Overview of diets used in larviculture of three Caribbean Conchs: Queen Conch Strombus gigas, Milk Conch Strombus costatus and Fighting Conch Strombus pugilis

Aquaculture 167 Ž1998. 163–178

Review

Overview of diets used in larviculture of three Caribbean Conchs: Queen Conch Strombus gigas, Milk Conch Strombus costatus and Fighting Conch Strombus pugilis Dalila Aldana-Aranda ) , Victoria Patino ˜ Suarez ´ Laboratorio de Biologıa Km. 6 Carretera Antigua a Progreso, ´ Marina CINVESTAV-IPN, Unidad Merida, ´ Merida, Yucatan, ´ ´ C.P. 97310, A.P. 73 Cordemex, Mexico Accepted 15 June 1998

Abstract The genus Strombus is widely distributed in the Caribbean. Six species are of commercial importance: S. gigas, S. raninus, S. costatus, S. alatus, S. gallus and S. pugilis. Economically, the Queen conch, S. gigas is the most important and consequently the most widely studied. However, since 1970 a decline of S. gigas populations due to over-fishing has been observed. Many authors have studied S. gigas hatchery rearing techniques in order to address this problem; however, for these hatchery techniques to be successful, an adequate diet must be provided for the larvae. Some information of the nutritional requirements of S. gigas larvae have been reported since 1965, but a nutritionally complete diet is still not available. In this work we summarize the different diets that have been used for S. gigas, S. costatus and S. pugilis larvae rearing. Twenty one different algae species have been used: Amphidinium carteri, Chaetoceros gracilis, Dunaliella tertiolecta, Emillania huxleyi, Heterocapsa pygmacea, Isochrysis ŽCaicos., Isochrysis ŽTahiti., Isochrysis sp., Monochrysis sp., Nannochloris, Nitzchia, Platymonas sp., P. tetraselmis, Prorocentrum minimun, Rhodomonas sp. , Skeletonema costatus, Tetraselmis chuii, Tetraselmis sp., T. suecica, Thalassiosira fluÕiatilis and T. weissflogii. There are other diets that have seldom been studied with Strombus veliger larvae, that could be a potential food source for these gastropods. The type and concentration of algae, larval rearing conditions are summarized along with the results attained in

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Corresponding author. Tel.: q52 99 812973; fax: q52 99 812917; e-mail: [email protected] 0044-8486r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 Ž 9 8 . 0 0 3 0 4 - 4

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D. Aldana-Aranda, V. Patino Aquaculture 167 (1998) 163–178 ˜ Suarezr ´

larval growth, metamorphosis, survival, ingestion and digestion rates. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Mollusca; Conch; Larvae; Diets

1. Introduction The genus Strombus is widely distributed along tropical and subtropical areas. In the Caribbean, six species of commercial importance are: S. raninus, S. gallus, S. alatus, S. pugilis, S. costatus and S. gigas. Of these species, S. gigas is the most abundant and the most important economically. An estimate of the total Caribbean conch catch is 4000 mt, which represents a value of US$40,000,000 ŽAppeldoorn, 1994.. Since 1970 a decline of S. gigas natural population due to overfishing has been observed ŽAdams, 1970; Berg, 1976; Brownell, 1977; Hesse and Hesse, 1977; Aldana-Aranda et al., 1989; Rathier, 1993; Appeldoorn, 1994; Stoner, 1996.. The milk conch, S. costatus, and the fighting conch, S. pugilis, are smaller species than S. gigas ŽAbbot, 1960.. Their abundance through the Caribbean coast and their total catch volume have not yet been estimated. Literature on these two species is also minor, even though both represent an important food and job source for the inhabitants of the region. In the Yucatan Peninsula, Mexico, S. costatus has the same market value and demand as S. gigas. It has also became an overfished resource ŽAldana-Aranda et al., 1989.. While S. gigas and S. costatus are intensively overfished, S. pugilis has an exploitation potential ŽBaqueiro and Medina, 1990; Aldana-Aranda and Baqueiro, 1995.. Many authors have studied S. gigas rearing techniques in order to protect natural populations and to restore some overfished areas, like the Yucatan Peninsula where Queen conch populations have decreased close to extinction ŽDe La Torre, 1982; Navarrete et al., 1993.. However, for these hatchery techniques to be successful, an adequate diet must be provided during the larval stage. In order to increase the scientific knowledge regarding veliger larvae mariculture we have reviewed the type of algae, food concentration and larval culture conditions that have been used in S. gigas, S. costatus and, more recently, S. pugilis larvae culture. We also summarize the results attained in terms of growth, development, metamorphosis, survival, ingestion and digestion rates.

2. Culture conditions for rearing Strombus veliger larvae Since S. gigas is one of the most important commercial species fished in the Caribbean, its culture has been widely studied. Larviculture techniques for S. costatus and S. pugilis compared to S. gigas are less well described: only four authors have examined S. costatus ŽBrownell, 1977; Ballantine and Appeldoorn, 1983; Aldana-Aranda et al., 1989; Davis et al., 1993., and three S. pugilis ŽBrownell, 1977; Bradshaw-Hawkins, 1982; Brito-Manzano, 1997.. In fact, the knowledge and experience from S. gigas larviculture techniques have been applied for rearing S. costatus and S. pugilis larvae.

D. Aldana-Aranda, V. Patino Aquaculture 167 (1998) 163–178 ˜ Suarezr ´

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The first attempt at S. gigas larvae rearing dates back to 1965, when D’Asaro Ž1965. described the development of the veliger larvae. Since then, larviculture researchers on Strombus sp. have shown that the success in larval rearing depends on factors such as, the quality of water culture, salinity, temperature, larval density, quality and quantity of food, cleanliness in all aspects of the culturing environment, as well as culture handling, since larval physical traumatism can produce high mortality rates. Tables 1–3 summarize the different conditions that have been used for rearing S. gigas, S. costatus and S. pugilis veliger larvae, as well as results expressed as growth rate, maximum length, metamorphosis and survival. Despite the dedicated efforts of scientists in Strombus sp. larviculture, the lack of consistency in culture conditions are still persistent. It is known that larval density, temperature as well as the quality and quantity of food have an important influence on survival, growth and metamorphosis rates ŽBrownell, 1977; Davis and Hesse, 1983; Hensen, 1983; Davis et al., 1987; Aldana-Aranda et al., 1989; Boidron-Metairon, 1992; Davis, 1994a; Garcıa-Santaella and Aldana-Aranda, ´ 1994. From Tables 1–3 we can observe that a temperature range of 23 to 318C has been used fo rearing Strombus sp. larvae. However, the best results in terms of growth and time to reach metamorphosis have been obtained in a temperature range of 27 to 308C. The lowest larval density used has been 10 larvae P ly1 and the highest 3500 larvae P ly1 ŽTables 1–3.. This highest density was used by Hensen Ž1983. for S. gigas initial culture, but was reduced to 200 to 600 larvae P ly1 by high mortality at the 28th day. This represents a survival of 6 to 17%. Heyman et al. Ž1989., working with S. gigas larvae fed Isochrysis ŽTahiti., used a initial density, between 500 and 600 larvae P ly1 , however at day 29 this density dropped by mortality to 100 larvae P ly1 . Aldana-Aranda et al. Ž1989. studied the optimal values of larval density for rearing S. costatus larvae. Five densities ranging from 100 to 500 larvae P ly1 in 100 steps were tested. Larvae were cultured with Isochrysis ŽTahiti.. The best results in growth and survival rates were for the lowest larval density. These authors pointed out that high larval densities contributed to slow growth rates and presumably slow development rates. At a low density of 10 larvae P ly1 Brownell Ž1977. reported a mean shell length of 2.2 mm at day 27 for S. gigas, 4.0 mm at day 36 for S. costatus and 2.9 mm at day 28 for S. pugilis larvae, compared to the minimun growth registered by larvae grown at 3500 larvae P ly1 by Hensen Ž1983..

3. Diets used in Strombus veliger larvae Over the past two decades larvae of conchs have been fed 21 different algae species: Platymonas tetraselmis, Isochrysis sp., Nitzchia, Skeletonema costatus, Monochrysis sp., Rhodomonas sp., Platymonas sp., Tetraselmis sp., Thalassiosira weissflogii, Nannochloris, Emillania huxleyi, Prorocentrum minimun, Tetraselmis suecica, Chaetoceros gracilis, Dunaliella tertiolecta, T. fluÕiatilis, T. chuii, Isochrysis ŽTahiti., Isochrysis ŽCaicos., Heterocapsa pygmacea and Amphidinium carteri ŽTables 1–3.. From 33 studies, 14 used Tetraselmis as a food source, while 23 used Isochrysis. Both algae have been the most commonly used diets for rearing conch larvae. Together they have been

166

Larval density, larvae ly1

T, 8C

Food

Algal concentration, 10 3 cells mly1

Growth, mm dy1

ML, mm

M, days

S, %

Source

10–100 10 10 – 100 38–3100 –

24–27 24–30 24–30 – – 28"1 –

k a a o, p, q, r b, c, d b, c, d, i f

20 100 100 10,000 – 100–250 1700

– 82 a – – 50 – –

– 2.2 – 1.2 0.9 a 1.9 –

60 28–33 27–35 22–24 14 28 15

– – – – – – –

3500

27"1

c, d, i

–

–

1.9 a

33 a

100

–

b, c, d

–

53

1.8

12–22

severe mortality –

D’Asaro Ž1965. Brownell Ž1977. Brownell et al. Ž1977. Barnett Ž1980. Ballantine Ž1981. Siddall Ž1981. Ballantine and Appeldoorn Ž1982. Hensen Ž1983.

250–350 150–500 –

29 26–30 –

d, h, i, j a d, h, i,

– 5–30 1–30

– – –

1.9 – 2

14–35 18–21 20–25

– – –

Ballantine and Appeldoorn Ž1983. Davis and Hesse Ž1983. Laughlin and Weil Ž1983. Siddall Ž1983.

D. Aldana-Aranda, V. Patino Aquaculture 167 (1998) 163–178 ˜ Suarezr ´

Table 1 Results in growth, maximum length ŽML., metamorphosis ŽM. and survival ŽS. of S. gigas larvae, under different culture conditions. The source of food is indicated as: Ža. Enriched natural cultures of phytoplankton, mainly Nitzchia spp. , S. costatus, and Chaetoceros spp., Žb. T. fluÕiatilis, Žc. T. chuii, Žd. Isochrysis ŽTahiti., Že. Isochrysis ŽCaicos., Žf. T. suecica, Žg. C. gracilis, Žh. Nanochloris, Ži. D. tertiolecta, Žj. T. weissflogii, Žk. P. tetraselmis, Žl. P. minimun, Žm. E. huxleyi, Žn. H. pygmacea, Žo. Isochrysis sp., Žp. Platymonas sp., Žq. Monochrysis sp., Žr. Rhodomonas sp., Žs. Tetraselmis sp.

23–31

c, d

0.1–10

–

–

19–30

– 5 200

26"1 27–29 28

d, i, l, m, n a c, d, cqd

– – 0.4–3.2

13–93 – 40

1.2 – 1

– – –

100–150 500–670

25–31 28–30

e, g, s d

– 7–40

– 24

1.1 0.93

20–30 30"10 25 100–200 60 100 200–300 275

– – 27 27–30 29"1 28 28–32 29"1

e, g e, g, s a, d, e e, g d, f f b, c, d b, d, f

8.5–15 – – 0.005–0.007 0.12 6.8 0.1–1.5 0.138–11

– – – 39 5–13 – – 24–37

1.1 1.2 1.3 1.2 0.73 – – 0.8

21–40 not reached 21–40 21"2 27"2 21 – – – –

60 – – – 21–52 – 15–20 25–82

100–200 20–60 20 250–500

28–30 – – 26–29

e, g e, g e, g a

– 20 – 5–25

– – –

1.3 1.2 – –

18–23 21 18–21 18–30

– – – –

a

Rate calculated for this table from other data in source.

massive mortality 83–96 – – 59"9

Corral and Ogawa Ž1985. Pillsbury Ž1985. Buitrago Ž1985. Aldana-Aranda and Torrentera Ž1987. Davis et al. Ž1987. Heyman et al. Ž1989. Ray and Davis Ž1989. Davis et al. Ž1990. Boidron-Metairon Ž1992. Davis et al. Ž1993. Ž1993. Domınguez ´ Aldana-Aranda et al. Ž1994. Baqueiro Ž1994. Garcıa-Santaella and ´ Aldana-Aranda Ž1994. Davis Ž1994a. Davis Ž1994b. Davis and Stoner Ž1994. Weil and Laughlin Ž1994.

D. Aldana-Aranda, V. Patino Aquaculture 167 (1998) 163–178 ˜ Suarezr ´

8–10

167

168

Larval density, larvae ly1

T, 8C

Food

Algal concentration, 10 3 cells mly1 dy1

Growth, mm dy1

ML, mm

M, days

S, %

Source

10 100 – 100 200 200 200

24–30 – – 24 28 32

a b, c, d f b, c, d d d d

100 – 1700 – 0.6 0.6 0.6

111a 50 – 51 26"9 35"13 –

4 ;1.25a – 1.1–1.8 0.685 0.812 –

– – – – 17 10

Brownell Ž1977. Ballantine Ž1981. Ballantine and Appeldoorn Ž1982. Ballantine and Appeldoorn Ž1983. Aldana-Aranda et al. Ž1989. Aldana-Aranda et al. Ž1989. Aldana-Aranda et al. Ž1989.

200 200 100–500 100–200

28"2 28"2 28"2 27–30

c, d, cqd d d e

0.6 0.2–1.2 0.6 0.005–0.007

29"9–40"12 17"6–30"4 26"11–50"16 28

0.72–0.87 0.559–0.704 0.691–0.965 1.277

26–30 21 16. 14–18 36 26 heavy mortality 28 28 – 32

8 14–35 5–18 –

Aldana-Aranda et al. Ž1989. Aldana-Aranda et al. Ž1989. Aldana-Aranda et al. Ž1989. Davis et al. Ž1993.

a

Rate calculated for this table from other data in source.

D. Aldana-Aranda, V. Patino Aquaculture 167 (1998) 163–178 ˜ Suarezr ´

Table 2 Results in growth, maximum length ŽML., metamorphosis ŽM. and survival ŽS. of S. costatus larvae, under different culture conditions. The source of food is indicated as: Ža. Enriched natural cultures of phytoplankton, mainly Nitzchia spp., S. costatus, and Chaetoceros spp., Žb. T. fluÕiatilis, Žc. T. chuii, Žd. Isochrysis ŽTahiti., Že. Isochrysis ŽCaicos., Žf. T. suecica

Larval density, larvae ly1

T, 8C

10 33 200

24–30 28 29"1

a

Food

a c, d b

Algal concentration, 10 3 cells mly1

Growth, mm dy1

ML, mm

M, days

Survival, %

Source

104 a – 23–41

2.9 1.18"0.19 0.912–1.496

32–36 31"7 27–31

– 28 13–44

Brownell Ž1977. Bradshaw-Hawkins Ž1982. Brito-Manzano Ž1997.

y1

100 – 1

Rate calculated for this table from other data in source.

D. Aldana-Aranda, V. Patino Aquaculture 167 (1998) 163–178 ˜ Suarezr ´

Table 3 Results in growth, maximum length ŽML., metamorphosis ŽM. and survival ŽS. of S. pugilis, under different culture conditions. The source of food is indicated as: Ža. Enriched natural cultures of phytoplankton, mainly Nitzchia spp., S. costatus, and Chaetoceros spp., Žb. T. suecica, Žc. Tetraselmis sp., Žd. A. carteri

169

170

D. Aldana-Aranda, V. Patino Aquaculture 167 (1998) 163–178 ˜ Suarezr ´

used in 68% of all diets, and they have been generally selected because they promote acceptable larval growth. The remaining algae have seldom exceeded 10%. Strombus larvae have been fed either as a single species or as a mixture, however best result in development and growth are observed for mixed diets, which include eather Isochrysis sp. or Tetraselmis sp. Barnett Ž1980. successfully reared S. gigas larvae through metamorphosis using a culture of four species: Isochrysis sp, Platymonas sp. Monochrysis sp. and Rhodomonas sp. In a later experiment, a mixture of R. lens and I. galbana was used and no apparent adverse effects were observed. A mixture of R. lens and P. tetrathele, promoted satisfactory growth and development, but no metamorphosis was observed ŽSiddall, 1981.. Siddall Ž1983. reported on S. gigas larvae, which were competent at 20–25 days after hatching and reached a shell length of 2 mm when the diet was composed of Isochrysis ŽTahiti., Nannochloris and D. tertiolecta. Aldana-Aranda et al. Ž1989. working with S. costatus larvae tested three different diets: Isochrysis, T. chuii and a mixture of these algae in equal proportions. They obtained mean shell length of 870 mm, 840 mm and 816 mm with the mixture Isochrysisq T. chuii, T. chuii, and Isochrysis, respectively, without significative differences among them, however the best growth rate per day was 39.69 mm with the mixed diet, 37.54 mm with T. chuii, and 35.84 mm with Isochrysis. Aldana-Aranda and Torrentera Ž1987. working with S. gigas larvae observed that a 1:1 mixture of I. galbana and T. chuii promoted better growth than either one alone. In 12 h post-hatched S. gigas larvae, the fastest ingestion and digestion rates were observed with a 1:1 mixture of T. chuii and I. galbana ŽAldana-Aranda et al., 1991.. Walne Ž1970. pointed out that a mixture of monocellular algae was a better food for bivalves larvae than the use of a single specie. Pillsbury Ž1985. studied the relative food value of algal diets for S. gigas larvae from a point of view of their lipid and fatty acid composition. Five algae were tested: I. aff. galbana, D. tertiolecta, E. huxleyi, P. minimum and H. pygmacea. Based upon larval growth and survival, Pillsbury valued the species I. aff. galbana, E. huxleyi, P. minimum and H. pygmacea as good diets, which supported rapid growth and high survival; while, D. tertilecta was valued as poor diet that supported only minimal growth. D. tertilecta was also the only diet that had a low lipid and fatty acid contents. Consequently, the author suggested that Queen conch larvae may require a diet rich in lipid. Other alga that has been found not to be suitable as diets during the larval period is T. fluÕiatilis, which caused high mortality in earlier S. gigas larvae ŽGarcıa-Santaella ´ and Aldana-Aranda, 1994.. Slow ingestion and digestion rates in S. gigas larvae have also been observed for this alga. The authors related this result to larval incompetence in catching larger algal cells and digesting their cell walls ŽAldana-Aranda et al., 1997a,b., but this alga has been successfully used in juvenile conchs ŽBallantine, 1981; Siddall, 1981.. The quantity of food that has been used to feed S. gigas, S. costatus and S. pugilis larvae varies enormously. Algal concentrations from 5 to 10 6 cells P mly1 P dy1 have been reported ŽTables 1–3.. High concentrations of food cause stress to the veliger larvae, which results in larval mucus production. In these masses of mucus, live and dead larvae, detritus ŽHeyman et al., 1989. as well as microalgae cells are entrapped. In this situation, larvae cannot swim, do not feed effectively, thus development and growth decline ŽSiddall, 1983.. This mixture of mucus, larvae and food is a rich medium to

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171

support bacterial and protozoan invasion, which harms the culture and contributes to larval mortality ŽHeyman et al., 1989; Davis, 1994b.. Aldana-Aranda et al. Ž1989. with S. costatus larvae tested different algal concentrations of Isochrysis ŽTahiti.: 100, 200, 400, 600, 800, 1000 and 1200 cells P mly1 P dy1 . At the lowest concentration the authors obtained high mortalities. They pointed out that the optimal growth rate was obtained when larvae were fed 600 cells P mly1 P dy1 , and it was not significantly different from that of larvae fed 800, 1000 and 1200 cells P mly1 P dy1 . The average growth rate for these concentrations was 28.15 mm P dy1 , signicantly different from 16.7 mm P dy1 obtained at feeding rate of 200 cells P mly1 P dy1 . Survival rate varied from 14 to 35% for the range 200–1200 cells P mly1 P dy1 . The lowest algal concentration reported for Strombus sp. larviculture is 5–7 cells P mly1 P dy1 ŽTables 1–3.. At this concentration of Isochrysis ŽCaicos., Davis et al. Ž1993. reared S. gigas and S. costatus larvae, which reached metamorphosis at 21 days and 32 days after hatching, with a shell length of 1170 and 1280 mm, and growth rate of 39 and 28 mm P dy1 , respectively. These authors used an initial larval density of 100–200 veligerP ly1 , which changed to 10–20 veliger P ly1 as larvae neared competence. Feeding began at initial density of 5 cells P mly1 P dy1 and was increased to 7 cells P mly1 P dy1 over the culture period. Besides the diet of the late-satge veliger was supplemented with 3 cell P mly1 P dy1 of C. gracilis Schutt. It is important to observe that using the same algal diet Ž Isochrysis and C. gracilis ., similar larval densities Ž20 and 60 veliger P ly1 at competence., but different algal concentrations Ž8500–20,000 cells P mly1 P dy1 ., Ray and Davis Ž1989. and Davis Ž1994a,b.. showed that S. gigas larvae reached metamorphosis in a range from 21–40, 18–23, and 21 days, with a shell length of 1100, 1300 and 1200 mm, respectively. In contrast, the highest algal concentration that has been used in S. gigas larvae is 10 7 cells P mly1 P dy1 ŽBarnett, 1980.. Barnett’s larvae reached metamorphosis at 22 to 24 days after hatching, and a maximum length of 1200 mm was reported at 18 days after hatching ŽTable 1.. This algal concentration compared with 5–7 cell P mly1 P dy1 ŽDavis et al., 1993. represents a huge difference, almost 100%; however the results in growth and development are very similar. This is of importance as an algal concentration 1,000,000 times lower shows as good results in growth and development. Overfeeding can produce a faster development, but the number of larvae that reach metamorphosis is too small. Davis and Hesse Ž1983. observed a mortality from 50 to 70% in S. gigas larvae at 9–11 days of culture, period which was considered by the authors as the most critical stage. This result was blamed on high concentrations of food. These authors observed that larvae safely passed through the critical stage with low food concentrations, which slowed down development, but gave hardier larvae and larger percentage survival through metamorphosis. Siddall Ž1983. used radioactively labelled phytoplankton to estimate maximum food density and maximum stocking density of S. gigas larvae. At food densities above 50,000 cells P mly1 or at stocking densities in excess of 1000 larvae P ly1 ingestion rates of food declined significantly. This author proposed a gradual feeding regime starting at 1000 cells P mly1 P dy1 2 days after hatching, reaching 25,000–30,000 cells P mly1 P dy1 10 days after hatching and beyond. Heyman et al. Ž1989. using a larval density of 500–670 larvae P ly1 of S. gigas larvae fed Isochrysis at 7–40 = 10 3 cells P mly1 P dy1 , estimated the average feeding rate of

172

D. Aldana-Aranda, V. Patino Aquaculture 167 (1998) 163–178 ˜ Suarezr ´

8.7 " 3.4 = 10 3 cells eaten larvae P ly1 P hy1 for larvae between days 12 and 17, and never observed a total depletation of algae available in the jar after 12 or 24 h. In larval development, the most critical time is during metamorphosis, which is a transformation in life habit: veliger larvae change from pelagic to benthic behaviour, with the formation of new juveniles ŽBallantine and Appeldoorn, 1983.. Working with S. gigas, S. costatus and S. pugilis larvae, Brownell Ž1977. established the development of proboscis, outward migration of eyes, and disappearance of velar lobes as the morphological indications of a complete metamorphosis. These criteria have been used by many authors; however Bradshaw-Hawkins Ž1982. only considered the fusion of the eyes and tentacles, and velar disappearance for having achieved metamorphosis in S. pugilis larvae. Besides Brownell’s criteria, Davis et al. Ž1990. also described that the conch of S. gigas larvae begin to crawl with the propodium when larvae have metamorphosed. Davis Ž1994a. described the pigmentation changes on the foot Žorange to green or black., and swim–crawl behavior Žlarva uses foot to move on sustrate with lobes extended. as signs of competency for metamorphosis. As we mentioned above, larval development, and consequently, the time to reach metamorphosis vary according to the culture conditions and also to the author’s criteria. In Table 1 we also show the metamorphosis rates, which varies between 12 and 40 days after hatching, of the three Strombus larvae analyzed.

4. Assessment of ingestion and digestion Aldana-Aranda et al. Ž1997a,b. point out that, besides Tetraselmis and Isochrysis, there are other algae that have seldom been used as food for conch veligers which may have potential for use by these larvae. These authors have been analyzing the nutritive value of different algae for the veliger larvae, in terms of ingestion and digestion, by means of epifluorescence microscopy. This technique has been of great use in this kind of studies, taking advantage of the transparency of tissues and shell of the veliger larvae, and the autofluorescence of chlorophyll and its derivates in the algal cells within their guts. In this way, a qualitative estimation of the quantity of whole algal cells ingested and their subsequent lysis and digestion can be followed. With this technique we can get information about the potential nutritive value of a particular algal specie based on their ingestion and digestion rates. Results from this kind of study have shown that the ingestion and digestion process varies according to larval age. These variations also depend on the type of food provided ŽTable 4.. According to the ingestion and digestion rates of different microalgae by Strombus larvae, Aldana-Aranda et al. Ž1997a,b. have shown C. coccoides as a potential diet for S. gigas larvae. Preliminary studies on the development and growth of S. gigas larvae fed this alga, show a metamorphosis rate at 22 days after hatching with a siphonal length of 979 " 99.67 mm, and a growth rate of 35.2 mm per day. In S. pugilis larvae, the first results on feeding behaviour, expressed also as ingestion and digestion rates, have been obtained. Two developmental stages, 1 and 30 days after hatching, have been tested with T. suecica. In both ages, T. suecica was well ingested; however, a significant difference in the digestion process was observed. In a period of

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173

Table 4 Ingestion and digestion rates ŽIR and DR. of S. gigas larvae of 1-day-old and 18-day-old fed different algae ŽAldana-Aranda and Patino-Suarez, in press; Aldana-Aranda et al., 1994.. IR and DR were classed as High, ˜ ´ qqq; Median, qq; and Low, q. Algae that were not digested are indicated as 0 Microalga

Larval age in days 1

Chaetoceros sp. Chlorella sp. T. chuii T. suecica C. coccoides I. aff. galbana D. tertiolecta T. fluÕiatilis

18

IR

DR

IR

DR

q y qqq qqq qqq qqq qqq q

0 y qqq qq q qqq qq 0

qqq qqq qqq q qq qq qq qq

qqq qqq qq qq qq qq q q

24 h, 1-day-old larvae did not digest T. suecica, compared to a 30-day-old larvae, which started the digestion of this alga 5 h after it was ingested ŽAldana-Aranda et al., 1997a.. Other recent studies have reported the influence of photoperiod on S. gigas and S. pugilis veliger larvae. In S. pugilis larvae, this parameter has been studied with respect to growth, development, metamorphosis and survival, which are summarized in Tables 1–3 ŽBrito-Manzano, 1997.. For S. gigas larvae, the effect of photoperiod has been measured by means of ingestion rates. A photoperiod 12 light:12 dark was used with two different feeding schedules: larvae fed in the light, and larvae fed in the darkness. No difference between these factors were observed ŽAldana-Aranda et al., in press..

5. Futher researches Since algae is the main food source for Strombus sp. larvae, it has to be taken into account the costs and time-consuming that represent their culture. In algal culture three main points have to be considered: electrical energy, nutrients and labor to maintain the culture in excellent quality. To rear Strombus sp. the cost of food must be taken thoughfully, overall in those hatcheries with massive production, since it could be economically unfeasible. The search of alternative diets that reduced the demands for time, labor and costs, has been mainly studied in bivalves, but the knowledge from these studies may be applied for Strombus sp. It is known that the nutritional value of algae is related to their shape, cell size, digestibility especially the cell wall, toxicity and biochemical composition ŽBayne, 1983; Webb and Chu, 1983; Fabregas et al., 1985; Fernandez-Reiriz et al., ´ 1989; Lucas, 1990; Delaunay et al., 1993.. Relatively high amounts of long chain polyunsaturated fatty acids ŽPUFA., epecially 22:6n y 3 Ždocosahexanoic acid. and 20:5n y 3 Žeicosapentanoic acid. have been detected in marine microalgae. In some of them, especially diatoms, the 20:5n y 3

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constitutes the major component of the PUFA Ž20–45%. ŽBlanchemain and Grizeau, 1996.. The presence of PUFA, mainly 22:6n y 3 and 20:5n y 3 has been detected as essential for growth and development of molluscs ŽHolland and Spencer, 1973; Lucas, 1982; Pillsbury, 1985; Chu and Greaves, 1991; Marty et al., 1992.. It has been stablished that the biochemical composition of a given specie of phytoplankton can be modified under different culture conditions, such as nutrient supply of the medium, photoperiod, irradiance, temperature, and changes according to the growth phases of the algae ŽFernandez-Reiriz et al., 1989; Sukenik et al., 1993; Blanchemain and Grizeau, ´ 1996; Lourenc¸o et al., 1997.. Under this knowledge, the biochemical compostion of microalgae has been physiologically manipulated to regulate the abundance and distribution of lipid and fatty acid ŽFernandez-Reiriz et al., 1989; Sukenik et al., 1993; ´ Thompson et al., 1996.. Another alternative is the use of dried algae ŽPillsbury, 1985., however to dry the cell, is necessary massive production of the live alga, so the time and costs for their preparation are greater. The importance of lipids as a source of energy and essential fatty acids for larval development has encouraged the development of artificial diets that provide the specific lipids supplements during larval rearing, such as microcapsulates, liposomes, lipid microspheres and lipid emulsions ŽParker and Selivonchick, 1986; Robinson, 1992; Coutteau et al., 1996; Knauer and Southgate, 1997.. Artificial diets for Strombus spp. larvae have not been studied. In this sense, Siddall Ž1983. tested microecapsulated mollusc lipid as an artificial food, observing that growth was reduced only 15% for larvae exclusively fed this diet compared to his best phytoplankton diet ŽTahitian Isochrysis, N. oculata, D. tertiolecta.. Recent studies have used mesocosm systems to study the effects of nutrition on the life-span of marine invertebrae larva in their natural environment ŽDavis et al., 1996; Brooke and Mann, 1996.. Davis et al. Ž1996. examined the field growth rates of S. gigas larvae fed two different natural assemblages of phytoplankton: 5 and 50 mm filtered ambient seawater. Initial stocking density was similar to that used in laboratory culture Ž20 veligersP ly1 or 4000 veligersP mesocosmy1 . and it was gradually reduced to 0.7 veligersP ly1 on day 13. A growth rate of 33 mm P dy1 is reported until day 9, without difference between the two phytoplankton assemblages. Metamorphosis was first observed on day 13, and 95% of the veliger were competent for metamorphosis at day 16. These results compared to larvae reared in the laboratory gave a 5-day advantage to the farmer ŽDavis et al., 1990, 1993; Davis, 1994b.. Veliger fed natural phytoplankton in the field not only showed faster growth rates, but had more vigor than those raised in the laboratory ŽDavis et al., 1996.. A comparative study of growth rates between S. gigas larvae fed natural phytoplankton from the Bahamian waters and cultured algae Ž Isochrysis sp. and C. gracilis . showed identical growth rates although the chlorophyll a level of cultured algae was 50 times higher than that in natural phytoplankton ŽDavis et al., 1996.. These results suggest that quality of food may be more important than quantity, that nutritional value of natural phytoplankton cells may be superior to cultured algae, and that natural food items must be used to establish growth rates likely to occur in the field. We suggest that the nutritional requeriment of Strombus spp. larvae should be analyzed both qualitative and quantitatively. With the qualitative analysis the nutritional value of the diets are studied according to their biochemical composition, and must be analyzed in function of the nutritional require-

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ments of the specie and life stage. With the quantitative analysis we can get important information about feeding according to the scope for growth after all metabolic demands have been covered ŽWiddows and Johnson, 1988.. From this point of view, the use of the energetic physiology could be an additional tool to get a better understanding of larval nutrition ŽLucas, 1993..

References Abbot, R.T., 1960. The genus Strombus in the Indo-Pacific. Indo-Pacific Mollusca 1, 33–146. Adams, J.E., 1970. Conch fishing industry of Union Island, Grenadines, West Indies. Trop. Sci. 12, 279–287. Aldana-Aranda, D., Baqueiro, E., 1995. Los moluscos en Mexico: estudio y aprovechamiento. Academia 27, ´ 33–43. Aldana-Aranda, D., Patino-Suarez, M.V., in press. Ingestion ˜ ´ ´ y digestion ´ de 7 microalgas por larvas de . de 1 dıa Strombus gigas ŽMolusco gasteropodo ´ ´ de edad, estudiado por epifluorescencia. Proc. Gulf Carib. Fish. Inst. . Aldana-Aranda, D., Torrentera, B. L, 1987. Croissance larvaire de Strombus gigas ŽMollusque: Gasteropode ´ en fontion de la nourriture et de la temperature. Haliotis 16, 403–411. ´ Aldana-Aranda, D., Lucas, A., Brule, ´ T., Salguero, E., Rendon, ´ F., 1989. Effect of temperature, algal food, feeding rate and density on larval growth of the milk conch Ž Strombus costatus . in Mexico. Aqualculture 76, 361–371. Aldana-Aranda, D., Lucas, A., Brule, E., Maginot, N., Le Pennec, M., ´ T., Andrade, M., Garcıa-Santaella, ´ 1991. Observations on ingestion and digestion of unicellular algae by Strombus gigas larvae ŽMollusca Gastropoda. using epifluorescence microscopy. Aquaculture 92, 359–366. Aldana-Aranda, D., Patino-Suarez, M.V., Brule, ˜ ´ ´ T., 1994. Ingestion and digestion of eight unicellular algae by Strombus gigas larvae ŽMollusca gastropode. studied by epifluo-rescence microscope. Aquaculture 126, 151–158. Aldana-Aranda, D., Patino-Suarez, M.V., Brule, ˜ ´ ´ T., 1997a. Nutritional potentialities of Chlamydomonas coccoides and Thalassiosira fluÕiatilis, as measured by their ingestion and digestion rates by the Queen Conch larvae Ž Strombus gigas .. Aquaculture 156, 9–20. Aldana-Aranda, D., Patino-Suarez, M.V., Brito, N.P., 1997b. Cineticas de alimentacion ˜ ´ ´ ´ de larvas de caracol de una, ˜ Strombus pugilis, de 1 y 30 dıas ´ de edad. Proc. Gulf Carib. Fish. Inst. 49, 469–484. Aldana-Aranda, D., Patino-Suarez, M.V., Baqueiro, E., Garcıa-Santaella, in press. El fotoperıodo en la ˜ ´ ´ ´ ingestion ´ y digestion ´ de las larvas de Strombus gigas. Proc. Gulf Carib. Fish. Inst. Appeldoorn, R., 1994. Queen conch management and research: status, needs and priorities. In: Appeldoorn, R.S., Rodrıguez, B. ŽEds.., Strombus gigas Queen Conch Biology, Fisheries and Mariculture. Fundacion ´ ´ Cientıfica Los Roques, Caracas, Venezuela, pp. 301–319. ´ Ballantine, D.L., 1981. Strombus gigas culture program in Puerto Rico. Natinal Marine Fisheries Service, Puerto Rico, 6 pp. Ballantine, D.L., Appeldoorn, R.S., 1982. Hatchery Culture and Reseeding of Queen Conch, Strombus gigas in Puerto Rico. No. FSE43-81-126-12. National Marine Fisheries Service, Puerto Rico, 20 pp. Ballantine, D.L., Appeldoorn, R.S., 1983. Queen conch culture and future prospects in Puerto Rico. Proc. Gulf Carib. Fish. Inst. 35, 57–63. Baqueiro, E., 1994. Cultivo de juveniles del caracol reina, Strombus gigas, en Quintana Roo, Mexico. In: ´ Appeldoorn, R.S. Rodrıguez, B. ŽEds.., Strombus gigas Queen Conch Biology, Fisheries and Mariculture. ´ Fundacion Los Roques, Caracas, Venezuela, pp. 295–300. ´ Cientıfica ´ Baqueiro, E., Medina, M., 1990. Diagnostico de la pesquerıa ´ ´ de caracol en Champoton ´ y Seyba Playa, Campeche. Instituto Nacional de la Pesca. Secretaria de Pesca, Mexico, No. 3, 17 pp. ´ Barnett, 1980. Preliminary research a Pride foundation report on the handling techniques used in the mariculture of Strombus gigas veliger larvae at Pine Cay in 1980. Foundation for Pride, Miami, FL, 9 pp. Bayne, B.L., 1983. Physiological ecology of marine molluscan larvae. In: Wilbur, K.M. ŽEd.. The Mollusca Development, Vol. 3, Academic Press, New York, pp. 299–343.

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