The experimental evidence of a maternal hatching inhibition at high population densities in the related species T. juponicus (Kahan et al., 1988) could also be a problem in T. $dvus. In our experiments, this aspect was not considered because females were maintained isolated. On the other hand, it is also possible that as in barnacles (Crisp, 1956; Crisp, 1969) some substances secreted by female can promote hatching of mature eggs. The evidence of chemical signalling has already been shown in T. j&us (Lazzaretto et al., 1990). In conclusion, the production of nauplii in culture by female T. fulvus seems to be promising in order to allow the utilization of this copepod as live food in aquaculture. Despite the evident differences observed by using different feeds, T. fulvus showed satisfactory production when fed either diet. Therefore, these results provide an improvement to the knowledge of the influence of diet on reproductive and life cycles of T. jidvus. Further research on the energetic content
118
A. Carli etal. /Aquaculwe
134 (1995) 113-119
of copepods fed with different diets will be necessary, in order to give an adequate caloric supply to the organisms of concern in aquaculture, such as larval fish and crustaceans. The improvement of mass culture techniques of T. filuus in order to obtain a suitable amount of available nauplii will also be necessary. Furthermore, the survival of T. jXuus in culture can be estimated proportional to costs, with the total number of nauplii obtained from each female representing rearing yield. Therefore, the relationship between cumulative number of nauplii and the days lived by each female could represent an efficient method to plan rearing strategy towards optimizing cost-benefits. These methods will be utilized in further studies concerning the preparation of T. jiduus cultures.
Acknowledgements Special thanks are due to Prof. Vincenzo giche e Metaboliche, Universid di Genova)
Balestra (Dipartimento Scienze Endocrinolofor the statistical evaluation of data.
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