Osteological development of wild-captured larvae and a juvenile Sebastes koreanus (Pisces, Scorpaenoidei) from the Yellow Sea
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The osteological development in Sebastes koreanus is described and illustrated on the basis of 32 larvae [6.11–11.10 mm body length (BL)] and a single juvenile (18.60 mm BL) collected from the Yellow Sea. The first-ossified skeletal elements, which are related to feeding, swimming, and respiration, appear in larvae of 6.27 mm BL; these include the jaw bones, palatine, opercular, hyoid arch, and pectoral girdle. All skeletal elements are fully ossified in the juvenile observed in the study. Ossification of the neurocranium started in the frontal, pterotic, and parietal regions at 6.27 mm BL, and then in the parasphenoid and basioccipital regions at 8.17 mm BL. The vertebrae had started to ossify at ~7.17 mm BL, and their ossification was nearly complete at 11.10 mm BL. In the juvenile, although ossification of the pectoral girdle was fully complete, the fusion of the scapula and uppermost radial had not yet occurred. Thus, the scapula and uppermost radial fuse during or after the juvenile stage. The five hypurals in the caudal skeleton were also fused to form three hypural elements. The osteological results are discussed from a functional viewpoint and in terms of the comparative osteological development in related species.
KeywordsSebastes koreanus Korean fish Larvae Juvenile Osteological development
Osteological development in teleost fishes involves a sequence of remarkable morphological and functional changes, occurring in different developmental stages (Löffler et al. 2008; Kang et al. 2012; Ott et al. 2012). These ontogenetic changes strongly influence the feeding, breathing, and swimming behaviors of both larvae and juveniles, and are therefore useful in functional and ecological analyses and as a basis for phylogenetic inferences about relationships among teleost taxa (Omori et al. 1996; Faustino and Power 1999; Koumoundouros et al. 2000, 2001a, b; Liu 2001; Lima et al. 2013; Voskoboinikova and Kudryavtseva 2014). Practically speaking, an accurate knowledge of skeletal development is essential for the detection and elimination of skeletal deformities appearing during artificial seedling production and to promote effective aquacultural and resource management (Koumoundouros et al. 1997a, b).
Sebastes koreanus Kim and Lee 1994, in the family Scorpaenidae (or Sebastidae sensu Nakabo and Kai 2013), is smaller than its congeneric species and is regarded as endemic to the Yellow Sea (Kim and Lee 1994; Kim et al. 2005; Choi and Yang 2008). The species may be a good model fish with which to understand the phylogenetic relationships within the suborder Scorpaenoidei, because the species is specifically adapted to the unique marine environment of the Yellow Sea. Comparisons of S. koreanus with other Sebastes species have shown that S. koreanus collected in the wild exhibit wide ontogenetic variations in their pigmentation patterns and in their head–spine development (Yu et al. 2015). In addition, the restricted distribution of the species makes populations susceptible to collapse as a consequence of environmental pollution and/or the influence of climate change on the Yellow Sea. In this respect, artificial seedling production presents a viable approach to the conservation of susceptible species. However, no studies have yet been conducted on the details of skeletal development in S. koreanus, except for studies of morphological development and parturition season (Yu et al. 2015) and a small study of osteological development of reared larvae and juveniles in the Korea Strait (Park et al. 2015). Also, we confirmed some differences in osteological development of S. koreanus when compared with reared larvae (Park et al. 2015) and wild-captured larvae from the Yellow Sea. Therefore, in this study, we describe in detail the early skeletal development of S. koreanus in the context of functional changes, based on wild-captured larvae and juvenile. We also compare the osteological development of S. koreanus with that of congeneric species.
All individuals were collected from the eastern margin of the Yellow Sea. Larvae of S. koreanus [6.11–11.10 mm body length (BL), n = 32] were collected off the Taean Peninsula in May 2011 using a bongo net (0.6 m mouth opening, with 330 and 500 μm mesh size; bottom depth 15–24 m). The juvenile of S. koreanus (18.60 mm BL, n = 1) was collected off Gang-hwa-do in July 2012 using a stow net. The individuals were preserved in 5 % formalin immediately after collection. The specimens fixed in formalin were washed with distilled water and then preserved in 99 % ethanol. Before staining, each sample was identified according to the morphological characteristics of Yu et al. (2015), which were measured to the nearest 0.01 mm with the stereomicroscope (Olympus SZX16, Japan). The methods of measurement followed Leis and Carson-Ewart (2000). The measured body parts included BL and total length (TL). The anatomical terminology relating to skeletal structures follows Russell (1976), and the terminology of developmental stages follows Kim et al. (2011). The skeletal staining technique was derived from the double staining protocol of Darias et al. (2010). After staining, the specimens were examined on their right and dorsal sides with a stereomicroscope and photographs taken with a camera lucida (Olympus SZX-DA, Japan) attached to the microscope. Drawings of the different skeletal parts were prepared from the photographs. We compared the skeletal structures of the larvae and the juvenile with those of adult S. koreanus specimens, to observe the precise locations and shapes of the skeletal elements. We also compared stained specimens with a stained Sebastes inermis complex juvenile (17.06 mm BL, n = 1) collected in the wild. The stained specimens were preserved in 100 % glycerin in glass bottles and were deposited at Pukyong National University (PKU).
The examined materials included preflexion larvae (6.11–6.27 mm BL, n = 2; PKUI 367–368), flexion larvae (6.43–8.40 mm BL, n = 19; PKUI 369–387), postflexion larvae (8.44–11.10 mm BL, n = 11; PKUI 388–398), and a juvenile (18.60 mm BL, n = 1; PKUI 21).
Developmental sequence of ossification in Sebastes koreanus
Jaw bones, palatine, and opercular series
Hyoid arch and pectoral girdle
Vertebrae and caudal skeleton
This study is the first to examine and describe in detail the sequence of osteological development in S. koreanus collected in the wild and to provide data with which to infer the phylogenetic relationships of species within the suborder Scorpaenoidei. In S. koreanus, ossification of the skeletal elements is first observed in the neurocranium, jaw bones, palatine, opercular, hyoid arch, and pectoral girdle of the preflexion larva with a length of 6.27 mm BL [6.45 mm total length (TL)], and then the only one juvenile (18.60 mm BL) has fully completed the skeletal development of all elements (Table 1). In a previous study of early skeletal development in the genus Sebastes, ossification was first observed in S. inermis complex at 7 days (7.0 mm mean TL) (Kim et al. 1993), in S. schlegelii at 6–8 days (6.85 mm TL) (Kim and Han 1991), and in S. oblongus at 3 days after release (8.0 mm TL) (Byun et al. 2012). In a study of early skeletal development in S. macdonaldi from southern California, ossification was first observed in the smallest larva (6.11 mm BL) (Moser 1972). Like this, the ossification of wild-captured S. koreanus larvae was first observed in larvae smaller than reared larvae of S. inermis, S. schlegelii, and S. oblongus (but not in S. macdonaldi). These differences in the size at the onset of ossification are probably related to the size at which the larvae are released from the adult, which is smaller than 6.11 mm BL in S. koreanus (this study), 6.12 mm TL in S. inermis (Kim et al. 1993), 5.52 mm TL in S. schlegelii (Kim and Han 1991), 7.2 mm TL in S. oblongus (Byun et al. 2012), and 4.5 mm BL in S. macdonaldi (Moser 1972). These differences may also be affected by external environmental factors, such as temperature and salinity (Fuiman 2002; Ložys 2004; Löffler et al. 2008; Ott et al. 2012), which may cause corresponding osteological differences (Matsuoka 1987; Wimberger 1993; Koumoundouros et al. 1997a) and meristic variations (Fowler 1970; Lau and Shafland 1982) between reared larvae and wild-captured larvae (Boglione et al. 2001). Therefore, despite the similar size of the released larvae of S. koreanus, S. inermis, and S. schlegelii, the first ossification size may also differ from each other as larvae of S. koreanus were collected in the wild while the other species were cultured in captivity. In addition, ossification was first observed in the reared larvae of Sebastiscus marmoratus and Sebastiscus tertius at 3.35 and 4.4 mm TL, respectively, even smaller than the larvae of species of Sebastes (Kim et al. 1997; Han et al. 2001). These results are also probably related to the size at parturition; at parturition, size is smaller than the larvae of Sebastes (Kim et al. 1997; Han et al. 2001).
In most cases, early skeletal development occurs first in elements that are necessary for feeding and respiration and therefore affect the survival of young larvae (Vandewalle et al. 1997; Wagemans and Vandewalle 1999). For example, the total resorption of the vitellus is essential for the transition from endogenous to exogenous feeding, because the efficiency of suction feeding increases with increasing prey size and the ossification of the related skeletal elements (Gluckmann et al. 1999). In S. koreanus, the skeletal elements that first start to ossify (at 6.27 mm BL) are the premaxillary, maxillary, dentary, preopercle, opercle, hyomandibular, and the fifth branchiostegal ray (Figs. 3 and 4), and the order of ossification is initially defined by the importance of the skeletal elements to feeding, swimming, and respiration. The cleithrum in the pectoral girdle ossifies in the same developmental stage (Fig. 5), and the early ossification of the clavicle produces an attachment site for the sternohyoideus muscle, which is important for swimming in subsequent growth stages (Wagemans and Vandewalle 1999; Koumoundouros et al. 2001a; Cloutier et al. 2011). Similar patterns of early skeletal development have been observed in other species (e.g., S. inermis, S. schlegelii, S. oblongus, Sebastiscus marmoratus, and Sebastiscus tertius). However, the timing of the ossification of the hyomandibular is highly variable (Kim and Han 1991; Kim et al. 1993; Kim et al. 1997; Byun et al. 2012). In most teleostei, the parasphenoid is the first element to ossify, except in some species in which the parasphenoid ossifies simultaneously with the frontals (Pagrus major; Matsuoka 1987) or the basioccipital (Heterobranchus longifilis; Vandewalle et al. 1997), or is ossified after the ossification of the frontals (Scophthalmus maximus; Wagemans et al. 1998). In S. koreanus, the first-ossified elements in the neurocranium (at 6.27 mm BL) are the parietal, frontal, and pterotic (Fig. 1). Subsequently, the parasphenoid and basioccipital begin to ossify at 8.17 mm BL (Fig. 1); these elements may help to reinforce the cranial floor to prevent damage to the neurocranium during feeding (Vandewalle et al. 1992) and to promote the balance needed for swimming (Weisel 1967). Therefore, the ossification of the parasphenoid and basioccipital during early skeletal development is important because they significantly affect feeding and swimming behavior, as do the jaw bones and clavicle, respectively.
The order of ossification of the neurocranial elements appears similar in different Sebastes species, but variations exist, particularly in the timing of the ossification of the parasphenoid. In many species of Sebastes and Sebastiscus, such as S. macdonaldi, S. inermis, S. schlegelii, Sebastiscus marmoratus, and Sebastiscus tertius, the parasphenoid is the first element to ossify (Moser 1972; Kim and Han 1991; Kim et al. 1993; Kim et al. 1997; Han et al. 2001), whereas in S. koreanus, the parasphenoid begins to ossify simultaneously with the basioccipital and exoccipital, just after the ossification of the parietal, frontal, and pterotic (present study), or in S. oblongus, the parasphenoid begins to ossify simultaneously with the supraoccipital, just after the ossification of the parietal and frontal (Byun et al. 2012). The pterotic and parietal also begin to ossify relatively early in some species, including S. koreanus, S. macdonaldi, Sebastiscus marmoratus, and Sebastiscus tertius, but no clear differences between the species of Sebastes and Sebastiscus are apparent (Moser 1972; Kim et al. 1997; Han et al. 2001). In S. koreanus, the early ossification of the hyoid arch appears on the ceratohyal and branchiostegal rays, but there is no additional ossification of elements between 8.17 and 11.10 mm BL (Fig. 4). In contrast, the ossification of the hyoid arch is clearly different in many species of Sebastes and Sebastiscus from that observed in S. koreanus and begins to occur at the same time as the ossification of the ceratohyal and epihyal (in S. inermis, S. schlegelii, Sebastiscus marmoratus, and Sebastiscus tertius), or the epihyal begins to ossify just after the ossification of the ceratohyal (as in S. oblongus) (Kim and Han 1991; Kim et al. 1993; Kim et al. 1997; Han et al. 2001; Byun et al. 2012).
Ossification of the pectoral girdle also shows a high degree of variability between different species of Sebastes and Sebastiscus. In S. koreanus, the ossification of the pectoral girdle begins with the clavicle, followed by the upper clavicle and soon thereafter by the supratemporal and posttemporal (Fig. 5). In contrast, in S. oblongus, the ossification of the clavicle first begins 3 days after release, and the ossification of the upper clavicle and posttemporal begin at 20 days, soon after the initial ossification of the supratemporal (Byun et al. 2012). In S. inermis, the first ossification on the clavicle begins 7 days after release, followed by the ossification of the postclavicle at 45 days and the upper clavicle at 50 days. The supratemporal begins to ossify at 65–69 days (Kim et al. 1993). In Sebastiscus marmoratus, the ossification of the clavicle first appears 5 days after release, and the ossification of the upper clavicle, posttemporal, scapula, and coracoid begin at 28 days (Kim et al. 1997). Therefore, it is difficult to determine a common ossification pattern for the pectoral girdle because of the observed variability between species. Also, S. koreanus ossified faster than other species of Sebastes and Sebastiscus, because this species might be to adapt to the harsh environment of the Yellow Sea such as strong current. According to Ishida (1994), adults of Sebastes, Sebastiscus, and Hozukius (suborder Scorpaenoidei) share the derived characteristic of a fusion of the scapula and uppermost radial in the pectoral girdle. However, although the fusion of the scapula and uppermost radial was observed in adults of S. koreanus, fusion was not observed in the S. koreanus juvenile (18.60 mm BL) (Fig. 5g). In some species of Sebastes (e.g., S. oblongus, Byun et al. 2012; S. schlegelii, Kim and Han 1991; Omori et al. 1996; S. macdonaldi, Moser 1972), fusion between the scapula and uppermost radial is not observed during skeletal development. Thus, it appears that the scapula and uppermost radial fuse slowly after (or starting in) the juvenile stage. Actually, the scapula and uppermost radial were closely adjoined along a thin boundary line in the S. koreanus juvenile observed in this study, presumably just prior to fusion. Since some ontogenetic studies have resolved taxonomic uncertainty that cannot be defined by adult morphology (Hubbs and Kampa 1946; White et al. 1983; Parin 1996), further ontogenetic study is needed to understand interrelationship between S. koreanus and Sebastes spp.
With respect to locomotion, the swimming of larvae immediately after their release from the adult is possible only through the antagonistic interactions of the notochord and trunk muscles (Ott et al. 2012). With growth, the notochord is gradually replaced by vertebrae, and the ossified vertebrae contribute stronger attachment sites for the powerful dorsalis trunci muscles, which are primarily responsible for swimming (Rojo 1991). In S. koreanus, after the ossification of the neural spine at 7.17 mm BL, the ossification of the vertebral centra mainly proceeds from the abdominal to the caudal vertebrae, and the urostyle is fully ossified just before the ossification of the caudal vertebrae is complete (Fig. 7). This pattern is similar to that observed in other species of Sebastes and Sebastiscus, except in S. schlegelii (Kim and Han 1991; Omori et al. 1996), e.g., in S. oblongus (Byun et al. 2012), S. inermis (Kim et al. 1993), Sebastiscus marmoratus (Kim et al. 1997), and Sebastiscus tertius (Han et al. 2001). Furthermore, in adults, the caudal skeleton in Sebastes and Sebastiscus species is formed by three hypurals (hy 1 + 2, hy 3 + 4, and hy 5), because the first and second hypurals and the third and fourth hypurals are fused (Ishida 1994). A similar trend is observed in the skeletal development of Sebastes and Sebastiscus species, including S. koreanus, S. inermis, S. schlegelii, S. koreanus, S. macdonaldi, Sebastiscus marmoratus, and Sebastiscus tertius (Fig. 7f) (Kim et al. 1993; Omori et al. 1996; Kim et al. 1997; Han et al. 2001; Byun et al. 2012). Therefore, the ontogenetic characteristics reflect the taxonomic characteristics of the adults well. In particular, the hypural cartilages fuse before ossification, unlike the fusion of the scapula and the uppermost radial (Omori et al. 1996).
Park et al. (2015) provided a brief overview of the external and osteological development of S. koreanus based on the artificial breeding of hatched larvae, using a gravid adult collected from the Korea Strait. However, compared with larvae and juvenile of S. koreanus collected from the wild in the Yellow Sea (present study; Yu et al. 2015), there are several differences, such as the pigmentation patterns and sequence of osteological development. We established three hypotheses regarding their differences. The first hypothesis is that it could be caused by the different population, as the present study, and Park et al. (2015) used different sampling sites (Yellow Sea vs. Korea Strait). In this respect, there is a possibility of existence of different population. Similarly, Kim et al. (2010) confirmed that the two populations of Ammodytes personatus larvae showed morphological differences in morphometric characters and pigmentation. The second hypothesis is that it could be caused by the difference in sea water temperature. The larvae and juvenile of S. koreanus (present study) were collected at the average sea water temperature 12.0–15.1 °C from Taean Peninsula (May and June), whereas larvae and juveniles were reared at the sea water temperature 13.5–15.5 °C. This slight water temperature difference (ca. 1.5 °C) could possibly cause the ontogenetic differences (Löffler et al. 2008; Ott et al. 2012). Finally, the third hypothesis is that it could be caused by differences of food source between reared larvae and wild-captured larvae. Particularly, calcium deficiency at the food source induces a delay in the ontogeny of skeletal development without affecting final bone mineralization (Fontagné et al. 2009). In addition, the different fixatives, formalin or alcohol, may have reduced or removed some pigmentation. Therefore, further research with microsatellite DNA and comparison of rearing is required to confirm the observed difference between reared larvae and wild-captured larvae.
In summary, although the sequences and periods of osteological development in Sebastes and Sebastiscus species show some variation, the early ossification of the skeleton proceeds in a sequence that prioritizes the elements required for feeding, swimming, and respiration. In this way, larvae are equipped with functional capacities that enhance the probability of their survival at this stage of their life cycle. Also, the ossification of parasphenoid and epihyal in the neurocranium appeared relatively later than congeneric species, this features indicated that S. koreanus is a unique species, which has been evolved in distinctive marine environment of the Yellow Sea.
This research was supported by the Marine Fish Resources Bank of Korea (MFRBK) under the Ministry of Oceans and Fisheries, Korea.
HJY performed the experiments and wrote the manuscript. JKK suggested all aspects of study design and commented on the earlier drafts of the manuscript. Both authors read and approved the final manuscript.
The authors declare that they have no competing interests.
- Byun SG, Kang CB, Myoung JG, Cha BS, Han KH, Jung CG. Early osteological development of the larvae and juveniles in Sebastes oblongus (Pisces: Scorpaenidae). Korean J Ichthyol. 2012;24:67–76.Google Scholar
- Choi Y, Yang AF. Intertidal fishes from the Shandong Peninsula, China. Korean J Ichthyol. 2008;20:54–60.Google Scholar
- Fuiman LA. Special considerations of fish eggs and larvae. In: Fuiman LA, Werner RG, editors. Fishery science: the unique contributions of early life history stages. Oxford, UK: Blackwell Science; 2002. p. 1–32.Google Scholar
- Gluckmann I, Huriaux F, Focant F, Vandewalle P. Postembryonic development of the cephalic skeleton in Dicentrarchus labrax (Pisces, Perciformes, Serranidae). Bull Mar Sci. 1999;65:11–36.Google Scholar
- Han KH, Lim SK, Kim KS, Kim CW, Yoo DJ. Osteological development of the larvae and juveniles of Sebasticus tertius (Barsukov et Chen) in Korea. Korean J Ichthyol. 2001;13:63–8.Google Scholar
- Ishida M. Phylogeny of the suborder Scorpaenoidei (Pisces: Scorpaeniformes). Bull Nansei Nat Fish Res Inst. 1994;27:1–11.Google Scholar
- Kang CB, Myoung JG, Kim YU, Kim HC. Early osteological development and squamation in the spotted sea bass Lateolabrax maculates (Pisces: Lateolabracidae). Kor J Fish Aquat Sci. 2012;45:271–82.Google Scholar
- Kim IS, Choi Y, Lee CL, Lee YJ, Kim BJ, Kim JH. Illustrated book of Korean fishes. Seoul, KR: Kyo-Hak Publishing Co; 2005.Google Scholar
- Kim IS, Lee WO. A new species of the genus Sebastes (Pisces; Scorpaenidae) from the Yellow Sea, Korea. Korean J Zool. 1994;37:409–15.Google Scholar
- Kim JK, Ryu JH, Kim S, Lee DW, Choi KH, Oh TY, et al. An identification guide for fish eggs, larvae and juveniles of Korea. Busan, KR: Hanguel Graphics Publishing Co; 2011.Google Scholar
- Kim YU, Han KH. The early life history of rockfish, Sebastes schlegeli. Korean J Ichthyol. 1991;3:67–83.Google Scholar
- Kim YU, Han KH, Byun SK. The early life history of the rockfish, Sebastes inermis. 2. Morphological and skeletal development of larvae and juveniles. Bull Korean Fish Soc. 1993;26:465–76.Google Scholar
- Kim YU, Han KH, Kang CB, Kim JK, Byun SK. The early life history of the rockfish, Sebastiscus marmoratus. 2. Morphological and skeletal development of larvae and juveniles. Korean J Ichthyol. 1997;9:186–94.Google Scholar
- Koumoundouros G, Divanach P, Kentouri M. Osteological development of Dentex dentex (Osteichthyes: Sparidae): dorsal, anal, paired fins and squamation. Mar Biol. 2001a;138:399–406.Google Scholar
- Koumoundouros G, Gagliardi F, Divanach P, Boglione C, Cataudella S, Kentouri M. Normal and abnormal osteological development of caudal fin in Sparus aurata L. fry. Aquaculture. 1997a;149:215–26.Google Scholar
- Koumoundouros G, Oran G, Divanach P, Stefanakis S, Kentouri M. The opercular complex deformity in intensive gilthead sea bream (Sparus aurata L.) larviculture. Moment of apparition and description. Aquaculture. 1997b;156:165–77.Google Scholar
- Koumoundouros G, Sfakianakis D, Maingot E, Divanach P, Kentouri M. Osteological development of the vertebral column and of the fins in Diplodus sargus (Teleostei: Perciformes: Sparidae). Mar Biol. 2001b;139:853–62.Google Scholar
- Leis JM, Carson-Ewart BM. The larvae of indo-pacific coastal fishes: an identification guide to marine fish larvae. Leiden, Netherlands: Brill; 2000.Google Scholar
- Liu CH. Early osteological development of the yellowtail Seriola dumerili (Pisces: Carangidae). Zool Stud. 2001;40:289–98.Google Scholar
- Matsuoka M. Development of skeletal tissue and skeletal muscle in the red sea bream, Pagrus major. Bull Seikai Reg Fish Res Lab. 1987;65:1–102.Google Scholar
- Moser HG. Development and geographic distribution of the rockfish, Sebastes macdonaldi (Eigenmann and Beeson, 1893), Family Scorpaenidae, off Southern California and Baja California. US Nat Mar Fish Serv Fish Bull. 1972;70:941–58.Google Scholar
- Nakabo T, Kai Y. Sebastidae. In: Nakabo T, editor. Fishes of Japan with pictorial keys to the species. 3rd ed. Tokyo, JP: Tokai Univ Press; 2013. p. 668–81.Google Scholar
- Parin NV. On the species composition of flying fishes (Exocoetidae) in the west-central part of tropical Pacific. J Ichthyol. 1996;36:357–64.Google Scholar
- Park JM, Cho JK, Han H, Han KH. Morphological and skeletal development and larvae and juvenile of Sebastes koreanus (Pisces: Scorpaenidae). Korean J Ichthyol. 2015;27:1–9.Google Scholar
- Rojo AL. Dictionary of evolutionary fish osteology. Boca Raton, FL: CRC Press; 1991.Google Scholar
- Russell FS. The eggs and planktonic stages of British marine fishes. London, U.K.: Academic Press Inc; 1976.Google Scholar
- Vandewalle P, Gluckmann I, Baras E, Huriaux F, Focant B. Postembryonic development of the cephalic region in Heterobranchus longifilis. J Fish Biol. 1997;50:227–53.Google Scholar
- Wagemans F, Vandewalle P. Development of the cartilaginous skull in Solea solea: trends in Pleuronectiforms. Ann Sci Nat. 1999;1:39–52.Google Scholar
- White BN, Lavenberg RJ, McGowen GE. Atheriniformes: development and relationships. Spec Publ Soc Ichthyol Herpetol. 1983;1:355–62.Google Scholar
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