MORPHOMETRIC CHARACTERISTICS OF ASIAN WALKING CATFISH CLARIAS BATRACHUS (L., 1758) (ACTINOPTERYGII: CLARIIDAE) FROM THE LABORATORY CULTURE

Beata Więcaszek, Stanisław Krzykawski
Department of Fish Systematics, West Pomeranian University of Technology, Szczecin, Poland

ABSTRACT

The material used to study meristic and morphometric characters consisted of 30 xanthic morph individuals collected from the laboratory culture, and one albino individual from the heated water aquaculture. The fishes measured from 23.3 to 43.0 cm in total length (TL). Taxonomic analysis involved 13 morphometric characters and 7 meristic ones. A high variability of metric characters, expressed as per cent of SL, was found in three characters: length and width of the occipital process, and the maxillary barbel length. The lowest variability showed the predorsal and prepelvic lengths as well anal and dorsal fin lengths, while in characters expressed as per cent of TL – anal fin length. The shape of the occipital process was found to change with the fish length. The length/width ratio of the occipital process ranged markedly from 40.74 to 79.17%, decreasing with the fish length increase. The following formula of meristic characters can be presented: Db 57–64 (61.50), Ab 39-50 (45.33), Vu 1 Vb 5–6 (5.83), Pu 1, Pb 7–8 (7.50), GR 17–21 (18.92), vt. 56–62 (59.00). Ranges of the ray count in dorsal and anal fins were lower compared to data in the literature from other regions. The rakers on the first gill arch are fine, while on the second arch they are squabby. On both gill arches rakers have  a slight serration on the inner side and they are rather rarely set. Catfish examined differed in some characters from the neotype of C. batrachus described in recent literature.

Key words: Clarias batrachus, morphometric characters, meristics, occipital process, walking catfish.

INTRODUCTION

The genus Clarias Scopoli, 1777 is a group of air-breathing catfishes found in inland waters throughout much of the Old World. They are easily identified by an anguilliform body, long-based dorsal and anal fins, eye with free orbital margin and located dorsolaterally, large and well-developed neurocranium and the presence of an accessory breathing organ comprised of modified gill arches. The bulk of Clarias diversity is found in Africa [25]. Some species like Asian walking catfish (or Philippine catfish, magur) C. batrachus or north African catfish C. gariepinus are of great commercial importance both in fisheries, aquaculture as well as in the aquarium trade. Of the 16 Asian species, none is better studied than C. batrachus, widely utilized in aquaculture, in the aquarium trade and has been the subject of numerous studies on its biochemistry [18].

The Asian walking catfish has been introduced in Europe and America for aquaculture purposes and as a pet fish [24]. It is available on the Polish market as a pet-fish in the aquarium trade and more seldom as a fish in the aquaculture. In Netherlands it has been an object of the aquaculture research since last century [15,10]. In south-east Asia C. batrachus has been used in local fish culture where it proved to be a fast growing protein source [14]. It is a favourite and important food fish in Bangladesh, India, Thailand, Vietnam, Malaysia and Indonesia, that is marketed live, fresh and frozen [5]. For the aquaculture purposes hybrids of two species of Clarias are often used. C. batrachus is less resistant to wide fluctuations in temperature and oxygen, as well it has lower growth rate and fecundity when compared to C. gariepinus. Therefore studies are often undertaken to improve the stock of Asian catfish through hybridization with introduced African catfish. Hybrid vigor of catfish was produced through cross breeding between C. batrachus ♀ and C. gariepinus ♂ [20].

The walking catfish is a potamodromous, demersal freshwater and brackish water species, widely distributed, known from Pakistan, eastern India, Sri Lanka (Ceylon), Bangladesh, Myanmar (Burma), Thailand, Malaysia, Singapore, Indonesia, and the Philippines [22,11]. C. batrachus and C. macrocephalus are the most abundant and widely distributed in Thailand, however C. batrachus has been recently identified as vulnurable to extinction [16]. According to Vidthayanon [27] is recently rare, due to replacing of introduced C. gariepinus. Valued in aquaculture by some Southeast Asian peoples, it seems likely that human activities are responsible for the presence of the species in some of the more disjunct portions of its current range.

C. batrachus is popular for aquaculture in its native range but not regarded as such in other Southeast Asian countries; generally the catfishes from the family Clariidae are not only important economically, but also regarded as invasive species. After Holčik [9] and according to the Convention of European Wildlife and Natural Habitats [3], two clariid species are present in Europe: C. gariepinus and C. batrachus (introduced mainly in the United Kingdom). The trade of them is restricted in Germany, considered there as invasive species [28]. In areas where C. batrachus has been introduced, it is thought certain to pose a threat to native fishes and aquatic environments although relatively little quantitative evidence has been published to date [6, 21]. However the history of the non-native population established in southern Florida during the early 1960's is well documented. The spread of the walking catfish from 1–2 points of introduction encompassed 20 counties in approximately 10 years; effectively the entire southern peninsula of  Florida [21]. It can walk and leave the water to migrate to other water bodies using its auxiliary breathing organs [24]. Walking catfish are not at this time valued as a food or sport fish in Florida. They are known to have invaded aquaculture farms, entering ponds where these predators prey on fish stocks. In response, fish farmers have had to erect protective fences to protect ponds [19]. C. batrachus are known also to prey on native tadpoles [2]. The species has been nominated as among 100 of the "World's Worst Invaders" by the Invasive Species Specialist Group (ISSG) [4].

C. batrachus has an elongate body with long dorsal and anal fins. Upper jaw is a little projecting. Spine of pectoral fins is rough on its outer edge and serrated on its inner edge [23]. There is a few varieties of C.  batrachus. Although most individuals are gray or gray-brown with small white spots, an albino with red eyes was once popular with aquarists and xanthic (or calico) morphs with fully pigmented eyes are known. Aberrantly coloured walking catfish, conspicuous to predators, are uncommon in the wild, however an albino variation which is unusual to most fish species, occurs in nature [21]. C. batrachus is still a rare fish species in Poland, nevertheless different species of Clarias, like C. batrachus, C. macrocephalus or C. angolensis and their hybrids (including C. gariepinus) are likely in the close future to be cultured in Europe and Poland, too [29].

The literature on the taxonomic status of the species is very scarce. The identity has been assumed to be without problem and has never seriously questioned. However morphological and karyological data indicate that four species are confused under the name C. batrachus [18]. To date, only few characters of the species are described in the literature. Thus the objective of the present work was to fill a gap in the knowledge on the C. batrachus under the fishculture condition in the heated waters and laboratory culture by studying in detail the morphometric and meristic characters.

MATERIALS AND METHODS

The material used to study consisted of 31 specimens collected in years 2006–2010, 30 xanthic (or marbled) morph individuals (20 females and 10 males) from the laboratory culture and 1 albino individual, a female (SL=36.2 cm) (Fig. 1) from the heated water aquaculture in Nowe Czarnowo – the Research Station of the West Pomeranian University of Technology, where it was reared together with the north African catfish C. gariepinus. The parent fish of the marbled morph were imported from Indonesia, then the spawning was induced. The parent fish and their reared offspring from two laboratory cultures were next studied.

At present it is difficult in Poland to attain a numerous sample of C. batrachus for the research purposes, however in the work of Ng and Kottelat [18] only 20 specimens were examined in order to make a taxonomic revision of the species.

The fishes measured from 23.3 to 45.1 cm in total length (TL), with the mean length of 33.28 cm, and from 21.5 to 37.9 cm in standard (or body) length (SL), with the mean length of 29.17 cm. For the morphometric studies two length classes TL were established: 21.5–25.1 cm and 29.5–37.9 cm.

Taxonomic analysis involved 13 morphometric characters and 7 meristic ones. Measurements were made with dial calipers and recorded to tenths of a centimeter for body characters and to tenths of millimeter for head characters. Counts and measurements were made on the left side of specimens. Subunits of the head are presented as proportions of head length (HL). Head length and measurements of body parts are given as proportions of standard length (SL) and total length (TL).

Measurements and counts follow those of Teugels [25] and Agnese et al. [1], slightly modified. The measurement design is presented in the paper of Więcaszek et al. [29].

Morphometric characters and their symbols, as well as results of the study are presented in Tables 1–4. The following metric characters were studied: total and standard lengths, head length (= the distance from tip of snout to end of occipital process), interorbital length, occipital process length and width, maxillary barbel length, prepectoral, prepelvic, predorsal and preanal lengths, length of dorsal and anal fin base, as well as the distance from the origin of dorsal fin to occipital process (OP-D). In addition, the length (OPL): width (OPW) of occipital process ratio was established.

The following meristic characters were determined: count of soft and hard ray in the dorsal, anal, pectoral and pelvic fins and count of gill-rakers on the first gill arch; total vertebral number included the four anteriormost vertebrae, associated with the Weberian apparatus, and the hypural fan centrum. The vertebral count was established in ten individuals (including the albino specimen) by X-ray (Roentgen) photography. Meristic characters and their symbols (with the results of study) are shown in Table 5. Symbols of meristic characters followed those of Holčik [8].

All the data were subjected to statistical treatment involving standard deviation (SD), mean (M) and standard error of the mean (m), and coefficient of variation (CV). The most frequent counts (MFC) are reported for meristic characteristics as well. According to Więcaszek et al. [29], the coefficient of variation is regarded as significant when ≥ 8%; characters showing CV> 8% were then considered highly variable.

The Student's test was applied to test the null hypothesis that the mean values of metric characters in two length-classes of fish examined did not significantly differ (at a significance level α = 0.05).

RESULTS

Results of studies of morphometric characters are presented in Tables 1–4, while meristic features are shown in Table 5. Table 1 summarises range and mean values of morphometric characters expressed as per cent total length (TL) of the whole sample, while Table 2 – the same characters, expressed as per cent standard (or body) length (SL) and head length (HL) of the whole sample, the sample without an albino individual and the albino individual alone.

Analysis of the coefficient of variation in Table 1, showed the CV values to be very high with respect to three characters: the occipital process length and width and the maxillary barbel length. The head length and preanal length showed a pretty high variability as well. The remaining nine characters are characterised by relatively low CV, especially the body length converted to per cent total length.

The same characters are the most variable, when analysed the CV values for the characters expressed as per cent of SL (Table 2). The lowest variability showed predorsal and prepelvic lengths, as well base length of anal and dorsal fins. The albino individual from the heated water aquaculture differed in some characters in comparison with the sample of the xanthic walking catfish from the laboratory culture. It had a smaller head and occipital process, as well shorter maxillary barbels; the dorsal and pectoral fins were shorter and situated closer to the fish mouth.

All metric characters pertinent with head, expressed as per cent head length, were highly variable (Tab. 2). The most variable are length and width of the occiptal process. The distance of the occipital process - dorsal fin is included 3.66–5.70 times in distance from tip of snout to end of occipital process (17.54–27.42% of HL) for the whole sample. The character is highly variable. The mean of the maxillary barbel length is nearly equal to the head length (103.43 % of HL). The CV of this character is statistically significant.

Table 3 summarises metric characters of C. batrachus studied, converted to per cent fish standard length (SL) and head length (HL) in the total length classes. Analysis of the coefficient of variation CV in Table 3 showed the CV values were very high with respect to four characters: the occipital process length and width, the maxillary barbel length and the distance from the dorsal fin to occipital process, however mainly in the lower length class (21.5–25.1 cm of TL). Similarly, all characters pertinent to the head, demonstrated high variability, however mainly in the lower length class (21.5–25.1 cm of TL), too.

When analysed the mean values in the length class, it was seen that the mean values of head length, interocular distance, occipital process width, prepectoral and predorsal distances were bigger in the higher length class, while the mean values of length of barbels, occipital process width and length of dorsal and anal fin were bigger in the lower length class. The mean values of prepelvic and preanal distances, as well as the distance from the dorsal fin to occipital process were very similar in both length classes. The Student t-test showed however the statistically significant differences between both fish length classes only in the head length, occipital process width and length, prepectoral distance and length of dorsal fin (Tab. 3).

Among the characters pertinent to the head, analysed in the length classes, only the mean value of width of occipital process was bigger in the higher length class, whereas in the remaining characters the mean values were lower (Tab. 3). The Student t-test confirmed the statistically significant differences between both fish length classes in occipital process width and length and the barbel length.

The ratio of the occipital process length to the occipital process width, is not a highly variable character, especially in the length class 21.5–25.1 cm (CV=3.09%) (Tab. 4). In the length class 29.5–37.9 cm it is markedly more variable (CV=8.90%). As seen from the Table 4, the shape of the triangular occipital process has been changed with the fish length increase. In the lower length class (21.5–25.1 cm) the occipital length is markedly bigger in the relation to the width, than in the higher length class (31.0–37.9 cm). Thus the base of the triangle (the shape of the occipital process) increases with the fish length increase, becoming more obtuse-angled, with the top rounded. The Student t-test confirmed the statistically significant differences between both fish length classes in occipital process width and length.

Data on seven meristic characters of the walking catfish individuals examined summarises Table 5. Two characters were stable in the sample studied, namely the count of unbranched rays (1) in the pectoral and pelvic fins. No spines were present in the dorsal and anal fins. The count of branched rays in the dorsal fin ranged from 57 to 64 (mean 61.50), whereas in the shorter anal fin – from 39 to 50 (mean 45.33). Lower individual variability was found in the ray count in dorsal fin compared to the anal one (CV 4.33 and 8.09%, respectively). In the dorsal fins 62 rays was noted most often, while in the anal fin – 46 rays.

The pectoral fin ray count ranged within 7–8 (mean 7.52), while pelvic fin ray count covered the range from 5 to 6 (mean 5.83). In the latter fin the most frequent count was 6. The degree of individual variability was similar in these two characters and amounted to 7.06 and 7.30%, respectively.

The number of gill rakers on the first gill arch of C. batrachus ranged from 17 to 21, with the mean count amounted to 18.92. Most specimens had 19 gill rakers. No relationship was found between the length and gill count. The rakers of C. batrachus on the first gill arch are fine, while on the second arch they are squabby. On both gill arches rakers have a slight serration on the inner side (Fig. 2). They are rather rarely set.

The vertebral count covered the range from 56 to 62; most individuals possessed 59 vertebrae. The meristics of albino individual fitted within the ranges obtained for the whole sample of the walking catfish, except the vertebral count, which was bigger compared to the marbled catfish sample.

Meristic characters in the sample examined were rather of low variability, except the count of gill rakers on the first gill arch (CV= 22.8%) and vertebral count (41.03%).  The most stable was the count of branch rays in the dorsal fin (CV= 4.33%).

In this study the following detailed formula of meristcs can be presented: Du 0, Db 57–64 (61.50), Au 0, Ab 39–50 (45.33), Vu 1, Vb 5–6 (5.83), Pu 1, Pb 7–8 (7.52), GR 17–21 (18.92), vt. 56–62 (59.00) (in brackets the mean values are given).

DISCUSSION

To date, C. batrachus is believed to be a common species widely distributed throughout South and Southeast Asia [13, 18]. However, while Ng and Kottelat [18] were trying to established the identity of this species, they discovered problems with the identity and nomenclature of C. batrachus. Ng and Kottelat [18] clarified the identity by the designation of a neotype. The variability observed in what is currently recognized as C. batrachus, according to morphological and karyological data indicate that four species are confused under the name C. batrachus. According to these authors, specimens described as neotype of C. batrachus inhabit only the drainages of Java. Next, for the species from the north-eastern India, previously identified as C. batrachus, should be used C. magur as the valid name. All of the material from the Mekong River drainage in the paper of Ng and Kottelat [18] are identified as C. aff. batrachus "Indochina", while catfishes from the Malay Peninsula and Borneo – as C. aff. batrachus "Sundaland".

Comparison of own data with those on C. batrachus from Java, given in Ng and Kottelat [18] showed many differences. They concerned the shape of head (narrower in the anterior part in fishes from Java), metric characters (fish in own study have shorter head and all distances between snout and fin origins, longer bases of dorsal and anal fins, closer located eyes and longer supraoccipital process). In meristic features, fish in own study have smaller count of rays in the dorsal and anal fins.

The character of serration on the inner part of spine in the pectoral fin excluded the affinity of the fish studied to the C. magur, described in paper of Ng and Kottelat [18], inhabiting, according to these authors, the north-eastern India.

In the paper of Taki [23], concerned the walking catfish from the Mekong River drainage, the following formula of meristics (containing only the dorsal and anal fin rays) was given: dorsal spines Du: 0; dorsal soft rays Db: 60–76; anal spines Au: 0; anal soft rays Ab: 47–58. Ranges of the ray count in dorsal and anal fins was clearly lower in own study (laboratory culture) and the albino individual fitted the lower ranges, compared to the sample from the Lao Mekong basin [23] (Tab. 6).

Mean values of morphometric characters of the walking catfish sample from the Laos (the number of specimens examined was not given), reported by Froese and Pauly [6] can be included in the ranges of characters of the walking catfish from this study, except the prepelvic length being a little bigger in catfish from Laos (Tab. 6). When the mean values from both samples were taken under consideration, it was found out that C. batrachus from Laos had a slightly shorter head, somewhat bigger the prepectoral and predorsal distances and markedly bigger the preventral and preanal distances.

Gill-rakers count is regarded by Teugels [25] as the most important and often the only reliable meristic feature to distinguish the species within the Clarias genus. Teugels [25] stated that gill rakers of the north African catfish C. gariepinus were fine, long (without serration) and closely set; their number increased with the size of fish. No description of rakers of C. batrachus has been yet available in literature. In this study it was found out that the rakers of C. batrachus were fine on the first gill arch, too (on the second row they were squabby), however with a slight serration on the inner side. They were rather rarely set.

The shape of the occipital process as well as the shape of the anterior and posterior fontanells is an important diagnostic characters in genus Clarias [17], too. The comparison between C. gariepinus and C. batrachus, geographically isolated, nevertheless genetically and morphologically similar [7] is shown in Fig. 3. The shape of occipital process and both fontanelles of the albino individual demonstrates Fig. 4.

Occipital process of the Asian walking catfish is more or less triangular. According to Kottelat [13] in catfish from Laos, its length is about 2 times in its width, thus the OPL/OPW (occipital process length/ occipital process width) is amounted to about 50%. In present study, the ratio OPL/OPW ranged markedly from 40.74 to 79.17%, decreasing with the fish length increase. Two lengths classes were clearly distinguished, with regard to the shape of the occipital process: 21.5–25.1 cm and 29.5–37.9 cm TL. In the first one the OPL/OPW ranged from 74.42–79.17%, while in the second length class – from 40.74 to 48.61%. Therefore the ratio OPL/OPW was in agree with the Kottelat's data [13] only for bigger fish, of TL ranged from 29.5–37.9 cm. For younger and smaller fish of TL from 21.5 to 25.1 cm, the length of the occipital process is about 1 1/3 of its width. The fact indicates for the necessity of comparison of the occipital shape in catfishes in the very similar ranges of fish size.

The distance between dorsal and occipital process (OP-D) is included 4–5.5 times (therefore from 18.18 to 25%) in the distance from tip of snout to end of the occipital process (HL) [13]. In this study the range of this character varied from 17.54–27.42%, with the mean of 22.76 %. Thus the distance between dorsal fin and occipital process (OP-D) is included 3.66–5.70 times in the distance from tip of snout to end of occipital process. No relationship was found between the fish length and this character.

The albino individual, originated from the underwater cages culture, compared to the xanthic specimens from the laboratory culture, was characterised by shorter head, smaller occipital process and shorter prepectoral and predorsal distances. In addition, its barbels and interocular distance were longer in relation to the head length. Turan et al. [27] stated, that such differences among the populations originated from the different habitats (for example from the natural reservoirs, underwater cages or the glass container with stagnant water in the laboratory) maybe related to the different biotic and abiotic characteristics, such as temperature, turbidity, food availability, water depth and flow. Walking catfish can be found in a variety of habitats, but they are most commonly encountered in muddy or swampy water of high turbidity [3]. All abiotic factors mentioned above were different, both between the sample from the own study and individuals from the Lao Mekong basin or Java rivers drainages, as well between the albino individual and the xanthic catfish sample from the laboratory culture.

CONCLUSIONS

The rakers of C. batrachus are fine on the first gill arch are fine, while on the second arch they are squabby. On both gill arches rakers have a slight serration on the inner side. They are set rather rarely.

The ratio of the occipital process length to the occipital process width, is not a highly variable character. The shape of the triangular occipital process has been changed with the fish length increase. In the lower length class (21.5–25.1 cm) the occipital length is markedly bigger in the relation to the width, than in the higher length class (31.0–37.9 cm). The base of the triangle (the shape of the occipital process) decreases with the fish length increase, becoming more obtuse-angled, with the top rounded.

Origin of the material studied in this paper, as well as analysis of the data obtained (including the head shape and serration on the spine in the pectoral fin) may indicate for the C. aff. batrachus "Sundaland" as valid name.

ACKNOWLEDGMENTS

Authors thank Piotr Robakowski (M.Sc, member of Ph.D. course) for a large part of material to study and Mr Michał Więcaszek for technical support in photos preparation.

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Accepted for print: 27.09.2010

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