Scorpaeniformes fish has two reproductive types, one is ovoviviparous, such as false kelpfish Sebastiscus marmoratus; and the other is fertilized externally, like I. japonicus. There are three types of teleost eggs, buoyant, sticky and demersal, and most marine teleostean spawn buoyant eggs. Usually one oil globule is contained in the egg, playing a role of floating. However, oil globule could not be found in I. japonicus, but the eggs were still floating in the seawater with a salinity of 30‰, indicating ups and downs of eggs of I. japonicus is related to the water content in the eggs. In this study, the full developmental sequence of the devil stinger I. japonicus from egg to juvenile in controlled aquarium conditions was stated. These results enabled us to compare the development and morphology of embryos of I. japonicus with those of other teleost fishes in detail. During the embryonic development of I. japonicus, the same events were observed as those seen in zebrafish Danio rerio (Kimmel et al. 1995), roughskin sculpin Trachidermus fasciatus (Takeshita et al. 1997; Wang et al. 2004), and cottid fish Hemilepidotus gilberti (Hayakawaa and Munehara 2001), and their stage definitions could be consistently adopted to describe the embryonic development of I. japonicus. Therefore, the embryonic development of I. japonicus can be considered to follow the general developmental pattern of teleosts. Egg size is an important consideration for egg and larval quality during incubation and rearing in aquaculture. The average diameter of most Scorpaeniformes fish eggs are around 1.2–2.0 mm, however, the size range is wide. Egg diameters of some Scorpaeniformes fish were reported as: 1.2–1.3 mm for I. japonicus (Kadomura et al. 2006; Kim et al. 2012), 1.3 mm for non-copulatory sculpin Hemilepidotus gilberti (Hayakawaa and Munehara 2001), 1.5–1.78 (1.98–2.21) mm for roughskin sculpin, Trachidermus fasciatus (Wang et al. 2004; Takeshita et al. 1997). The egg of devil stinger is spherical, floating and has approximately 1.40 mm average diameter, which is similar to its previous reports. The egg size and fecundity are determined by several factors, i.e., broodstock age, broodstock size, feed and water quality (Celik et al. 2012).
In most fish species the blastomeres are regular in size and shape (Hall 2008). In the devil stinger, first five cleavages divided the blastodisc into 32 equal-sized blastomeres at the animal pore and horizontal cleavage occurred between 64 and 128 cell stages (after the fifth division). In zebrafish Danio rerio (Kimmel et al. 1995), Atlantic cod Gadus morhua (Hall et al. 2004), and cichlid fish Cichlasoma dimerusn (Meijide and Guerrero 2000), the first horizontal cleavage occurs at the sixth cleavage, between the 32 and the 64 cell stages. It occurs between the 16 and 32 cell stages in the medaka Oryzias latipes (Iwamatsu 1994) and common snook Centropomus undecimalis (Yanes-Roca et al. 2012). It occurs even earlier in the Holostean fish Amia calva (between the 8 and the 16 cell stages) (Ballard 1986; Nakatsuji et al. 1997) and in the ice goby Leucopsarion petersii (between the 4 and the 8 cell stages) (Nakatsuji et al. 1997). Theoretical knowledge of embryonic development stages might be useful for incubation management with regard to environmental variables, thus larvae malformation and low productivity in captivity can be prevented (Celik et al. 2012). Furthermore, the information on embryonic and early larval development is important for large-scale seed production and aquaculture (Koumoundouros et al. 2001; Saillant et al. 2001). Teleost gastrulation was morphologically characterized by the presence of a germ ring (Arezo et al. 2005). In this study, gastrulation was observed at 11:36 hours and 50% epiboly began 13:45 h. I. japonicus embryo reached the eight-somite stage at 25:46 hours and reached the pre-hatching stage at 42 h with muscular contractions.
The development of teleost fins during incubation process is various among different species. For example, the fins of Salmonidae fish begin to develop before hatching, but the fins of other fish, such as Nibea albiflora, Paralichthys olivaceus, Scomberomorus niphonius and Engraulis japonicus, start to develop after hatching, and the pectoral fin rays form late (Kendall et al. 1984). In I. japonicus, pectoral fin buds developed early at the late embryonic stage, showing a fan-shape film with black spot after hatching. The pectoral fin was larger than the head 3 days after hatching, and three melanin spots spread on the edge of the fin films. The larvae were inactive but short periods of swimming were observed. They started swimming freely within 3–4 days. While many marine fish larvae had two kinds of energy reserves, yolk and oil globule (Bjelland and Berit 2006), devil stinger has only yolk sac. The yolk sac is depleted within 3–4 days and the larvae start to feed exogenously before complete absorption of the yolk sac. Mouth opening was on the third day. Primordium of tail fin appeared at 6 DAH, at the moment the pectoral fin had developed very large, and ten nicks formed on the edge of the fin rays, with fuscous melanin in each ray. After 20 days, bright gold yellow stripes appeared on the large fan-shape pectoral fin. In juveniles, the last two fin rays were separate from others. Possibly the development of pectoral fin in I. japonicus was corresponding with its functions. During larval stage, the fish were pelagic in the middle-upper waters, and fan-shape pectoral fin played a role in balance. When ten nicks showed up in the pectoral fin, they made the swimming of the larvae more accurate and flexible, guaranteeing their feeding successful. In the post-larvae, they changed the free-swimming to nestling on the bottom, because the large pectoral fin made their swimming slow. However, the powerful pectoral fins make the fish move quickly for a short distance intermittently, facilitating its successful feeding. In the juvenile stage, fish transferred to benthonic life style completely, and swam slowly on the bottom of the water supporting by the two separate pectoral fin rays. The development of pectoral fins in I. japonicus is useful for enhancing the active search and predation efficiency of food organisms, which is similar to the pectoral fins of yellow croaker Larimichthys crocea and river loach Triplophysa bleekeri (Li and Yan 2009; Wang et al. 2010).
Early larval development of I. japonicus was divided into four main periods: Yolk-sac larva: the presence of a yolk sac ventrally in the body, between hatching and 4 DAH. Yolk sac was absorbed and larvae swam actively 3–4 days after hatching, and the onset of exogenous feeding occurred 3 days later. Post yolk-sac larva: this period began at absorption of yolk sac and ended at the start of upward flexion of the notochord (between 4 and 12 DAH). Flexion larva: this period (the period during notochord flexion) was characterized with the hypural bones assuming a vertical position, between 13 and 15 DAH. Postflexion larva: the period between completion of flexion and the juvenile stage, 16–25 DAH. Our findings may provide a basis for further studying the complete early life history of I. japonicus and commercial production of this fish. The results of this study can contribute to a better understanding of the embryonic and larval development of other commercial scorpionfish larvae. They can be used to explain some aspects of the early life history at culture conditions and to develop better larval culture methodologies in hatchery. Similarly, they will be helpful to increase success rates in the larval culture of some scorpionfish fish species.
In the present study, based on salinity tolerance rest, salinity tolerance of I. japonicus was comparative wide, ranging from 10–30‰, the optimum salinity range was 10–20‰. Reducing the salinity appropriately did not negatively affect the development and growth of the larvae, but increased the survival of the larvae. This result was similar to the other fish species, such as Nibea miichthioides (Huang et al. 1997) and Pagrosomus major (Wang 2002). The SAI and MST are popular indexes for evaluating the vitality and quality of the larvae during the marine fish larviculture. In the present study, their values were higher at the salinities of 10–20‰, and were lower when salinity was below 10‰ or above 25‰. During the observation, larval development was normal under such salinity levels, indicating SAI and MST could be regarded as useful indicators for evaluating the optimum salinity range. Lin (2008) reported that suitable salinity range for I. japonicus larvae was 19–31‰, but he did not test the difference of salinity tolerance among different developmental stages, which was observed in the present study. The suitable salinity range for newly hatched larvae was 10–30‰. However, the suitable salinities for the yolk-sac larvae, mouth open larvae, and post yolk-sac larvae were almost the same, ranging from 10‰ to 20‰. The flexion larvae showed stronger low salinity tolerance compared with earlier stages, but this capacity decreased when the larvae finished metamorphosis. Except flexion larvae, all larvae were not able to survive at salinity 5‰, but lowering salinity appropriately could increase the survival of larvae in all developmental stages. Thus, in the present study, as a coastal fish species, the suitable salinity range for larviculture of I. japonicus was proved good at 10–20‰.
The SAI and MST displayed a similar trend under different salinities for all developmental stages. The SAI and MST are related to not only the nutrient storage, but also the living conditions. For example, when the salinity is suitable, the larvae only need to consume a little energy for osmoregulation, allocating large amount of energy to organ development and growth, thus survive longer under such conditions. However, when larvae are subject to lower or higher salinities, they need to spend more energy maintaining osmotic balance, and the other physiological functions are also affected, resulting in slow growth, reduced SAI and MST. In the present study, newly hatched larvae showed high SAI and MST at salinity 10–25‰, and larvae in other developmental stages showed higher values of the two parameters at salinity 10–20‰, indicating that the suitable salinity for the larviculture of I. japonicus should be reconsidered. Thus, the current salinity condition (30‰) in larviculture of Japanese devil stinger should be improved, and it is beneficial to reduce salinity moderately.