External and Internal Morphological... (Hagi Yulia Sugeha and Marlina Ummas Genisa)

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1 External and Internal Morphological... (Hagi Yulia Sugeha and Marlina Ummas Genisa) EXTERNAL AND INTERNAL MORPHOLOGICAL CHARACTERISTICS OF GLASS EELS Anguilla bicolor bicolor FROM THE CIBALIUNG RIVER ESTUARY, BANTEN, INDONESIA KARAKTERISTIK MORFOLOGI EKSTERNAL DAN INTERNAL JUVENIL Anguilla bicolor bicolor DARI MUARA SUNGAI CIBALIUNG, BANTEN, INDONESIA Hagi Yulia Sugeha 1* and Marlina Ummas Genisa 2 1 Research Center for Oceanography, Indonesian Institute of Sciences, Jakarta, Indonesia 2 Graduate Program for Biology, Gadjah Mada University, Yogyakarta, Indonesia * address: hysugeha@gmail.com Received 18 December 2013, Accepted 16 February 2015 ABSTRACT Anguilla bicolor bicolor is one of the tropical eel species collected commercially along the southern coasts of the western Indonesian region, particularly during the glass eel stage. However, basic biological information on the morphological characteristics of the species during their early life stages is limited. This study was conducted in the Cibaliung River estuary, Banten, Indonesia, from May to October 2005, and examined the external and internal morphological characteristics of the species during the glass eel stage, based on length measurement and vertebrae count analyses. A total of 140 tropical anguillid glass eels were collected during the six months of investigation with the biggest catch in October. The 116 specimens were identified as A. bicolor bicolor based on the characteristics of ADL/%TL (anodorsal length as a percentage of total length: range = %; mean ± SD = 1.78 ± 1.16) and the number of ADV (anodorsal vertebrae: - 3-4; 1.15 ± 1.24). The other external characteristics (predorsal length as a percentage of total length = PDL/%TL; preanal length as a percentage of total length = PAL/%TL) and internal characteristics (predorsal vertebrae = PDV; preanal vertebrae = PAV; abdominal vertebrae = ABV) overlapped and could not be used as morphological key characteristics to identify the species in the sampling area. Anguilla bicolor bicolor glass eels from the Cibaliung River estuary were about g (0.13 ± 0.02) in body weight and were about mm (53.01 ± 1.75) in total length. Melanophore pigmentation development of the specimens was dominated by the pigmentation stage VA (94.2%), compared to the pigmentation stage VB (5.8%), suggesting they were in the glass eel stage when entering the mouth of the Cibaliung River. Based on length-weight relationship analysis it was found that A. bicolor bicolor glass eels recruited to the Cibaliung River estuary have a positive linear growth pattern with a regression equation: y = x and r² = with P-value < Keywords: Anguilla bicolor bicolor, glass eel, Cibaliung River, morphological characteristics ABSTRAK Anguilla bicolor bicolor adalah salah satu spesies ikan sidat tropis yang sering ditangkap di sepanjang pantai selatan perairan Indonesia Barat pada tahap juvenil untuk tujuan komersial. Akan tetapi, informasi biologis yang mendasar terkait karakteristik morfologis spesies tersebut pada tahap awal daur hidupnya masih terbatas. Studi ini dilaksanakan di muara Sungai Cibaliung, Banten, Indonesia dari Mei hingga Oktober 2005, dan telah mengkaji karakteristik morfologis ekternal dan internal A. bicolor bicolor selama tahap juvenil (glass eel), berdasarkan analisis hasil pengukuran panjang tubuh dan penghitungan jumlah tulang vertebra. Sebanyak 140 juvenil dari ordo Anguilliformes telah dikoleksi selama 6 bulan penelitian dengan koleksi terbesar didapat pada bulan Oktober. Berdasarkan karaktersitik ADL/%TL (panjang anodorsal sebagai persentase terhadap panjang total: kisaran = %; rata-rata ± SD = 1,78 ± 1,16) dan ADV (vertebra anodorsal: -3--4; 1,15 ± 1,24), sebanyak 116 spesimen di antaranya teridentifikasi sebagai A. bicolor bicolor. 37

2 Oseanologi dan Limnologi di Indonesia. Vol. 41, No. 1, April 2015: Karakteristik morfologis ekternal lainnya (persentase panjang predorsal terhadap panjang total = PDL/%TL; persentase panjang preanal terhadap panjang total = PAL/%TL) dan karakteristik internal lainnya (vertebra predorsal = PDV; vertebra preanal = PAV; vertebra abdominal = ABV) saling tumpang tindih dan tidak bisa digunakan sebagai karakteristik kunci morfologis untuk mengidentifikasi spesies tersebut dari wilayah perairan studi. Juvenil A. bicolor bicolor dari muara Sungai Cibaliung memiliki kisaran berat tubuh 0,09-0,18 g (0,13 ± 0,02) dan kisaran panjang tubuh mm (53,01 ± 1,75). Perkembangan pigmentasi melanofor didominasi oleh pigmentasi tahap VA (94,2%) jika dibandingkan dengan pigmentasi tahap VB (5,8%) yang mengindikasikan bahwa keseluruhan specimen A. bicolor bicolor berada pada tahap juvenil saat memasuki muara Sungai Cibaliung. Berdasarkan analisis hubungan panjang berat, diketahui bahwa juvenil A. bicolor bicolor yang memasuki muara Sungai Cibaliung memiliki pola pertumbuhan linier positif dengan persamaan regresi: y = x and r² = dengan nilai P < Kata kunci: Anguilla bicolor bicolor, juvenil, muara Sungai Cibaliung, karakteristik morfologis INTRODUCTION The glass eel stage is a part of the life cycle of catadromous eels from the genus Anguilla. Eels of this genus born in the ocean but grow in the freshwater and returning to the ocean for maturation and reproduction. Glass eels have a transparent elongated body shape when they arrive in the estuary after several months as larvae, namely leptocephali, in the ocean. The arrival of tropical glass eels in the estuary usually occurs during a very short period, following the patterns of tidal streams into the rivers (Sugeha et al., 2001a, b; 2008a, b; Sugeha, 2010). Furthermore, it was reported that the inshore migration of tropical glass eels occured during the night of a new moon when the tidal cycles reached their minimum and maximum levels (Sugeha et al., 2001a, b; 2008a, b). The timing of the inshore migration of tropical glass eels is different from that of temperate glass eels that occurs during both day and night and is less affected by the phases of the moon (Sugeha et al., 2001a, b; 2008a, b). Most species of tropical glass eels migrate inshore throughout most of the year with the peak migration takes place during the dry season (Sugeha et al., 2001a, b; 2008a, b), while temperate glass eels migrate inshore for half the year, mainly in the winter (Jellyman et al., 1977; Todd, 1981; Sloane, 1984). Previous studies reported the occurrence of nine species and subspecies of tropical eels from the genus Anguilla in Indonesian waters (Sugeha et al., 2008a) that included Anguilla bicolor. The species is distributed along the Indo- Pacific region from the coastal areas of Madagascar in the Indian Ocean to the west coasts of Sumatra, the south coasts of Java, through the sea of Sulawesi, Halmahera, Nusa Tenggara and Western Papua, Vietnam, The Philippines, to Papua New Guinea in the Pacific Ocean (Ege, 1939; Jespersen, 1942; Watanabe et al., 2004; Jamandre et al., 2007; Sugeha et al., 2008a, b; Sugeha & Suharti, 2008; Arai et al., 2012; Arai et al., 2013). The species of A. bicolor is divided into two subspecies based on their spawning origin i.e. A bicolor bicolor from the Indian Ocean (Ege, 1939; Robinet et al., 2002; Aoyama et al., 2007) and A. bicolor pacifica from the Pacific Ocean (Ege, 1939; Arai et al., 1999; Aoyama et al., 1999). The two subspecies overlapped morphologically in anodorsal length as a percentage of total length (ADL/%TL) and in the number of anodorsal vertebrae (ADV) as explained by Watanabe et al. (2004) and Sugeha & Suharti (2008). So, they could not be distinguished using morphological characteristics alone. However, the two subspecies inhabit different geographic areas of Indonesian waters. Anguilla b. bicolor is distributed in the western Indonesian region (Watanabe et al., 2004, 2005; Aoyama et al., 2007; Sugeha & Suharti, 2008), while A. b. pacifica is distributed from the central to the eastern Indonesian region (Watanabe et al., 2004; Aoyama et al., 1999; Sugeha & Suharti, 2008). Based on mitochondrial DNA sequencing analysis of the control region (D-loop), Sugeha et al. (2010b) reported the possibility of a panmictic population (single spawning population) for A. b. bicolor and a non-panmictic population (multiple spawning populations) for A. b. pacifica inhabiting the Indonesian waters. Based on microsatellite analysis, Minegishi et al. (2011) suggested the occurrence of genetic divergence between A. bicolor from the Indian and the Pacific Oceans. However, significant genetic variation of the species was observed within each ocean (Minegishi et al., 2011) that might support the hypothesis of multiple spawning populations of the 38

3 External and Internal Morphological... (Hagi Yulia Sugeha and Marlina Ummas Genisa) species in the Pacific Ocean (Sugeha et al., 2010a; Sugeha, 2010). The expansion in the geographic range of species distribution was reported to occur in various marine organisms (Perry et al., 2005; Sorte et al., 2010). This included the tropical eel of genus Anguilla (Sugeha et al, 2008a). It has been suggested that the phenomenon of species shifting in Anguilla was triggered by several factors including the impact of global warming, climate change and deterioration of natural environment as a result of human activities (Casselman & Cairns, 2009; Sugeha et al., 2008a). The occurrence of gene flow in the population genetic structure of Indonesian A. bicolor (Sugeha et al., 2012) and the variation of morphological characteristics of the species in different sampling locations (Sugeha & Suharti, 2008) may be biological factors that trigger the occurrence of species shifting within the genus Anguilla in Indonesian waters. Therefore, it is important to carry out and record the morphological characteristics of Anguilla during the glass eel stage as it is an essential part of their life cycle that plays a crucial role in the successful migration of the organism from the ocean to the freshwater. Anguilla b. bicolor is one of the tropical eel species commercially collected along the southern coast of the western Indonesian region, particularly during the glass eel stage. Glass eels are collected just after their arrival in the mouth of the river. They are then incubated for several months until past the critical period of elver (fully pigmented juvenile) and are cultured in ponds as an export commodity to East Asian countries (Aida et al., 2003). Economically, it has become an important income for coastal fishing communities, but ecologically it is harmful to the natural life cycle of catadromous eels. Ecologically and biologically, tropical eels are different from temperate ones (Miller et al, 2009). In order to prevent the decline of tropical eels, we need to collect and to understand the basic biological information including morphological characteristics, particularly during their early life stages. The knowledge of morphological characteristics will not only be important for scientific progress, but also will be important as basic information for the application of the artificial reproduction of eels, including their development at the early life stages from larvae to juveniles. Based on this consideration, we conducted a study of the glass eels A. b. bicolor from the estuary of Cibaliung River to record their external and internal morphological characteristics during this stage. METHODOLOGY Specimens of glass eels were collected from the estuary of Cibaliung River, Banten, Indonesia (Figure 1), on the nights of the new moon from May to October 2005, using two scoop nets (1µm in mesh size). Collected glass eels were fixed in formalin 10% and transported to the laboratory for future examination. Body length and weight measurements (Figure 2) were done prior to external examination. In order to examine external morphology characteristics, body length measurements i.e. total length (TL), predorsal length (PDL), preanal length (PAL) and anodorsal length (ADL) were done to the nearest millimeter (Sugeha et al., 2001; 2006; 2008a,b; Sugeha & Suharti, 2008). Development of pigmentation in early life stages of Anguilla consists of pigmentation stages I, II, III, IV, VA, VB, VIAi, VIAii, VIAiii, VIAiv and VIB (Bertin, 1956). In this study, pigmentation stages of the glass eel were observed under a stereo microscope (40X magnification). 39

4 Oseanologi dan Limnologi di Indonesia. Vol. 41, No. 1, April 2015: Figure 1. Sampling location of the tropical glass eels in the estuary of Cibaliung River, Banten. Gambar 1. Lokasi pengambilan sampel juvenil sidat tropis di muara Sungai Cibaliung, Banten. Figure 2. Body shape and length characteristics of the tropical short-finned glass eel without ADL characteristics as the length between PDL and PAL. In this figure the ADL is not shown since PDL and PAL are just equal. Gambar 2. Karakteristik bentuk dan panjang tubuh juvenil sidat tropis bersirip pendek tanpa karakteristik ADL sebagai panjang antara PDL dan PAL. Pada gambar ini ADL tidak terlihat karena awal PDL dan PAL adalah sejajar. Internal morphology characteristics (Figure 3) consisting of the number of total vertebrae (TV), predorsal vertebrae (PDV), preanal vertebrae (PAV), anodorsal vertebrae (ADV) and abdominal vertebrae (ABV) were counted under the stereo microscope (40X magnification). Vertebrae counts were done after bleaching, transparancing and staining the specimens of glass eels in a series of bone-staining analyses using alizarine red solution (Sugeha et al., 2001a, 2006, 2008a, b; Sugeha & Suharti, 2008; Sugeha & Arai, 2010; Sugeha, 2010). All measurement data were statistically counted using Statistical Analysis ToolPak available in Microsoft Excel Program Ver.10, including the body-length relationships that were analyzed to understand the growth status of the species while entering the Cibaliung River estuary. Figure 3. Sectional counts of the number of total vertebrae (TV), preanal vertebrae (PAV), predorsal vertebrae (PDV) and abdominal vertebrae (ABV) in the glass eel stage of Anguilla under the stereo microscope (40X magnification). Gambar 3. Pembagian penghitungan jumlah total vertebra (TV), vertebra preanal (PAV), vertebra predorsal (PDV) dan vertebra abdominal (ABV) genus Anguilla pada tahap juvenil di bawah mikroskop stereo (pembesaran 40X). 40

5 External and Internal Morphological... (Hagi Yulia Sugeha and Marlina Ummas Genisa) RESULTS A total of 140 tropical anguillid glass eels were collected during the six months of investigation from May to October 2005 (Table 1) and the biggest catch was in October. A total of 116 specimens were identified as Anguilla bicolor bicolor based on the characteristics of ADL/%TL (anodorsal length as a percentage of total length: range = %; mean ± SD = 1.78 ± 1.16) and the number of ADV (anodorsal vertebrae: -3-4; 1.15 ± 1.24) that were seen in Figure 4 and Figure 5, respectively. The other external characteristics (PDL/%TL; PAL/%TL) and internal characteristics (PDV; PAV; ABV) overlapped and could not be used as morphological key characteristics for identifying the species (Table 2). The remaining specimens (24 individuals) belonged to the Congeridae and Muraenidae families or to other members of the genus Anguilla that could not be morphologically identified to the species level and needed to be genetically analyzed. Table 1. Number of glass eels (order Anguilliformes) collected monthly in the estuary of Cibaliung River, Banten, Indonesia. Tabel 1. Jumlah juvenil (ordo Anguilliformes) yang dikoleksi setiap bulan di muara Sungai Cibaliung, Banten, Indonesia. No. Sampling date Number of catches 1 May 20-21, June 7-8, July 7-8, August 6-7, September 6-7, October 26-27, Total catches 140 Table 2. External and internal morphological characteristics of A. b. bicolor glass eels collected from the estuary of Cibaliung River. Remarks: BW = body weight, TL = total length, PDL = predorsal length, PAL = preanal length, ADL = anodorsal length, TV = total vertebrae, PDV = predorsal vertebrae, PAV = preanal vertebrae, ADV = anodorsal vertebrae, ABV = abdominal vertebrae. Tabel 2. Karakteristik morfologi eksternal dan internal juvenil A. b. bicolor yang dikoleksi di muara Sungai Cibaliung. Keterangan: BW = Berat Tubuh; TL = Panjang Tubuh, PDL = panjang predorsal; PAL = panjang preanal; ADL = panjang anodorsal; TV = vertebra total; PDV = vertebra preanal; PAV = vertebra predorsal; ADV = vertebra anodorsal; ABV = vertebra abdominal. Characteristics BW TL PDL PAL ADL (g) (mm) (mm) (mm) (mm) TV PDV PAV ADV ABV Mean ST DV Min Max N

6 PDV, PAV, ADV, and ABV Characters Proportion of length characters (%) Oseanologi dan Limnologi di Indonesia. Vol. 41, No. 1, April 2015: N =115 PDL/%TL PAL/%TL ADL/%TL Total Length (mm) Figure 4. Scatter diagram of the proportion of length characteristics (PDL, PAL, ADL) to the total length (TL) of A. b. bicolor glass eels collected from the estuary of the Cibaliung River, Banten, Indonesia. Gambar 4. Diagram pencar proporsi karakteristik panjang (PDL, PAL, ADL) terhadap panjang total (TL) juvenil A. b. bicolor yang dikoleksi dari muara Sungai Cibaliung, Banten, Indonesia N = 112 PDV PAV ADV ABV Character of Total Vertebrae (TV) Figure 5. Scatter diagram of the proportion of the number of predorsal vertebrae (PDV), preanal vertebrae (PAV), anodorsal vertebrae (ADV) and abdominal vertebrae (ABV) to the number of total vertebrae (TV) of A. b. bicolor glass eels collected from the estuary of Cibaliung River, Banten, Indonesia. Gambar 5. Diagram pencar proporsi jumlah vertebra predorsal (PDV), vertebra preanal (PAV), vertebra anodorsal (ADV) dan vertebra abdominal (ABV) terhadap jumlah total vertebra (TV) juvenil A. b. bicolor yang dikoleksi dari muara Sungai Cibaliung, Banten, Indonesia. Anguilla b. bicolor glass eels from the estuary of Cibaliung River were about g (0.13 ± 0.02) in body weight and mm (53.01 ± 1.75) in total length (Table 2). Based on the length-weight relationship analysis it was found that these glass eels had a positive linear growth pattern with a regression equation y = x and r² = with P-value < 0.05 (Figure 6). 42

7 Number of individu Body Weight (gr) External and Internal Morphological... (Hagi Yulia Sugeha and Marlina Ummas Genisa) N = 116 Y = X R square = P-value < Total Length (mm) Figure 6. Length-weight relationship of A. b. bicolor glass eels collected in the estuary of Cibaliung River, Banten, Indonesia. Gambar 6. Hubungan panjang berat juvenil A. b. bicolor yang dikoleksi di muara Sungai Cibaliung, Banten, Indonesia. Melanophore pigmentation development of the species was dominated by the pigmentation stage VA (94.2%) compared to the pigmentation stage VB (5.8%), suggesting they were in the glass eel stage when entering the mouth of the Cibaliung River (Figure 7). The glass eel pigmentation stage VA was represented by the appearance of melanophore spots in the caudal tip, while the pigmentation stage VB was represented by the appearance of melanophore spots in both caudal tip and skull (Figure 8) N = 116 Stage VA Stage VB Pigmentation development Figure 7. Proportion of pigmentation development of A. b. bicolor glass eels collected from the estuary of Cibaliung River, Banten, Indonesia. Gambar 7. Proporsi perkembangan pigmentasi juvenil A. b. bicolor yang dikoleksi dari muara Sungai Cibaliung, Banten, Indonesia. 43

8 Oseanologi dan Limnologi di Indonesia. Vol. 41, No. 1, April 2015: Figure 8. Image of pigmentation development of A. b. bicolor glass eels collected from the estuary of Cibaliung River. Left: pigmentation in the skull; Right: pigmentation in the caudal; Bottom: pigmentation stages found in this study. Gambar 8. Gambar perkembangan pigmentasi juvenil A. b. bicolor yang dikoleksi dari muara Sungai Cibaliung. Kiri: pigmentasi di kepala; Kanan: pigmentasi di ekor; Bawah: tahap-tahap pigmentasi yang ditemukan dalam penelitian ini. DISCUSSION Eels are elongated fish with a tubular body shape termed anguilliform. There are many kinds of eels, but eels of the genus Anguilla are the only member of the family Anguillidae from about 15 families in the order Anguilliformes (Miller & Tsukamoto, 2004; Inoue et al., 2010). Most recent study reported the occurrence of 19 species of Anguilla in the world (Watanabe et al., 2009). Most eels have a connecting fin that is formed from the tip of the dorsal fin to the tip of the anal fin through the caudal fin and the categorization of the adult eels is based on the fin length characteristics (Ege, 1939). Tabeta et al. (1976a) used the vertebrae count as the internal morphological characteristics to identify the species in the glass eel stage. Tzeng & Tabeta (1983) categorized the A. b. pacifica glass eels using morphological characteristics. According to Tabeta et al. (1976a, b) and Tzeng & Tabeta (1983) the length of the fin from the tip of the dorsal fin to the tip of the caudal fin is called the predorsal length (PDL) and the length from the tip of the anal fin to the tip of the caudal fin is called the preanal length (PAL). The distance between the PDL and the PAL is called the anodorsal length (ADL). Based on the proportion of the anodorsal length (ADL) to the total length (TL), the genus Anguilla consists of two types of eel i.e. shortfinned eels and long-finned eels (Ege, 1939). In short-finned eels, the proportion of anodorsal length to the total length ranges from -3 to 3%, whereas in long-finned eels the range is 8 to 16%. These fin characteristics were used as a guide by many eel biologists for more than 50 years. Recent studies reported that the genus Anguilla from Indonesia comprises of three fin types of eels i.e. short-finned eels (Sugeha et al., 2001a; Sugeha et al., 2008a,b; Sugeha & Suharti, 2008; Sugeha, 2010), moderate-finned eels (Sugeha & Suharti, 2008; Sugeha, 2010) and longfinned eels (Sugeha & Suharti, 2008; Sugeha, 2010). The short-finned eels have ADL/%TL 6%, the moderate-finned eels have 7 to 13%, and the long-finned eels have 14% (Sugeha et al., 2012). Based on the new categorization, the lack of range 4-7% in ADL/%TL found in previous studies (Sugeha et al., 2001a; Watanabe et al., 2004) could be clarified. Furthermore, the new category of moderate-finned eels will help in the recognition of some tropical eel species matching neither short-finned nor long-finned eel characteristics. In this study, analysis of both external and internal morphological characteristics of A. b. 44

9 External and Internal Morphological... (Hagi Yulia Sugeha and Marlina Ummas Genisa) bicolor from the western tip of Java Island was carried out. Based on the measurement of anodorsal length as a percentage of total length ( %; mean ± SD = 1.78 ± 1.16), it was proven that the species belonged to the shortfinned eel type. According to Ege (1939) and Watanabe et al. (2004), the short-finned eel characteristics are found in both subspecies A. b. bicolor and A. b. pacifica, in both subspecies A. a. australis and A. a. schmidtii and in A. obscura. However, since the three species are distributed in different geographic areas (Ege, 1939; Watanabe et al. 2004; Sugeha et al., 2008a; Sugeha & Suharti, 2008; Jellyman 1977; Todd, 1981) it is suggested that the short-finned eels reported in this study belong to A. b. bicolor. Besides the external morphological characteristics of body length measurement, the internal morphological characteristics of vertebrae count were also examined. Tabeta et al. (1976a), Watanabe et al. (2004), Sugeha et al. (2001a; 2008a,b; Sugeha & Suharti, 2008), Sugeha & Arai (2010), and Sugeha (2012) reported that the eel species from the genus Anguilla could be separated by the internal morphological characteristics based on the number of anodorsal vertebrae. It is suggested that the short-finned eels have -3 to 3 anodorsal vertebrae. In this study we found that the glass eels from the estuary of Cibaliung River had -3 to 4 (mean ± SD = 1.15 ± 1.24) anodorsal vertebrae. So, it is reasonable to identify the species as short-finned eels from western Indonesian waters, namely A. b. bicolor. The occurrence of A. b. bicolor in the Indonesian waters was also reported by Watanabe et al., 2009; Sugeha & Suharti, 2008; Minegishi et al., 2011; Tanaka et al., Pigmentation development was clearly observed in the glass eels from the Cibaliung River estuary. As previous studies reported, the tropical glass eels tend to tolerate warm coastal temperatures earlier and for a longer period than temperate glass eels (Aida et al., 2003; Miller et al., 2009; Sugeha & Suharti, 2008; Sugeha, 2012). For this reason, they appear in the mouth of the river when the pigmentation has just developed into the glass eel pigmentation stage VA and or stage VB. At the pigmentation stage VA, the melanophore pigment has just appeared in the caudal tip of tropical short-finned eels (Sugeha & Suharti, 2008), in the lateral line of tropical longfinned eels (Sugeha et al., 2010) and in the caudal tip and lateral line of tropical moderate-finned eels (Sugeha, 2010). At the pigmentation stage VB, the melanophore pigment has also appeared in the skull of each type of eels. In general, the tropical glass eel pigmentation development of stage VA and stage VB are recognizable, and this indicates that the tropical glass eels are just entering the estuary after experiencing metamorphosis from oceanic eel larvae (leptocephali) into inshore glass eels. During that period of metamorphosis, the somatic growth gradually decreases after a fast growing period during the larval stage in the open ocean (Arai et al., 1999). After having several days of adaption to the critical condition of inshore migration into the estuarine area, the glass eels become completely pigmented and are called elvers that will start to grow in the freshwater as yellow immature eels (Tesch, 2003). In this study, the length-weight relationship of the A. b. bicolor glass eels during the onset of their inshore migration to the Cibaliung River estuary was examined. Based on the regression equation y = x (R² = 0.989; P-value < 0.05) it is suggested that these A. b. bicolor glass eels have a positive growth linear regression which is the growth of total length followed by the growth of body weight. It is reasonable, since the glass eels stage is also believed to be the early stage of the growth in the freshwater. At this stage they start to enter the freshwater and begin their feeding behavior as freshwater fish. Anguilla b. bicolor glass eels recruited in the Cibaliung River estuary were about g (0.13 ± 0.02) in body weight and mm (53.01 ± 1.75) in total length. This body size was relatively similar to the same subspecies from other estuaries in western Indonesia (Sugeha & Suharti, 2008). Since the spawning ground of A. b. bicolor is hypothesized to be in or around the Indian Ocean (Robinet et al., 2002; Aoyama et al, 2007), it is reasonable to suggest this subspecies of glass eels experienced similar oceanic environmental conditions before entering the same estuary as their mother came from. However, Anguilla b. bicolor are bigger in body size than their sister subspecies A. b. pacifica from the eastern Indonesia (Sugeha & Suharti, 2008). Differences in body size between the two subspecies could be reflecting the differences in larval durations and migration behavior of both subspecies. Furthermore, A. b. bicolor was hypothesized to spawn in the Indian Ocean (Robinet et al., 2002; Aoyama et al., 2007), while A. b. pacifica was hypothesized to spawn in the Pacific Ocean (Aoyama et al., 1999). Differences in the origin of spawning may also affect the early life history traits of eels. The adaptation ability of 45

10 Oseanologi dan Limnologi di Indonesia. Vol. 41, No. 1, April 2015: each subspecies to such huge and extreme conditions in both Indian and Pacific Oceans probably affects the body sizes of glass eels when they enter the estuary. Anguilla b. bicolor from the Indian Ocean may experience shorter oceanic migration distances compared to A. b. pacifica from the Pacific Ocean. Eel larvae which experience a shorter oceanic migration distance have lower energy expenditure compared to eel larvae that experience a longer oceanic migration distance. This may be the reason for the smaller size of A. b. pacifica compared to A. b. bicolor (Sugeha & Suharti, 2008). Consequently, A. b. pacifica may need a longer time to reach the inshore area than A. b. bicolor. This means A. b. pacifica are older than A. b. bicolor when entering the estuarine area. This hypothesis needs to be clarified by examining the age and growth of both subspecies inhabiting Indonesian waters, based on otolith microstructure analyses of the inshore migrant glass eels. Further analyses of the short-finned eel A. b. bicolor from the estuary of the Cibaliung River revealed that the subspecies migrated inshore during the late dry season to the early wet season. Setiawan et al. (2001) reported that the A. b. bicolor glass eel migrated inshore to Cimandiri River throughout the year with two peaks of recruitment (dry and wet season) based on backcalculating of the hatching date from the otolith daily increment rings. However, in this study it was found that the peak migration of A. b. bicolor glass eels was in October or in early wet season. This study supports the previous idea (Sugeha et al., 2008a) that recruitment season of tropical glass eels in the western Indonesia occurs during wet season. A complete one-year study on the migration season in this area is important to clarify the seasonal migration of A. b. bicolor. CONCLUSION Species identification of the short-finned eel Anguilla bicolor bicolor is not possible if only based on observation of morphological characteristics of body length, vertebrae count and pigmentation development, but also should be consider sampling location with refers to the geographic distribution range of the species in the world. ACKNOWLEDGEMENTS This study was granted by Program Competitive-LIPI, Subprogram Census of Marine Life for 2004 to 2005 fiscal years. Our sincere thanks go to Dr. Michael J. Miller and Dr. Mari Rhydwen who kindly conducted a critical reading of the manuscript and corrected the English writing. We are also grateful to the fisherman in the local harbour of Cibaliung River, Banten Province, Indonesia, for helping us in collecting glass eel specimens. REFERENCES Aoyama J, N Mochioka, T Otake, S Ishikawa, Y Kawakami, P Castle, M Nishida and K Tsukamoto Distribution and dispersal of anguillid leptocephali in the western Pacific Ocean revealed by molecular analysis. Marine Ecology Progress Series 188: Aoyama J, S Wouthuyzen, MJ Miller, T Inagaki, Y Minegishi, M Kuroki, S Suharti, T Kawakami, O Sumadhiharga and K Tsukamoto Distribution of leptocephali of the freshwater eels, genus Anguilla, in the waters off Sumatra in the Indian Ocean. Environmental Biology of Fishes 80: Aida K, K Tsukamoto and K Yamauchi, editors Eel Biology. Spinger Verlag Tokyo. 497 pp. Arai T, J Aoyama, D Limbong and K Tsukamoto Species composition and inshore migration of the tropical eels, Anguilla spp, recruiting to the estuary of the Poigar River, Sulawesi Island. Marine Ecology Progress Series 188: Arai T, N Chino, SZ Zulkifli and A Ismail Notes on the occurrence of the tropical eel Anguilla bicolor bicolor in Peninsula Malaysia, Malaysia. Journal of Fish Biology 80: Arai T, N Chino and DQ Le Migration and habitat use of the tropical eels Anguilla marmorata and A. bicolor pacifica in Vietnam. Aquatic Ecology 47: Bertin L Eels - a biological study. Cleaver- Hume Press, London. Casselman JM and DK Cairns, editors Eels at the edge: science, status, and conservation concerns. American Fisheries Society, Symposium 58, Bethesda, Maryland. 460 pp. Ege V A revision of the genus Anguilla Shaw. A systematic, phylogenetic and 46

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