Morphological description of saddleback silver-biddy (gerres limbatus) larvae and juveniles from the Tien Yen estuary, Viet Nam

JOURNAL OF SCIENCE OF HNUE Chemical and Biological Sci., 2014, Vol. 59, No. 9, pp. 88-94 This paper is available online at MORPHOLOGICAL DESCRIPTION OF SADDLEBACK SILVER-BIDDY (Gerres limbatus) LARVAE AND JUVENILES FROM THE TIEN YEN ESTUARY, VIETNAM Tran Trung Thanh1, Ta Thi Thuy2 and Tran Duc Hau1 1Faculty of Biology, Hanoi National University of Education 2Faculty of Natural Sciences and Technology, Hanoi College of Education Abstract. Larvae and juveniles of Saddleback silver-biddy (Gerres limbatus Cuvier, 1830) were collected using a small seine net from the bank waters of the Tien Yen Estuary, Quang Ninh Province, in northern Vietnam. These larvae and juveniles are compressed and relatively slender. Weak spinations are present on the head and on the preopercle. Teeth are visible on the tip of the lower jaw. A protractile mouth with a long ascending premaxillary process is observed in the fish. The gut is short and coiled. A small and inconspicuous gas bladder is present above the anterior portion of the gut. The gap between the vent and anal fin is large and decreases with growth. The main counting meristic characters are as follows: D = IX, 10; A = III, 7; P = 13 - 15; 24 myomeres. The dorsal profile has four melanophores patches. Morphological comparisons with other congeneric species are also given in the present study. Keywords: Saddleback silver-biddy, early stages, morphology, Tien Yen Estuary. 1. Introduction The family Gerreidae consists of 8 genera and 44 species, mainly distributed in sea water, occasionally in brackish water and rarely in freshwater [7]. In Vietnam, 9 species of the genus Gerres are dominant [8] and many of their larvae and those showing juvenile morphology have been reported [2, 4]. A description of a G. lucidus juvenile (about 11 mm BL) was reported by Jeyaseelan [2] but the G. lucidus was considered to be a young specimen of G. limbatus [1]. Until now, a juvenile specimen of G. limbatus has been sketched only by Jeyaseelan [2] and there has been no data on its morphological characteristics, thus little is known about the early stage of this species. A survey was Received November 6, 2014. Accepted December 3, 2014. Contact Tran Duc Hau, e-mail address: tdhauzoo@yahoo.com 88 Morphological description of saddleback silver-biddy (Gerres limbatus) larvae and juveniles... carried out in the Tien Yen Estuary, Quang Ninh Province, at which time a number of larvae and juvenile Gerres specimens were collected and, based on morphological characteristics, these fish were identified as G. limbatus. Herein, this paper will describe the morphology of G. limbatus in both the larval and juvenile stage (3.9 - 10.9 mm). 2. Content 2.1. Materials and methods A total of 103 specimens of G. limbatus (3 flexion larvae, 3.9 - 4.2 mm, 91 postflexion larvae, 4.0 - 7.4 mm and 9 juveniles, 9.8 - 10.9 mm BL) were collected in the bank waters of the Tien Yen River system (N 2133’20”N, 10725’20”E) in northern Vietnam from March 2013 to February 2014 using a small seine net (1  4 m, 1 mm mesh-aperture) [5]. Fish samples were fixed in 5% formalin and then sorted. Specimens were preserved in 70% ethanol and subsequently measured by sizes to determine developmental stage [3]. In this study, unlabeled lengths indicate body length (BL) (notochord length for flexion larvae and standard length for postflexion larvae and juveniles). Proportional measurements follow Leis & Rennis [6]. Observations and drawings of G. limbatus were made with a binocular microscope and camera lucida. Samples used in this study were deposited at the Faculty of Biology, Hanoi National University of Education. Identification of the present samples of G. limbatus follow Jeyaseelan [2], Kinoshita [4] and Leis & Rennis [6]. 2.2. Results and discussion 2.2.1. Identification of G. limbatus larvae and juveniles Larvae and juveniles of G. limbatus (Figure 1) were identified at the species level based on the following combination of characteristics: weak head spination, distinctive pigmentation, protractile mouth with its long ascending premaxillary process, short coiled gut, inconspicuous gas bladder, large gap between the vent and anal fin, and tiny teeth in the tip of the lower jaw. Meristics are as follows: D IX, 10; A III, 7; P 13-15; 24 myomeres. The melanophores patterns are characterized by the four pigmentation patches which become four diffused dark saddles along the upper half of the body. Because the characteristics agree with the description in Jeyaseelan [2], and compare well with congeners Leis & Rennis [6] and Kinoshita [4], the fish are identified as G. limbatus. 2.2.2. Description of the G. limbatus larvae and juveniles Morphology: larvae and juveniles are laterally compressed and relatively slender (BD = 16.4-28.8% BL) (Figure 2e). The head is moderate in size, broad (HL = 25.4-36.1% BL) (Figure 2f). The mouth is short (4.9-13.3% BL), and becomes protractile having an 89 Tran Trung Thanh, Ta Thi Thuy and Tran Duc Hau ascending process of the premaxillary (5.9-15.4% BL) (Figure 2b). Tiny teeth are present in the tip of the lower jaw. The eye is round and large (10.3-15.4% BL) (Figure 2a). The proportional parts increase with growth except for the gap between vent and anal fin which decreases (Figure 2). Small preopercular spines could be found in some observed specimens (Figure 1D). The gap between the vent and anal fin base is large and becomes smaller with growth, from ca. 22 to 6% BL (Figure 2c). This means that the gut becomes longer and the vent migrates to the middle; the pre-anal length (PAL) is from 38.0 to 54.3% BL (Figure 2g). A gas bladder is present above the anterior portion of the gut. Figure 1. Developmental stages of G. limbatus collected from the Tien Yen Estuary A. 4.2 mm flexion larvae, B. 5.1 mm postflexion larvae, C. 6.4 mm postflexion larvae, D. 10.9 mm juvenile 90 Morphological description of saddleback silver-biddy (Gerres limbatus) larvae and juveniles... The developmental stages of this fish are as follows: from 3.9 to 4.2 mm BL, flexion larvae (Figure 1A). The body has pectoral, dorsal and anal fins, but their rays are incipient. Pelvic buds are not present (Figure 1A). At 5.1 mm, the three former fin rays begin to ossify in the postflexion larvae (Figure 1B). The pelvic fin starts to form. At 6.4 mm BL, the anal and pelvic fin rays are completed but the pectoral fin rays are incomplete in the postflexion larvae (Figure 1C). The soft rays of the dorsal fin are completely formed. The dorsal fin spines start to form at ca. 6.0 mm BL and 9 fin rays a represent at 6.4 mm BL, but they have not attained full length or complete ossification. The nostrils are not divided in a 6.4 mm BL specimen (Figure 1C). From 9.8 to 10.9 mm BL, all fins are fully developed in juveniles (Figure 1D). The caudal fin becomes deeply forked. Gerres filamentosus, Gerres limbatus, Gerreomorpha japonicas (synomym of Gerres japonicas), Gerres erythrourus, Gerreomorpha decacanthus (synomym of Gerres decacanthus) and Gerres oyena found in Vietnamese estuaries [8, 9], the first four species being present in the Tien Yen Estuary [9]. Because the larvae and juveniles of the above six species have been described, a comparison could be made in this study and the present samples could be identified as G. limbatus. First, 10 dorsal fin spines are present in G. japonicas and G. decacanthus, and 9 are present in G. filamentosus, G. limbatus, G. oyena and G. erythrourus [4, 8]. Therefore, the present specimens belong to the latter group. To distinguish the target species from the latter group, the melanophores patterns were examined. Pigmentation: At 4.2 mm BL, there are no melanophores on the head of flexion larvae (Figure 1A). There are three small star-shaped melanophores along the ventral aspect of the gut, one just posterior to the vent, one at the midline between the vent and the anal fin, and one just anterior to the anal fin base. Three melanophores are found along the anal fin base and four along the caudal peduncle. Two melanophores are present at the upper part of the caudal fin rays. Internal melanophores are visible at the dorsal margin of the gas bladder. At 5.1 mm BL, melanophores appear on the supraoccipital and on the epiotic of postflexion larvae (Figure 1B). Some small melanophores are present scattered on the operculum. More melanophores are distributed along the anal fin base and along the peduncle. At 6.4 mm BL, melanophores appear on the premaxillary and 4 large star-shaped melanophores on the supraoccipital of postflexion larvae (Figure 1C). Small melanophores are present on the upper margin of the operculum near the pectoral fin base. Two star-shaped melanophores appear on the base of the last two soft rays of the dorsal fin. X-shaped melanophores are present along the anal fin base. At 10.9 mm BL, star-shaped melanophores are visible on the upper margin of the operculum and present scattered on the operculum of juveniles (Figure 1D). The dorsal profile has four melanophores patches: one on the supraoccipital, one on the beginning of the dorsal base, one on the middle of 91 Tran Trung Thanh, Ta Thi Thuy and Tran Duc Hau dorsal base and one from the 7th soft ray of the dorsal fin to the base of the caudal fin. X-shaped melanophores become more distinctive on the anal fin base and on the peduncle. The juvenile has heavy pigment, with some large patches on the trunk (Figure 1D). Figure 2. Sequence of changes in the ratio of each measured part to the body length of G. limbatus larvae and juveniles 92 Morphological description of saddleback silver-biddy (Gerres limbatus) larvae and juveniles... In previous studies [2, 4, 6], it has been suggested that species of Gerres genus develop melanophores with growth, especially on the margin of the trunk. The character of the melanophores could be used to distinguish species among the 9-dorsal spine group (i.e., G. filamentosus, G. limbatus, G. oyena and G. erythrourus). The present samples differ from the larvae and juveniles of G. oyena measuring 5.0, 5.1, 7.5 and 9.8 mm BL [4] and having a heavier pigment on the anal fin base and the ventral peduncle. The 12.6 mm BL G. filamentosus juvenile in [4] also has lighter pigment than that of the 10.9 mm BL present juveniles (Figure 1D), thus the present specimens are neither G. filamentosus nor G. oyena. In this group, the present samples resemble that of G. erythrourus of the same size (10.7 mm BL juvenile stage) [4]. However, a difference can be found between the two types in the distribution of the melanophores. The 8.1 mm BL larvae of G. erythrourus have two star-shaped melanophores on the anal fin base [2] while a number of x-shaped melanophores appear along the anal fin base at 6.4 mm BL larvae in the present study (Figure 1C). It is important to note that the star-shaped melanophores appear on the beginning of the anal fin in the present specimens, unlike that of G. erythrourus. In additional, there is a difference in the ratios of PAL (mode at 42% in the present study vs. 48% in Kinoshita [4]), HL (28 vs. 33) and BD (20 vs. 26) (Figure 2e-g). Consequently, based on the above comparisons, the present sample specimens are considered to be G. limbatus. When compared with other species of the Gerres genus, i.e., G. setifer and G. oblongus [2], there are also some differences in the morphology of their larvae. G. setifer (4.2 - 9.6 mm BL) has 10 dorsal spines (vs. 9 in the present samples) and has less developed melanophores than the present specimens. Larvae and juveniles of G. oblongus (5 - 11 mm BL) are similar to G. limbatus in fin rays counts and distribution of the melanophores on the head and anal fin base, but different from G. limbatus in that they are without melanophores distributed on the trunk [2]. 3. Conclusion Descriptions of the larvae and juveniles of G. limbatus collected from the Tien Yen Estuary are given in the present study. This is the first data to be presented on the larval morphology of the fish. 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