ANNALES • Ser. hist. nat. • 14 • 2004 • 2 original scientific article UDC 597.5:591.13(262.35-18) received: 2004-11-14 FEEDING HABITS OF THE MEDITERRANEAN POOR COD TRISOPTERUS MINUTUS CAPELANUS (LACEPEDE) (PISCES: GADIDAE) FROM THE EASTERN CENTRAL ADRIATIC Jakov DULCIC Institute of Oceanography and Fisheries, HR-21000 Split, P.O.BOX 500, Croatia E-mail: dulcic@izor.hr Zrinka DULCIC Sardina d.d., HR-21420 Postira, Croatia ABSTRACT The stomach contents of poor cod Trisopterus minutus capelanus (Lacepede) were taken at monthly intervals off the eastern Central Adriatic coast (Croatia) between March 2003 and January 2004. A total of 273 specimens were analysed to determine diet according to fish size and season. The basic food consists of crustaceans (Mysidacea and Decapoda) and teleosts. Feeding habits varied with size: decapods and fish were more abundant in the stomachs of larger specimens. Moderate seasonal variation in food habits was recorded. Key words: Trisopterus minutus capelanus, poor cod, feeding habits, Adriatic Sea ABITUDINI ALIMENTARI DELLA BUSBANA TRISOPTERUS MINUTUS CAPELANUS (LACEPEDE) (PISCES: GADIDAE) DELL'ADRIATICO CENTRO - ORIENTALE SINTESI Tra marzo 2003 e gennaio 2004 e stato effettuato un esame a intervalli mensili del contenuto dello stomaco di busbane Trisopterus minutus capelanus (Lacepede) pescate nelle acque croate del Medio Adriatico. Per accertare le abitudini alimentari di questa specie, in relazione alle loro dimensioni e alla stagione, sono stati esaminati 273 esemplari. Il cibo principale era costituito da crostacei (Mysidacea e Decapoda) e teleostei. Le abitudini alimentari variavano a seconda delle dimensioni: decapodi e pesci erano piu abbondanti nello stomaco degli esemplari piu grandi. Nelle abitudini alimentari sono state registrate anche moderate variazioni stagionali. Parole chiave: Trisopterus minutus capelanus, busbana, abitudini alimentari, Mare Adriatico 45 ANNALES • Ser. hist. nat. • 14 • 2004 • 2 Jakov DULCIC & Zrinka DULCIC: FEEDING HABITS OF THE MEDITERRANEAN POOR COD TRISOPTERUS MINUTUS CAPELANUS (LACEPEDE) ..., 189-196 INTRODUCTION The poor cod, Trisopterus minutus capelanus (Lace-pede), is one of the most abundant endemic fish species in the Mediterranean Sea. Its distribution extends to the eastern Atlantic Ocean off the coast of Morocco, whereas north of Gibraltar the subspecies Trisopterus minutus minutus is found. However, recent genetic analysis (Mattiangeli et al., 2000) supports the classification of Atlantic and Mediterranean poor cod as distinct taxa and the close relationship between the Mediterranean poor cod and bib Trisopterus luscus (Linnaeus, 1758). The poor cod is among the most abundant gadoid fishes in the Central Adriatic. Here it is actively exploited by bottom trawlers on muddy or sandy-muddy bottoms at depths ranging from 40 to 250 m (Jardas, 1996). Despite its commercial value and abundance in the fisheries of Mediterranean countries, little is known about the trophic ecology of the Mediterranean poor cod. Planas & Vives (1952) made some observations on its diet off the Mediterranean coast of Spain, while Poli-tou et al. (1989) presented some preliminary information on the feeding habits of the species in the Euboikos and Pagassitikos Gulfs in Greece. Politou & Papaconstanti-nou (1994) reported on the feeding ecology of poor cod from the eastern coast of Greece. Biagi et al. (1992) made some observations in the northern Tyrrhenian Sea, while Gramitto (1999) presented data on the feeding habits and estimation of daily ration of poor cod in the Adriatic Sea. Morte et al. (2001) studied the poor cod's feeding habits off the eastern coast of Spain (western Mediterranean). The main goal of this study was to present information on the feeding ecology of poor cod T. m. capelanus in the Central Adriatic (eastern coast), including the systematic and detailed study of its prey, and the influence of predator size and seasonal variations in the stomach contents. MATERIAL AND METHODS Monthly samples of T. m. capelanus were collected from diurnal commercial catches landed in the port of Sibenik; the catches were taken by trawl (type "tartana") at 50 to 175 m depth in the area of Vis Island and Jabuka Pit (Blitvenica area) between March 2003 and January 2004. A total of 273 specimens (108 females, 41 males) with a total length between 8.3 cm and 24.0 cm were measured, dissected and their stomachs removed and preserved in 4% formaldehyde. Upon opening, stomach contents were preserved in a 70% ethanol solution. A few fish showing evidence of regurgitation were excluded from the study. In the laboratory, identification of prey was carried out at the lowest possible taxonomic level. We regis- tered the number and wet weight of food items after removing surface water by blotting it on tissue paper. The distribution of each feeding item in the diet was determined by the frequency of occurrence (F %), numerical composition (Cn %) and biomass composition (Cw %) (Hyslop, 1980). The percentage of empty stomach (V) was also recorded. Indices of relative importance (Pinkas et al., 1971) (IRI= (%N + %W) %F) and main food items (MFI = ((%N + %F)/2) %W) (Zander, 1982) were calculated for each consumed prey item. To assess potential diet changes with respect to size, fish were divided into two size classes according to age classes: small (<14 cm) and large (>14.5 cm). The proportional food overlap between the size classes and seasons was calculated using the Schoener overlap index (1970): a=1-0.5 (X Ipxi-pyil) i-1 where pxi and pyi are the biomass composition indices of prey (i) in the diets of size classes x and y, respectively. The index has a minimum value of zero when no overlap occurs and a maximum value of one when all prey are shared in equal proportions by two size classes. Statistical differences in diet composition as a function of size and season were assessed using the chi-square $2 test (Sokal & Rohlf, 1981). The significance of variation of the mean number of prey and weight per stomach was tested by analysis of variance (ANOVA). RESULTS Emptiness index Of the 273 stomachs of T. m. capelanus examined, 20 were empty (7.32 %). This percentage did not vary significantly over the year ($2 = 1.17, df = 3, P >0.05). However, the emptiness index varied significantly with poor cod size ($2 = 7.56, df = 1, P <0.05). Somewhat higher value of this index was obtained in the smaller group (smallest specimens). Overall composition of the diet and variation in stomach contents relative to fish length The stomach contents of the poor cod consisted of at least 37 different prey species, with a low average number of prey per stomach (2.49) and low average weight of prey per stomach (0.37). The food consisted almost exclusively of crustaceans and fish, the former constituting a much greater part then the latter. The crustaceans were mainly mysids and decapods (Peneus sp., Alpheus glaber). The fish were mainly gobiids. Less abundant crustaceans included copepods, euphausiids and stomatopods. Other taxa found in the stomach con 190 ANNALES • Ser. hist. nat. • 14 • 2004 • 2 Jakov DULCIC & Zrinka DULCIC: FEEDING HABITS OF THE MEDITERRANEAN POOR COD TRISOPTERUS MINUTUS CAPELANUS (LACEPEDE) ..., 189-196 Tab. 1: Prey registered in the stomach of T. m. capelanus (small size group <14 cm) (F % - frequency of occurrence, Cn % - numerical composition, Cw % - biomass composition) (No. = 96). Tab. 1: Plen, zabeležen v želodcu moliča T. m. capelanus (skupina manjših primerkov <14 cm) (F % - frekvenca pojavljanja, Cn % - številčna sestava, Cw % - sestava biomase) (No. = 96). Food items F (F %) N (Cn %) W (Cw %) Polychaeta Aphrodite acuelata 1 (1.04) 1 (0.35) 0.01 (0.06) Tunicata Appendicularia 1 (1.04) 1 (0.35) 0.01 (0.06) Crustacea Mysidacea 41 (42.70) 115 (41.21) 0.72 (4.53) Euphausiacea 2 (2.08) 2 (0.71) 0.01 (0.06) Stomatopoda Squilla sp. 5 (5.20) 6 (2.15) 0.23 (1.44) Decapoda Penaeidae Penaeus sp. 35 (36.45) 77 (27.59) 5.97 ( 37.57) Penaeus kerathurus 1 (1.04) 1 ( 0.35) 0.12 (0.75) Parapenaeus longirostris 3 (3.12) 3 ( 1.07) 0.52 (3.27) Total Penaeidae 39 (40.62) 81 (29.03) 6.61 (41.59) Alpheidae Alpheus sp. 7 (7.29) 8 ( 2.86) 1.15 (7.23) Alpheus glaber 10 (10.41) 12 ( 4.30) 1.21( 7.61) Total Alpheidae 17 (17.70) 20 ( 7.16) 2.36 ( 14.85) Pasiphaeidae Pasiphea sivado 3 ( 3.12) 4 ( 1.43) 0.16 ( 1.00) Pandalinae Plesionika martia 1 (1.04) 1 (0.36) 0.04 (0.25) Pandalina brevirostris 1 (1.04) 2 (0.71) 0.04 (0.25) Total Pandalinae 2 (2.08) 3 (1.07) 0.08 (0.50) Hippolytidae Lysmata seticaudata 1 (1.04) 1 (0.36) 0.08 (0.50) Nephropidae Nephrops norvegicus 2 (2.08) 2 (0.72) 0.86 (5.41) Paguridea Pagurus sp. 1 (1.04) 1 (0.36) 0.01 (0.06) Albuneidae Albunea carabus 1 (1.04) 1 (0.36) 0.02 (0.12) Portunidae Macropipus sp. 1 (1.04) 1 (0.36) 0.04 (0.25) Xanthidae Xantho sp. 2 ( 2.08 ) 2 (0.72) 0.04 (0.25) Goneplacidae Goneplax rhomboides 8 (8.33) 9 (3.23) 2.07 (13.02) Decapoda 15 (15.63) 21 (7.53) 1.78 (11.20) Total Crustacea 140 (145.83) 269 (96.76) 15.06 (94.77) Pisces Gobius sp. 3 (3.12) 3 (1.07) 0.49 (3.08) Pisces 4 (4.16) 4 (1.43) 0.32 (2.01) Total Pisces 7 (7.29) 7 (2.51) 0.81 (5.09) Algae (Phaeophyta) 1 (1.04) 1 (0.36) 0 190 ANNALES • Ser. hist. nat. • 14 • 2004 • 2 Jakov DULCIC & Zrinka DULCIC: FEEDING HABITS OF THE MEDITERRANEAN POOR COD TRISOPTERUS MINUTUS CAPELANUS (LACEPEDE) ..., 189-196 Tab. 2: Prey registered in the stomach of T. m. capelanus (large size group >14.5 cm) (F % - frequency of occurrence, Cn % - numerical composition, Cw % - biomass composition) (No. = 177). Tab. 2: Plen, zabeležen v želodcu moliča T. m. capelanus (skupina večjih primerkov >14,5 cm) (F % - frekvenca pojavljanja, Cn % - številčna sestava, Cw % - sestava biomase) (No. = 177). Food items F (F %) N (Cn %) W (Cw %) Polychaeta Aphrodite acuelata 4 (2.26) 5 (1.24) 0.05 (0.08) Bryozoa 4 (2.26) 4 (0.99) 0.03(0.05) Crustacea Mysidacea 18 (10.16) 41 (10.22) 0.04 (0.07) Copepoda Nauplia 1 (0.56) 1 (0.25) 0 Stomatopoda Squilla desmaresti 2 (1.13) 2 (0.49) 0,03 (0,05) Squilla sp. 1 (0.56) 1 (0.25) 0,14 (0,24) Total Stomatopoda 3 (1.69) 1 (0.25) 0,17 (0,30) Decapoda Penaeidae Penaeus sp. 59 (33.33) 103 (25.68) 9.56 (17.01) Penaeus kerathurus 5 (2.82) 5 (1.24) 1.3 (2.31) Stenopus spinosus 2 (1.13) 2 (0.49) 0.19 (0.33) Metapenaeus monoceros 1 (0.56) 1 (0.25) 0.09 (0.16) Gennades elegans 1 (0.56) 1 (0.25) 0.11 (0.19) Parapenaeus longirostris 4 (2.26) 4 (0.99) 1.03 (1.83) Total Penaeidae 72 (40.67) 116 (28.92) 12.28 (21.85) Alpheidae Alpheus sp. 29 (16.38) 33 (8.23) 6.42 (11.42) Alpheus glaber 26 (14.69) 28 (6.98) 6.15 (10.94) Total Alpheidae 55 (31.07) 61 (15.21) 12.57 (22.36) Pasiphaeidae Pasiphea sivado 5 (2.82) 9 (2.24) 0.72 (1.28) Pandalinae Pandalina brevirostris 1 (0.56) 1 (0.25) 0.01 (0.07) Nephropidae Nephrops norvegicus 11 (6.21) 11 (2.74) 5.68 (10.10) Palinuridea Polycheles typhlops 1 (0.56) 1 (0.25) 0.3 (0.53) Portunidae Macropipus depurator 2 (1.13) 2 (0.49) 0.11 (0.19) Xanthidae Xantho sp. 2 (1.13) 2 (0.49) 0.06 (0.10) Goneplacidae Goneplax rhomboids 23 (12.99) 27 (6.73) 4.54 (8.07) Crangonidae Crangon crangon 1 (0.56) 1 (0.25) 0.07 (0.12) Palaeomonidae Paleomon sp. 3 (1.69) 3 (0.74) 0.08 (0.14) Galatheidea Munida sp. 3 (1.69) 3 (0.74) 0.12 (0.21) Isopoda Ligia italica 1 (0.56) 1 (0.25) 0.23 (0.40) 190 ANNALES • Ser. hist. nat. • 14 • 2004 • 2 Jakov DULCIC & Zrinka DULCIC: FEEDING HABITS OF THE MEDITERRANEAN POOR COD TRISOPTERUS MINUTUS CAPELANUS (LACEPEDE) ..., 189-196 Epicaridea 1 (0.56) 1 (0.25) 0.01 (0.01) Decapoda 33 (18.64) 53 (13.21) 3.73 (6.63) Total Crustacea 236 (133.33) 337 (84.03) 40.72 (72.45) Pisces Gobius sp. 23 (12.99) 27 (6.73) 8.01 (14.25) Engraulis encrasicolus 1 (0.56) 1 (0.25) 2.8 (4.98) Other fish 22 (12.43) 23 (5.73) 4.59 (8.16) Fish eggs 1 (0.56) 1 (0.25) 0 Total Pisces 47 (26.55) 52 (12.96) 15.4 (27.4) Algae (Phaeophyta) 3 (1.69) 3 (0.74) 0 tents, but of lesser importance, were bryozoans, poly-chaetes, tunicates and algal remains. According to IRI, Penaeidae represented the highest portion. Tables 1 and 2 show the frequency of occurrence, numerical composition and biomass composition of all prey items found in small (<14 cm) and large (>14.5 cm) size groups. Tables 3 and 4 show the values of IRI and MFI for small (<14 cm) and large (>14.5 cm) size groups. Tab. 3: T. m. capelanus. The values of IRI and MFI (No. = 96). Tab. 3: T. m. capelanus. Vrednosti IRI in MFI indeksov (No. = 96). According to IRI and MFI, the main food of smaller poor cod specimens consisted of Mysidacea, Penaeidae, Alpheidae, Goneplacidae and other Decapods, while larger specimens fed on Decapoda (Penaeidae, Alphei-dae) and pisces (Gobiidae). Although the average number of prey per stomach decreased from the smallest individuals to the largest, no significant differences were found (F = 5.85, df = 272, P >0.05). The average prey weight per stomach, however, increased significantly (F = 234.33, df = 272, F <0.001) from the smallest to the largest size classes. The value of various prey types varied with poor cod size: in Penaeidae, Alpheidae and Pisces it increased with the increasing poor cod size but decreased in mysids and Goneplacidae. There was a clear tendency for mysids ($2 = 124.82, P <0.001), Goneplacidae ($2 = 78.54, P <0.001) and "other crusta- ceans" ($2 = 38.68, P <0.001) to be mostly consumed by the small specimens. The chi-square test revealed significant differences among poor cod size classes with respect to decapods ($2 = 96.43, df = 2, P<0.001) and teleosts ($2 = 98.39, P <0.001) due to the low number of prey in the small size class. The importance of mysid species decreased as predator size increased. An opposite trend was evident for the main decapod and teleost species, whose IRI increased considerably as the predator grew. Tab. 4: T. m. capelanus. The values of IRI and MFI (No. = 177). Tab. 4: T. m. capelanus. Vrednosti IRI in MFI indeksov (No. = 177). Food items MFI IRI Polychaeta 0.156 3.01887 Bryozoa 0.08694 2.37489 Crustacea Penaeidae 760.46 2065.55 Alpheidae 517.622 1167.69 Nephropidae 45.2673 79.8582 Goneplacidae 79.6823 192.465 Other Decapoda 105.731 370.1587 Total Crustacea 7874.94 20866.1 Pisces 541.483 1071.96 Seasonal variation in diet The average number of prey per stomach was significantly lower in the winter (F = 64.70, df = 272, P <0.001), whereas the mean stomach weight was constant through the year (F = 4.55, df = 272, P >0.05). The relative importance of T. m. capelanus prey groups changed seasonally, although decapod crustaceans were the dominant food in all seasons. According to IRI (4133.29) and MFI (1318.781) values, Penaeidae were dominant during the spring, while Alpheidae (IRI = 982.348, MFI = 397.788) were dominant during the autumn. Mysids were present in the stomach contents of poor cod throughout the year ($2 = 20.33, df = 3, P <0.001). Teleosts appeared most frequently in spring Food items MFI IRI Polychaeta 0.044056 0.438912 Tunicata 0.044056 0.38912 Crustacea Mysidacea 190.1429 1953.897 Penaeidae 1448.818 2869.374 Alpheidae 184.7364 389.9475 Goneplacidae 75.29081 135.4406 Other Decapoda 405.0783 938.0181 Total Crustacea 11496.21 27932.8 Pisces 24.97957 55.46411 Algae 0 0.373357 190 ANNALES • Ser. hist. nat. • 14 • 2004 • 2 Jakov DULCIC & Zrinka DULCIC: FEEDING HABITS OF THE MEDITERRANEAN POOR COD TRISOPTERUS MINUTUS CAPELANUS (LACEPEDE) ..., 189-196 (MFI = 865.833, IRI = 1887.436, $2 = 12.50, P<0.01). Goneplacidae were frequent in the spring and autumn diet (MFI = 30.965, IRI = 69.229, MFI = 98.292, IRI = 205.142), while Nephropidae dominated during the spring (MFI = 128.341, IRI = 274.248). Diet overlap The diet overlap, calculated on the basis of prey weight (Cw %), among fishes of different length groups in each season was moderate (0.62). Only in spring, a relatively high dietary overlap (0.76) was noted between size classes (Tab. 5). Tab. 5: Proportional food overlap coefficients of T. m. capelanus between seasons and size classes. Tab. 5: Koeficienti proporcionalnega prehranskega prekrivanja moliča T. m. capelanus med letnimi časi glede na posamezne velikostne razrede te vrste. Our study of T. m. capelanus in the eastern Central Adriatic shows that decapods (mostly shrimps, Penaei-dae, Alpheidae, Goneplacidae) constitute the main identifiable prey. Mysids, although dominant in number, constitute only a small percentage by weight. Teleosts such as gobiids are also important, but only in larger size classes. In general, our results agree with previous studies carried out in the Adriatic (Zupanovic & Jardas, 1989; Gramitto, 1999) and other Mediterranean areas (Biagi et al., 1992; Politou & Papaconstantinou, 1994; Morte et al., 2001). Euphausiacea, found in the stomachs during this study, were not found in the stomachs of poor cod collected on the continental shelf of the Central Adriatic, where they were replaced by other crustaceans (Mysida-cea and Amphipoda) (Gramitto, 1999). The found eggs and pleopods and occasional fragments of legs of large Decapods, or bites of fish meat together with detritus suggest that poor cod also feeds on dead organisms found on the sea floor, possibly trawler discards. The sporadic finds of remains of T. m. capelanus may be either the result of this behaviour or cannibalism, which had been previously confirmed by Gramitto (1999). The poor cod has a well developed barbel and long soft pelvic rays, indicating that it is adapted to feed mainly near the bottom. Moreover, its mouth is designed for quick suction of prey above the bottom (Mattson, 1990). The diet consists of benthic species (A. glaber, Nephrops norvegicus, Gobius sp.) or species living just above the bottom. Most of these preys live buried in the substratum. Prey-search must therefore be active, and the fish has to sweep its pelvic fin over the bottom to locate prey, as do other members of the family Gadidae (Marshall & Cohen, 1973). The preference for bigger prey types as predator size increases in this species results in the replacement of some small food organisms (Mysidacea and Euphasia-cea) by others of bigger size (A. glaber, teleosts). Similar results have been reported for other Mediterranean areas (Politou & Papaconstantinou, 1994; Gramitto, 1999; Morte et al., 2001), although there are certain differences in the diet. Armstrong (1982) reported a gradual increase of N. norvegicus of the 0+ age class in the stomach contents with increasing predator size. In this study, N. norvegicus was found in the stomachs of both size classes (<14.0 cm, >14.5 cm) and confirms the previous findings. Gramitto (1999) reported that small Norway lobsters were found in the stomachs of large poor cod specimens only four times in the Adriatic, even though the fish feeds intensively on other burrowing decapods of similar size living on "Nephrops grounds", such as A. glaber. The same author explained this with the behaviour of both predator and prey. The low percentage of empty stomachs agrees with the observations of previous investigators for other areas (Biagi et al., 1992; Politou & Papaconstantinou, 1994; Gramitto, 1999; Morte et al., 2001). The low values of the vacuity index and the finding of filled stomachs all day round suggest that the poor cod is a continuous feeder, as hypothesised by Biagi et al. (1 992) for the Tyrrhenian Sea. The analysis of stomach fullness in the 24 hours showed that T. m. capelanus is a continuous feeder, also if some periodicity can be detected probably linked to different availability of the prey, due to their diet behaviour pattern (Gramitto, 1999). Reproduction, which takes place in the winter and early spring, seems to have little effect on vacuity, as this was constant throughout the year. Similar findings, as far as the Adriatic is concerned, have been reported by Gramitto (1999), whereas Politou & Papaconstantinou (1994) found a seasonal cycle with a maximum vacuity in winter off the eastern coast of Greece. The poor cod's feeding habits changed seasonally. These changes could be due to the different depths of sampling sites, which are difficult to assess when sam- <14.0 cm >14.5 cm Winter <14.0 cm 0.78 >14.5 cm 0.25 0.57 SPrinJ <14.0 cm 0.96 >14.5 cm 0.56 0.76 Summer <14.0 cm 0.74 >14.5 cm 0.15 0.45 Autumn <14.0 cm 0.78 >14.5 cm 0.22 0.51 DISCUSSION 190 ANNALES • Ser. hist. nat. • 14 • 2004 • 2 Jakov DULCIC & Zrinka DULCIC: FEEDING HABITS OF THE MEDITERRANEAN POOR COD TRISOPTERUS MINUTUS CAPELANUS (LACEPEDE) ..., 189-196 pies are provided by professional fishermen. In this study, however, the fish were collected over a narrow depth range (50 to 175 m), where the structure of bottom communities is probably quite similar. Other factors should also be considered, such as temporal variation in abundance and/or prey availability of prey. A number of authors have shown that, as the density of particular prey type declines, a predator may switch to another, more abundant prey (Hume & Northcote, 1985; Davidson, 1986). Unfortunately, no data on the distribution and abundance of crustaceans are currently available to determine the food web in the area of investigation. PREHRANJEVALNE NAVADE MOLIČA TRISOPTERUS MINUTUS CAPELANUS (LACEPEDE) (PISCES: GADIADE) V VZHODNEM SREDNJEM JADRANU Jakov DULČIČ Inštitut za oceanografijo in ribištvo, HR-21000 Split, P.O.BOX 500, Hrvaška E-mail: dulcic@izor.hr Zrinka DULČIČ Sardina d.d., HR-21420 Postira, Hrvaška POVZETEK V mesečnih intervalih med marcem 2003 in januarjem 2004 je bila pregledana vsebina želodcev moličev Tri-sopterus minutus capelanus (Lacepede), ujetih v hrvaških vodah vzhodnega srednjega Jadrana. Da bi ugotovili prehranjevalne navade te vrste glede na velikost primerkov in letne čase, je bilo skupaj pregledanih 273 primerkov. Njihovo osnovno hrano so sestavljali raki (Mysidacea in Decapoda) in prave kostnice, glede na velikost primerkov pa je bilo v želodcih večjih moličev ugotovljenih več deseteronožcev in rib kot v želodcih manjših primerkov. 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