Monacanthid fishes are generally inhabiting both temperate and tropical shallow waters, there are about 95 species are known worldwide (Nelson, 1984). The bluefin leatherjacket Thamnaconus modestus (Günther, 1877) belonging to the family of Monacanthidae, is an important commercial bottom-dwelling marine filefish which distributes in the northwestern Pacific Ocean (Su and Li, 2002). As a traditional commercial fishery species, bluefin leatherjacket was widely fished in China, Korea and Japan in the 1990s (Xu et al., 1992). However, due to over fishing, bluefin leatherjacket has become an endanger species in 1990s(Qian, 1998; Chen and Zhan, 2000). In the past two decades, a series of studies have been conducted, including stocks and distributions identification (Xu et al., 1992; Ding, 1994; Chen and Zhan, 2000), ecology factors (Maekawa, 1989), cryopreservation of sperm (Kang et al., 2004), and genetics (Xu et al., 2010). Although several studies have been done on the morphological development and growth evaluation of bluefin leatherjacket (Zhao and Chen, 1980 ; Chen and Zheng, 1984), most of these studies were conducted under field survey. Results from these studies are not consistent due to the limitation of the sampling scales. Up to present, little information is available on the developmental characters of bluefin leatherjacket at early stage. Furthermore, artificial breeding of this species is still under trial scales.
The aim of this study was to explore the spawning in captivity and larvi-culture of bluefin leatherjacket in commercial scales. Our study will provide fundamental information on the morphological and functional development of bluefin leatherjacket larvae reared under intensive conditions. Information such as the age and size appearing transformation, culture condition’s evaluation for mass production of high quality seedling will benefit the further development of the breeding protocol on this species.
1 Results and Discussion
1.1 Spawning and Egg Production
Spawning commenced on 15th June, 2010 and continued to 9th August, 2010. The peak of spawning was from 1st July to 25th July, corresponding to the temperatures ranging from 21.0 ℃ to 22.0 ℃. The mean diameter of the eggs was (0.607±0.03) mm, and a total of 4.345 million eggs were collected through the spawning season, of which 2.845 million were considered as well developed (Figure 1). 1.8 million newly hatched larvae were obtained in this study, and the hatching rate was 41.7%.
Figure 1 Spawning frequency and egg numbers produced by T. modestus
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Like most species from the family of Monacanthidae, the eggs of bluefin leatherjacket were subjected to adhesive egg. The adhesiveness of eggs from Monacanthidae is very strong (Zhao et al., 1985). In nature, this type of eggs was normally adhered to the substrate such as reef, algae and cay. There are a large number of viscin threads distributing on the egg surface of bluefin leatherjacket (Zhao et al., 1985; Safran and Omori, 1990). Previous studies indicated that by using viscin threads the fertilized eggs of bluefin leatherjacket can stick on the substrates such as shell, coral reef, large algae, and rocks in the natural environment (Qian, 1998; Su and Li, 2002). Results from our previous investigation indicate that the plastic plate and black soft plastic film attracted more eggs in the artificial breeding environment, and the viscin threads of eggs were not well stuck to the rough surface materials (Guan et al., 2012). Therefore, plastic corrugated plates were used to collect the eggs in the present study.
In order to manipulate the natural hatching environment and reduce human interruption, after eggs collection, the eggs’ together with attached plastic corrugated plates were directly incubated in the larval rearing tanks without any treatments. Because of the characteristic of sticky, the fertilized eggs were easily to be lumped with sands or other substrates.Copepods and nematodes were observed living on the egg lumps in the present study. The hatching rate was only 41.7%, which was lower than those species with floating eggs (Ma et al., 2012a; Nocillado et al., 2000). It is unclear whether the hatching rate was affected by lump-forms and parasites in the present study. In order to increase the hatching rate, future research should towards developing the hatching protocols for adhesive eggs, and technology such as eggs debonding should be considered in the future study.
1.2 Description of Development During Endogenous Feeding Period
After approximate 1 h 30 min, fertilized eggs entered into 2-cell stage, and 32-cell stage was recorded at 3 h 25 min (Table 1, Figure 2). After 6h further development, the fertilized eggs entered into blastula stage, the accumulated temperature reached to 132.93 (℃×h). When the accumulated temperature reached to 210 (℃×h), fertilized eggs entered into gastrula stage. After 48h development when the accumulated temperature reached to 1 000 (℃×h), fertilized eggs became pre-hatch embryo, and 50% eggs hatched when the accumulated temperature reached to 1050 (℃×h) (Table 1, Figure 2).
Table 1 Embryonic development and accumulative temperature of Thamnaconus modestus
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Figure 2 Embryonic development of T. modestus. Stages and development information refer to Table 1
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1.3 Yolk and Oil Globule Absorption
Growth and morphological characteristics of the larvae during the first five days are presented in Table 2. The mean total length (TL) of newly hatched larvae was (2.08±0.09) mm with an ellipsoid yolk-sac of (0.5331±0.0366) mm long and (0.2593±0.0393) mm wide. One oil globule with the diameter of (0.20±0.01) mm was presented at the anterior end of the yolk-sac. After hatching, fish larvae had un-pigmented eyes, and the mouth did not open. Melanophores were presented on the snout tip, the trunk and on the tissue surrounding the inner side of the yolk-sac. Xanthophores were presented on the trunk, abdomen and around the head. The mouth opened on 3dph, while first was started between 3 and 4 dph. Yolk sac was completed absorbed on 2 dph, while oil globule was absorbed completely on 4 dph (104.4 d·℃) when larval size reached to (2.54±0.06) mm (TL, Table 2).
Table 2 Yolk-sac and oil globule volumes in larvae of T. modestus (means ± SD)
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Generally, egg’s diameters of fish from the family of Monacanthid were less than one millimeter, for instance the egg’s diameters of Rudarius ercodes is about 0.52 mm (Kawase and Nakazono, 1994; Kawase and Nakazono, 1995), Oxymonacanthus longirostris is about 0.7 mm, and Thamnaconus modestus is about 0.64 mm (Nakazono and Kawase, 1993). In the present study, the egg diameter of bluefin leatherjacket was (0.607±0.03) mm, which was smaller than Nakazono and Kawase’s (1993) finding. The slight difference may cause by the genetic difference between two populations as fish from Nakazono and Kawase’s (1993) study was from Japanese Sea while our broodstocks were came from Bohai Sea. Because of the small size of egg, total length of newly hatched larval bluefin leatherjacket was only (2.08±0.09) mm. The character of small size larvae potentially increased the difficulty of larvi-culture during the first feeding period. For instant, as the mouth size of bluefin leatherjacket was very small (unpublished data), the size of live feeds could be the issue affecting the successful first feeding.
Live feeds supply during the first feeding for those small size fish at hatching has become the major bottleneck continually hinder the larval culture in the marine fish hatchery. Similar like grouper larvae, bluefin leatherjacket have very small mouth gap, under some particular situation, common rotifers may not fulfill the size of requirement for the first feeding bluefin leatherjacket larvae. Therefore, the production of live feeds with small size characteristics is an important hatchery operation in bluefin leatherjacket breeding. As the geographical difference, super small strain rotifers were not available in our region. Therefore, to explore new live feeds substrates is essential. In the present study, by using fertilized egg and trochophore-stage larvae of Pacific oyster (Crassostrea gigas) during the initial feeding stage, this issue was partially solved.
1.4 Growth and Seeding Rate
The SGR from 0 dph to50 dph was 6.00% day-1. The larvae grew exponentially and their growth can be described by the equation.
Y=3.30394-0.31862x+0.02222x2 (r2=0.90362, n=427).
Where Y is total length in millimeters and x is days post hatching (Figure 3). In this study, two heavy mortality periods were observed, which were found between 5 dph~10 dph and 30 dph~60 dph. After 70 days rearing, 0.33 million juveniles (mean total length>40 mm) were obtained and the ratio of seedling/ newly was (5.62±0.83) %.
Figure 3 Growth of T. modestus during the first 50 days
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From the physical aspect, sufficient nutrient supply depends on the efficiency of the digestive system. During the early development stage, lack of sufficient digestive capabilities is common in this phase and larvae mainly depend on pinocytosis and intracellular digestion and absorption (Ma et al., 2012b). Previous study indicates that the nutrition is presumably the predominant factor affecting the growth of fish in early life stage (Yin, 1995). In species such as yellowtail kingfish (Seriola lalandi), growth is normally found accelerated after the digestive system is fully functional (Ma, unpublished data). In the present study, bluefin leatherjacket grew slowly in the original developmental stage (0 dph~16 dph), the specific growth rate was only about 4.7%/day. The growth turning point was found around 20 dph, when the growth was accelerated. This may indicate that a more efficient digestive system was formed around this stage.
In present study, two heavy mortality periods were observed, which were found between 5 dph~10 dph and 30 dph~60 dph. During the first mortality period (5 dph~10 dph), nearly 80% of fish were lost. Within this period, dead fish were examined under stereo microscope. Dead fish was showing nutrition defeated sign that the body of fish was dark, and emaciated. Although feeds can be found in fish gut, little of them were digested. We suspected that this may cause by the D shaped larvae of Pacific oyster as the chitin shell of D shaped larvae cannot be digested by fish larvae, and its unique character may also block the digestive tract of fish larvae. Therefore, in the rest of our production run we started to control the development stage of the Pacific oyster larvae to make sure less D shaped larvae were presented in the rearing tanks and a protocol has also been developed (Guan et al., 2012).
The second mortality period was started from 30 dph, which occurred after weaning started. Previous study have indicated that mortality happened within this period was contributed by cannibalism (Hoey and McCormick, 2004), feeding decrease or suspend (Folkvord and Otterib, 1993), stress response enhancement (Wedemeyer, 1972), disease and so on. In the present study, cannibalism began from 30 dph, chasing, biting, and chocked fish can be easily found in each rearing tanks. In order to reduce cannibalism, in the rest of our production run we started to grade the fish weekly, and increased the water exchange rate during the day time which the cannibalism was reduced.
In summary, the present study demonstrated that bluefin leatherjacket was feasible breeding under artificial environment. Aspects regarding to bluefin leatherjacket breeding have been successfully explored. Although the final seeding rate was low, as the first breeding experiment under commercial scales, our results can still provide the fundamental information on artificial breeding on this species. Future study should towards increasing the hatching rate and understand the digestive physiology of bluefin leatherjacket.
2 Materials and Methods
2.1 Broodstocks Management
Broodstocks (395 fish, mean total length=(277.8±30.0) mm) were caught in Bohai Sea, P.R. China, and have been acclimated in the artificial environment for 15 months in a commercial hatchery (Yantai, Shandong Province, P.R. China). A total of 216 fish was used in this study. The fish were kept in the indoor cement tank (diameter 7.0 m; water depth 0.8 m~1.0 m) and the culture density was maintained at 4 fish/m3. Ambient seawater filtered by 100 µm filter was supplied to the rearing tank at 10 m3/h. Salinity was maintained at 30‰ through this study. A nature photoperiod was used in this study. The photoperiod at the start of the spawning season (June) was 13 h light: 11 h dark, and increased to a maximum day length in August (14 h light: 10 h dark) at the end of spawning season (at the end of August). Vigorous aeration was provided through diffuser stones. Fish were hand-fed once per day to the level of satiation in the morning, with approximately (7~10)% amounts (by weight) of mixture diet (30 % white-hair rough shrimp (Trachypenaeus curvirostris, Stimpson), 40 % pacific oyster (Crassostrea gigas),15% Mytilus edulis and 15 % squid (Loligo sp.). Pre to the spawning season, broodstocks (32 fish per tank) were transferred from the culture tank to spawning tanks (concrete, 28.8 m3) with a sex ratio of 1:1 (female: male). Natural spawning occurred when the ambient water temperature increased from 19℃ to 21℃ in the spawning seasons.
2.2 Eggs Collection
The collection of eggs for larvi-culture started from early June to the end of July. The collection was stopped when egg quantity and quality decreased to a degree that made larvi-culture unviable for commercial proposes. The adhesive eggs from natural spawning were collected by using plastic corrugated plates (50 cm×50 cm) which were placed on the bottom of each spawning tank. These plates were placed at 10:00 am before each spawning event, and removed in the following day. Eggs were transferred to the larvae rearing tanks together with the attached plastic corrugated plates. At the same time, photographs of each plate were taken with a calibration scale for samples. The egg number per unit (5 cm×5 cm) on the plate was counted and calculated. A stereo microscope (Olympus SZ-61) was used to observe and measure the eggs. Eggs appear to be cloudy, un-global or developed abnormally were considered as non-viable. A sample of 10 cm×10 cm egg collection plate was taken out daily to estimate the normal egg rate. 60 fertilized eggs were sampled from each batch, and the diameters were measured to the nearest 0.01 mm.
The eggs were incubated in the larval rearing tanks (concrete, 28.8 m3) at an initial density of 3.0×105 eggs/tank. Constant aeration was supplied to each tank. The salinity and temperature were maintained at 30‰, 21℃, respectively. After hatching, the plastic corrugated plates with un-hatched eggs were removed from the rearing tanks, and the yolk sac larvae were reared in the same tank.
2.3 Larval rearing
The rearing density of fish larvae was maintained at 6000 larvae/m3~8000 larvae/m3, and a total of 19 tanks were used in this study. The rearing tanks were supplied with sand-filtered seawater. Larvae were reared with a 24 h-light photoperiod, and the illumination was provided by fluorescent lamps suspended over the rearing tank, and light intensity was 1500-2000 lux (measured on the surface of water). The salinity and dissolved oxygen were maintained at 30‰, 5 mg/L ~7.0 mg/L respectively. The water temperature was gradually increased from 21.0℃ (0 dph) to 26.0℃ (16 dph), and then maintained at 26℃ until the experiment was finished (Figure 4). Water exchanging in the rearing tanks was started on 4 dph, the exchange rate was gradually increased from 10% (4 dph) to 100% (20 dph). On 20 dph, all the juveniles were collected and transferred to new rearing tanks (concrete, 28.8 m3) with a stock density of 2000 fish/m3.
Figure 4 Feeding scheme and rearing temperature for T. modestus during the first 45 days post hatching
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The feeding regime was summarized in Figure 4 From 1dph, Nannochloropsis sp. was supplied to the rearing tank every morning to achieve an initial concentration of 3×105 cells/mL. Larvae were fed with fertilized egg and trochophore-stage larvae of Pacific oyster (Crassostrea gigas) three times per day from 3 dph to 10 dph at a density of 10 inds/mL~20 inds/mL. From 8 dph to 24 dph, rotifers (Brachionus sp., small strain) enriched with Algamac 3 050 (Aquafauna Bio-Marine, USA) were supplied to each rearing tanks at a density of 10 inds/mL~15 inds/mL. Starting from 19 dph, Artemia nauplii enriched with Algamac 3 050 (Aquafauna Bio-Marine, USA) were added into fish tanks, the feeding density was gradually increased from 5 nauplii/mL (19 dph) to 20 nauplii/mL (25 dph), from 32 dph, micro-particulate feeds (Otohime B2, C1, C2, Marubeni Nisshin Feed Co., Ltd. Japan) were used starting from smaller size to large size. The amounts of micro-particulate feed were distributed by hand and adjusted according to fish demand at 1-h intervals from 0900 hours to 1 900 hours each day. Weaning was completed on 36 dph. Outlet screens were cleaned, and tank bottoms were siphoned daily to remove dead fish, uneaten feeds and faeces.
2.4 Description of Embryonic Development During Endogenous Feeds Period
Approximately 5 000 eggs were taken from the egg collectors within an hour of spawning. The eggs were incubated at ambient water temperature (21℃) in a 200 L semi-conical tank supplied with 5µm filtered seawater flowing at 0.8 L/min. Light density was maintained at 1500 lux~2000 lux (measured at the water surface) by using fluorescent lamps with a photoperiod of 14 h light: 10 h dark. Samples of eggs (20 eggs) were taken at various intervals and development stage, and were recorded by using a digital camera connected to microscope (Olympus SZ-61, Japan). This study was repeated for four different batches of eggs. As the development time was not significantly different between each batch (P>0.05), the time to developmental checkpoint was taken as the earliest time that over 50% of the sample of embryos or larvae had reached a particular stage.
2.5 Morphological Measurements of Larvae and Juveniles
Fish samples were taken every day from 0 dph to 10 dph, and then were taken on 14 dph, 16 dph, 19 dph, 24 dph, 28 dph, 32 dph, 36 dph, 40 dph, and 50 dph. On each of the sampling day, 2~30 larvae were used for morphological measurements. Larvae were randomly selected and dipped at different zones from the rearing tank until 15 dph, after 16 dph were netted at different zones. All the fish were anaesthetized (MS-222, Tricaine methane sulfonate, 20 mg/L~30 mg/L) and measured under dissecting microscope (Olympus SZ-61). The volume of yolk sac (VYS, mm3) was calculated using the formula for an ellipsoidal volume: VYS= π/6×L×H2, where L was the major axis, and H was the minor axis of the yolk sac (Blaxter and Hempel, 1963). The volume of oil globule (VOG) in cubic millimeter was calculated: VOG=4/3π(d/2)3, where d is diameter of the spherical oil globule. Total length (TL) was measured from the tip of the lower jaw to the posterior margin of the caudal fin.
Growth was determined by the specific growth rate (SGR) as %/day using the following equations (Hopkins KD, 1992). SGR=100 (LnSLf –LnSLi)/Δt, where SLf and SLi were the final and initial fish total length (mm), respectively, and Δt was the time between sampling intervals (Chen et al., 2006a). The seeding rate (SR) was calculated using the formula: SR = [number of young fish (Total length ≥50mm)/ number of normal newly hatched larvae] ×100%.
2.6 Statistical Analysis
The data in this article were expressed as mean ± SD, and an independent T-test was used to compare the developing time of the embryo between different batches of eggs (PASW statistics 18.0, IBM, Chicago, IL, USA).
Acknowledgements
This research was sponsored by Science and Technology Development Program of Shandong Province “Breeding of bluefin leatherjacket, Thamnaconus modestus in commercial scales” (2009GG10005017), and Development Program of Fine Seeds Program in Shandong Province “Finfish breeding program for industry scales”. The authors wish to thank staffs from Yantai Baijia fishery Co., Ltd. for technical assistance in broodstocks management and larval fish rearing. The authors would like to thank Mr. Ji-lin Lei, Dr. Yong-jiang Xu and Dr. Yun-wei Dong for early planning on this study.
Chen L., and Zheng Y., 1984, On the early decelopment, the spawning ground and spawning season of Navodon septentrionalis (Gunther) in the Donghai, Ctaa Ecologica Sinica, 4: 73-79
Chen P., and Zhan B., 2000, Age and growth of Thamnaconus septentrionalis and rational exploitation, J. Fish. Sci. China,7: 35-40
Ding M., 1994, On the stock of filefish Navodon septentrionalis and their distributions in the East China Sea, J. Fish. China, 18: 45-56
Folkvord A., and Otterib H. 1993, Effects of initial size distribution, day length, and feeding frequency on growth, survival, and cannibalism in juvenile Atlantic cod (Gadus morhua L. ), Aquaculture, 114: 243-260
http://dx.doi.org/10.1016/0044-8486(93)90300-N
Guan J., Guan S. G., and Liu H. G., 2012, The usage of shellfish larvae as first feeding feeds for Blue Leatherjacket, Thamnaconus modestus larvae. Liu L., Patten No. ZL 201110121086.X., P. R. China
Guan J., Guan S. G., Liu H. J., and Zheng Y. Y., 2012, An artificial spawning ground and a method of eggs collection of bluefin leatherjacket, Thamnaconus modestus, Patten No. 201110184727.6 (submitted), P. R. China
Kang K. H., Kho K. H., Chen Z. T., Kim J. M., Kim Y. H., and Zhang Z. F., 2004, Cryopreservation of filefish (Thamnaconus septentrionalis Gunther, 1877) sperm, Aquac. Res., 35(15): 1429-1433
http://dx.doi.org/10.1111/j.1365-2109.2004.01166.x
Kawase H., and Nakazono A., 1994, Embryonic and pre-larval development and otolith increments in two filefish, Rudarius ercodes and Oaramonacanthus japonicus (Monacanthidae), Japan. J. Ichthy., 41: 57-63
Kawase H., and Nakazono A., 1995, Predominant maternal egg care and promiscuous mating system in the Japanese filefish, Rudarius encodes (Monacanthidae), Environ. Biol. Fish., 43(3): 241-254
http://dx.doi.org/10.1007/BF00005856
Ma Z., Qin J. G., and Nie Z. L., 2012b, Morphological changes of marine ï¬sh larvae and their nutrition need. In: Pourali, K. and Raad V. N. (Eds.), Larvae: Morphology, Biology and Life Cycle. Nova Science Publishers, Inc., New York, NY, USA., pp. 1-20
Ma Z., Qin J. G., Hutchinson W. and Chen B. N., 2012a, Food consumption and selective by larval yellowtail kingfish Seriola lalandi cultured at differernt live feed densities. Aquac. Nutr.
http://dx.doi.org/10.1111/anu.12004
Maekawa C., 1989, Relationship between water temperature and catch quantity of filefish Thamnaconus modestus, Bull. Kanag. Prefect. Fish. Exp. Stati., 10: 27-30
Nakazono A., and Kawase H., 1993, Spawning and biparental egg-care in a temperate filefish, Paramonacanthus japonicus (Monacanthidae), Environ. Biol. Fish., 37(3): 245-256
http://dx.doi.org/10.1007/BF00004632
Nelson J. S., 1984, Fishes of the world, 2nd ed. John Wiley and Sons, New York, pp.523, PMid:6501463
Nocillado J. I., Penaflorida V. D., Borlongan I.G., 2000, Measures of egg auqality in induced spawns of the Asian sea bass, Lates calcarifer Bloch, Fish Physiol. Biochem., 22: 1-9
http://dx.doi.org/10.1023/A:1007881231664
Qian S., 1998, The biological characteristics and resource status of the Yellow-fin Filefish in the East China Sea, J. Fish. Sci. China, 5(3): 25-29
Safran P., and Omori M., 1990, Some ecological observations on fishes associated with drifting seaweed off Tohoku coast, Japan, Marine Biology, 105(3): 395-402
http://dx.doi.org/10.1007/BF01316310
Su J., and Li C., 2002, Fauna Sinica: Tetraodontiformes, Pegasiformes, Gobiesociformes and Lophiiformes, Science Press, Beijing, P.R. China
Wedemeyer G., 1972, Some Physiological Consequences of Handling Stress in the Juvenile Coho Salmon (Oncorhynchus kisutch) and Steelhead Trout (Salmo gairdneri), J. Fish. Res. Board Can., 29(12): 1780-1783
Xu G-B., Chen S-L., and Tian Y-S., 2010, New polymorphic microsatellite markers for blueï¬n leatherjacket (Navodon septentrionalis Gunther, 1877), Conserv Genet, 11(3): 1111-1113
http://dx.doi.org/10.1007/s10592-009-9891-3
Xu X., Zheng Y., and Liu S., 1992, Estimation of stock size of filefish Thamaconus Modestus in the East Sea and Yellow Sea, Oceanol. Limnol. Sin., 23: 651-656
Yin M., 1995, Feeding and growth of the larva stage of fish, J. Fish. China, 19: 335-342
Zhao C., and Chen L., 1980, Artificial fertilization and larvae of the filefish, Thamnaconus septentrionalis, Fish. Sci. Tech. Infor., 1980: 1-3
Zhao C., Zhang R., Lu H., Lian C., Zang Z., and Jiang Y., 1985, Fish egg and larvae of China coastal sea, Shanghai scientific and technical publishers, Shanghai, pp.206