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Morphometric Comparison of blue catfish Ictalurus furcatus (Lesueur, 1840) from Northern and Southern Atlantic Drainages of Mexico.

Abstract. -- A morphometric comparison was performed on specimens of blue catfish (Ictalurus furcatus) from northern (Lower Rio Bravo) and southern (Chiapas) Mexico in order to identify diagnostic characters that allow their discrimination. The discriminant function analysis determined three characters to be highly diagnostic to separate the two groups of specimens: the southern group [SG] has a shorter anal base (mean = 3.4 times in standard length [SL], range = 3.1 to 3.7) vs northern group [NG] (mean = 2.9 times in SL, range = 2.7 to 3.1), a lesser head width (mean = 6.0 times in SL, range 5.1 to 6.8) vs NG (mean = 5.6 times, range = 5.3 to 6.0), and a lower number of anal rays (mean = 26, range = 24 to 28) vs NG (mean = 31, range = 29 to 34). Additionally, 14 other characters were also different (P < 0.01) between both groups. All these characters support the taxonomic validation of Ictalurus meridionalis (Gunther 1864) for the individuals of SG that are currently included in I. furcatus Lesueur. Studies on comparative osteology and molecular genetics of both forms are needed for the clarification of their taxonomic status.

Resumen. -- Se realizo una comparacion morfometrica en el bagre azul (Ictalurus furcatus) del norte (Bajo Rio Bravo) y sur (Chiapas) de Mexico, con el objetivo de identificar caracteres diagnosticos que permitan su discriminacion. El analisis de funcion discriminante determino que tres caracteres son altamente diagnosticos para separar los dos grupos geograficos. El grupo sureno [GS] tiene una base anal mas corta (promedio = 3.4 veces en longitud patron [LP], intervalo = 3.1 a 3.7) vs grupo norteno [GN] (promedio = 2.9 veces en LP, intervalo = 2.7 a 3.1), una cabeza mas angosta (promedio = 6.0 veces en LP, intervalo 5.1 a 6.8) vs GN (promedio = 5.6 veces en LP, intervalo = 5.3 a 6.0), y un menor numero de radios anales (promedio = 26, intervalo = 24 to 28) vs NG (promedio = 31, intervalo = 29 to 34). Ademas, otros 14 caracteres tambien fueron diferentes (P < 0.01) entre ambos grupos. Todos estos caracteres sustentan la validacion taxonomica de Ictalurus meridionalis (Gunther 1864) para individuos del GS que son actualmente referidos como /. furcatus Lesueur. Estudios sobre osteologia comparada y genetica molecular de ambas formas son necesarios para clarificar su estatus taxonomico.


The blue catfish Ictalurusfurcatus (Lesueur 1840) is one of the Nearctic freshwater fish species of the Atlantic slope with a distribution extended to Neotropical localities as far southern as the Rio Usumacinta and the Rio Belize, Belize, where it was originally called Ameiurus meridionalis Gunther 1864 [= Ictalurus meridionalis] (Miller et al. 2005).

This southern form was described on the basis of its lower number of anal rays, its shorter barbels and its smaller eye than the northern form (Gunther 1864; Jordan and Evermann 1896-1900; Meek 1904; Alvarez del Villar 1970). However, Lundberg (1992) considered the nominal species I. meridionalis from the Rio Usumacinta as conspecific with I. furcatus, a situation that Miller et al. (2005) stressed as an interesting theme for additional study.

Our ichthyological explorations to the "Reserva de la Biosfera de Montes Azules" in the Mexican State of Chiapas, during 1979-1985 (Lozano-Vilano and Contreras-Balderas 1987) and 2004-2006 (Lozano-Vilano et al. 2007), had already detected in the field some differences in the body proportions of the southern blue catfish specimens when compared with the northern form, mainly the basal length of the anal fin, as well as the head width and length. Additionally, the geographical distribution of blue catfish in Mexico exhibits a notable disjunct pattern in the drainages of Veracruz, where the northern and southern populations are widely separated (cf. Miller et al. 2005).

In the present work, we compared 28 morphologic characters (27 morphometric and 1 meristic) in blue catfish from northern (Lower Rio Bravo) and southem (Rio Lacantun) Mexico, in order to determine the magnitude and signification of the differences.


Thirty-four individuals of blue catfish from different sites in southern (Chiapas, B in Fig. 1: 1, Rio Lacanja; 2, Rio Tzendales; 3, Arroyo Miranda; 4, Rio Lacantun at Estacion Chajul; 5, Rio Chajul; 6, Arroyo San Pablo; 7, El Colorado; and 8, Arroyo Manzanares) and northern Mexico (Lower Rio Bravo, A in Fig. 1: 1, Rancho Taffinder; 2, Rio Alamo; 3, El Astillero; 4, Garceno; 5, La Gloria; 6, Rodriguez de Anahuac; and 7, Presa Don Martin) were chosen for the comparative analysis (Appendix 1). The range and average length of the two groups of specimens (southern and northern) were similar (Table 1).

Twenty-two body distances based on box truss protocol (Bookstein et al. 1985, Fig. 2) and five distances of the head region and the number of anal rays were considered in the analysis (Hubbs and Lagler 1947). Each specimen was measured with a digital caliper (precision 0.01 mm) connected to a personal computer. The measurements were as follows (landmark number in parenthesis): (1-2), tip snout to mouth commissure; (1-3), tip snout to nostril; (2-3), mouth commissure to nostril; (2-4), mouth commissure to dorsal fin origin; (2-5), mouth commissure to pectoral fin origin; (3-4), nostril to dorsal fin origin; (3-5), nostril to pectoral fin origin; (4-5), dorsal fin origin to pectoral fin origin; (4-6), basal length of dorsal fin; (4-7), dorsal fin origin to pelvic fin origin; (5-6), pectoral fin origin to posterior insertion of dorsal fin; (5-7), pectoral fin origin to pelvic fin origin; (6-7), posterior insertion of dorsal fin to pelvic fin origin; (6-8), posterior insertion of dorsal fin to posterior insertion of adipose fin; (6-9), posterior insertion of dorsal fin to anal fin origin; (7-8), pelvic fin origin to posterior insertion of adipose fin; (7-9), pelvic fin origin to anal fin origin; (8-9), posterior insertion of adipose fin to anal fin origin; (8-10), posterior insertion of adipose fin to posterior insertion of anal fin; (8-11), posterior insertion of adipose fin to mid caudal base; (9-10), basal length of anal fin; and (10-11), posterior insertion of anal fin to mid caudal base. Other lineal measures and counts were: head length, head width (at level of occipital), eye diameter, internostril width, interorbital width, and the number of anal rays.


The original body measurements were standardized by means of the regression of Elliott et al. (1995), which removes the size component from the shape measurements (allometry), and is calculated for each character by the following equation:

Ms = Mo [(Ls/Lt).sub.b]; where Ms = standardized measurement of the character, Mo = original measurement of the character (mm), Ls = average standard length (mm) of all the specimens from the two groups examined, Lt = standard length (mm) of specimen, and "b" was estimated for each character from the observed data using the non-linear regression equation, M = a[L.sup.b]. Parameter "b" was estimated as the slope of the regression log Mo on log Lt using all fish.

The standardized morphometric values of the 34 examined specimens were analyzed between groups (northem: Lower Rio Bravo basin, and southern: Rio Lacantun basin) by means of"forward stepwise discriminant" function analysis (DFA) using Statistica 5.0 (StatSoft, Inc., Tulsa, OK, 1995). The DFA allowed us to detect which combination of characters discnminated best between groups. Finally, each character was compared statistically between groups using a Student's "t" test.


Results and Discussion

The means and standard deviations of the 28 morphological characters of 34 examined specimens of blue catfish are depicted in Table 1. The "t" student test for each character between groups (northern vs. southern) revealed 17 characters to be statistically different.

The DFA applied to 34 specimens of blue catfish from both groups selected 9 of 28 morphologic variables examined (Table 2). Overall value of Wilks' lambda was 0.03007, indicating a significant discrimination (p<0001) between groups. The highest Wilks' values were associated with the basal length of anal fin (0.0361), posterior insertion of adipose fin to posterior insertion of anal fin (0.0576), and the head width at level of occipital (0.0436). The contribution of each one of the nine characters selected by the model to the overall discrimination appears in the Table 2. The characters with high weight to the discrimination between groups were the variables: 8-10 ([Y.sub.1] = -1.18316), head width ([Y.sub.1] = 0.93789) and 9-10 ([Y.sub.1] = 0.57657). Predicted or correct classification of individuals was 100% in both groups, which indicates that the individuals maintain the identity of each group as shown in figure 3.

The count of anal rays was statistically different between groups, with the lowest number (mean = 26, range = 24 to 28) for the southern group (SG) and the highest for the northern group (NG, mean = 31, range = 29 to 34) (Table 1). When both groups were compared without transformation of the body measurements, the following body proportions were obtained: in the NG, the head width contained in average 5.6 times in standard length [SL] (range 5.3 to 6.0), while in that of the SG 6.0 times (range 5.1 to 6.8). The head of the NG specimens was notably longer and wider than in those SG specimens (Fig. 4). The basal length of anal fin in the NG specimens contained in average 2.9 times (range = 2.7 to 3.1) in SL in comparison with that of the SG (3.4 times, range = 3.1 to 3.7).


In the comparative analysis of blue catfish between specimens from the northern and southern populations of Mexico (Lower Rio Bravo and Rio Lacantun, respectively) 17 morphological characters were found to be significantly different (Table 1). Several of these differences had already been referred in the literature (e.g., Gunther 1864; Jordan and Evermann 1896-1900; Meek 1904; Alvarez del Villar 1970), such asa lower number of anal rays and a smaller head in southern populations (I. "meridionalis") in comparison with those of northern populations (I. furcatus). Also, the length of the barbels is different between these two populations; being longer in northern specimens (reaching the origin dorsal fin) and shorter in southern specimens (reaching the end of head or slightly beyond).

We also detected that the coloration of live specimens of "I. meridionalis" is gray silvery on the dorsal part of the body with steel reflections and whiter ventrally; however, this coloration pattern is different than that reported by Jordan & Everrnann (1896) and Meek (1904), which was brownish with steel blue reflections on the dorsum, and silvery on the ventral region. Of the 17 characters found to be significant by means of the student's "t" test, three of them were highly useful for separating both groups and were associated with distances in the anal region and the head width. Although the number of anal rays could be a character with latitudinal clinal variation, we did not detect this trend because specimens examined here from an intermediate area (Rio Tanquilin, San Luis Potosi) had a lower number of anal rays (mean = 22) when compared with southern specimens of Chiapas (mean = 26).


In spite of the range and average of length for both species examined being statistically similar, we expect that the variation of each character compared will decrease as the sample size increases. We suggest that a larger sample size for each southern and northern group, including intermediate populations, as well as a detailed study on the osteology and molecular genetic analysis should be conducted to evaluate the taxonomic status of the southern form "Ictalurus meridionalis."


We thank C. Ramirez Martinez, responsible of the project "Ordenamiento de la actividad pesquera en la ribera del Rio Lacantun de la Reserva de la Biosfera de Montes Azules". Our thanks to A.J. Contreras-Balderas for helping us in the field work and an early review of the manuscript. A. Z. Guerrero, J. M. Leza, D. L. Romero, A. A. Macossay, G. Lara, J. H. Lozano, M. Villalobos, G. Mendez, C. Chankayub, and R. Lombera for their help in the sampling and field work. Two anonymous reviewers and Dan Guthrie made useful comments that improved the manuscript. Funds were provided by the Centro Interdisciplinario de Biodiversidad y Ambiente, A. C.; Fondo Mexicano para la Conservacion de la Naturaleza, A. C.; Natura; Ecosistemas Mexicanos, A. C.; and Universidad Autonoma de Nuevo Leon (PAICYT, grant N1117-05).

Appendix 1

The material examined of blue catfish for the comparative analysis is deposited in the Coleccion Ictiologica del Laboratorio de Ictiologia, Facultad de Ciencias Biologicas, Universidad Autonoma de Nuevo Leon (UANL). CHIAPAS: UANL-15739, Rio Tzendales, 16 [degrees] 17'52" N, 90 [degrees] 53'12" W, 24 Aug. 2004; UANL-15718, Arroyo San Pablo, 16 [degrees] 06'07" N, 91 [degrees] 00'52" W, 23 Aug. 2004; UANL-15790, idem, 30 Nov. 2004; UANL-16984, Arroyo San Pablo, idem, 10 Dic. 2005, 30 Nov. 2004; UANL-16883, Rio Manzanares, 16 [degrees] 10'14" N, 90 [degrees] 50'36" W, 14 Sep. 2005; UANL-15760, Arroyo Miranda exit to Lacantun, 16 [degrees] 08'44" N, 90 [degrees] 55'50" W, 25 Aug. 2004; UANL-15822, Rio Lacantun at Estacion Chajul, 16 [degrees] 06'35" N, 90 [degrees] 56'23" W, 1-2 Dec. 2004; UANL-15993, Rio Lacanja, 16 [degrees] 24'14" N, 90 [degrees] 47'52"W, 10 Feb. 2005; UANL-16698, Rio Lacanja, idem, 9 Jun. 2005; UANL-16726 (ex UANL 16716), El Colorado, 16 [degrees] 07'13" N, 91 [degrees] 07'50" W, 9 Jun. 2005; UANL-16883, Rio Manzanares, 16 [degrees] 10'14" N, 90 [degrees] 50'36" W, 14 Sep. 2005; UANL-17046, Rio Chajul, 16 [degrees] 06'11" N, 90 [degrees] 57'22" W, 11 Dec. 2005. TAMAULIPAS: UANL-826, Rancho Taffinder, 8 km NNW Nueva Ciudad Guerrero, 26 [degrees] 37'42"N, 99 [degrees] 14'59"W, 14 Oct. 1966; UANL-4226, mouth of the Rio Alamo, 5.2 km E Ciudad Mier, 26 [degrees] 25'34"N, 99 [degrees] 06'41"W, 15 Feb. 1982; UANL-8968, Rio Salado at El Astillero, 26 [degrees] 51'30"N, 99 [degrees] 35'28"W, 6 Jun. 1985; NUEVO LEON: UANL-8157, Rio Salado at Garceno, 27 [degrees] 10' 19"N, 100 [degrees] 04' 10" W, 7 Ago. 1984; UANL-8557, Rio Salado at La Gloria, 26 [degrees] 53'14"N, 99 [degrees] 48'20"W, 10 Nov. 1984; UANL-8935, Rio Salado at La Gloria, idem, 6 Jun. 1985; UANL-11628, Rio Salado at Rodriguez de Anahuac, 27 [degrees] 13'59"N, 100 [degrees] 07'43"W, 1982 [no date]; COAHUILA: UANL-15179, Presa Venustiano Carranza (Don Martin), 27 [degrees] 31'24"N, 100 [degrees] 37'51"W, 9 May 2002. Other material examined. VERACRUZ: UANL-1823, Rio Papaloapam at Los Amates, 18 [degrees] 17'00" N, 95 [degrees] 52'00" W, 14 Dec.1972. TABASCO: UANL-2911, Rio Sonapa 18 Km W Huimanguillo, 17 [degrees] 52'00"N 93 [degrees] 28'00"W ,14 Feb. 1978. SAN LUIS POTOSI: UANL-1269, Rio Tanquilin SW Camoca, 21 [degrees] 16'00" N, 99 [degrees] 03' 00" W, 28 Oct., 1971.

Literature cited

Gunther, A. 1864. Catalogue of the fishes in the British Museum. Catalogue of the Physostomi, containing the families Siluridae, Characinidae, Haplochitonidae, Sternoptychidae, Scopelidae, Stomiatidae in the collection of the British Mus. V. 5:i-xxii+1-455.

Alvarez, J. 1970. Peces Mexicanos (Claves). Ser. Inv. Pesq. Nal., Inv. Biol. Pesq., Mexico. 166 pp.

Bookstein, F.L., B. Chemoff, R.L. Elder, J.M. Humphries, G.R. Smith Jr., and R.E. Strauss. 1985. Morphometrics in evolutionary biology. Acad. Nat. Sci. Phil. Sp. Pub. 15, 277 pp.

Elliott, N.G., K. Haskard, and J.A. Koslow. 1995. Morphometric analysis of orange roughy (Hoplostethus atlanticus) off the continental slope of southern Australia. Jour. Fish Biol., 46:202-220.

Hubbs, C.L. and K.F. Lagler. 1947. Fishes of the Great Lakes region. Cranbrook Inst. of Sci. Bull., 26: 1-186, pls. 26, figs. 251.

Jordan, D.S. and B.W. Evermann. 1896-1900. The fishes of North and Middle America. Bull. U. S. Nat. Mus., 47(I-IV): 1-3313.

Lesueur, C.A. 1840. Pimelodus furcatus. In: Histoire naturelle des poissons (G. Cuvier and A. Valenciennes). Tome quinzieme. Suite du livre dix-septiem. Siluro'ides. v. 15, Pp. i-xxxi + 1-540, Pls. 421-455.

Lozano-Vilano, M.L. and S. Contreras-Balderas. 1987. Lista zoogeografica y ecologica de la ictiofauna continental de Chiapas, Mexico. The Southwestern Naturalist, 32:233-236.

--, M.E. Garcia-Ramirez, S. Contreras-Balderas, and C. Ramirez-Martinez. 2007. Diversity and conservation status of the ichthyofauna of the Rio Lacantun basin in the Biosphere Reserve Montes Azules, Chiapas, Mexico. Zootaxa, 1410:43-53.

Lundberg, J.G. 1992. The phylogeny of ictalurid catfishes: a synthesis of recent work. Pp. 392-420. In: Systematics, historical ecology, & North American freshwater fishes (R.L. Mayden, ed.). Stanford University Press, Stanford.

Meek, S.E. 1904. The freshwater of Mexico, North of the Isthmus of Tehuantepec. Field. Col. Mus. Publ., 93:i-xiii+1-254.

Miller, R.R., W.L. Minckley, and S.M. Norris. 2005. Freshwater Fishes of Mexico. Univ. Chicago Press, Chicago. Pp. I-XXV, 1-490.

Gorgonio Ruiz-Campos, (1) Maria de Lourdes Lozano-Vilano, (2) and Maria Elena Garcia-Ramirez (2)

(1) Laboratorio de Vertebrados (Coleccion Ictiologica), Facultad de Ciencias, Universidad Autonoma de Baja California, Apdo. Postal 233, Ensenada, Baja California, 22800, Mexico. U.S. mailing address: PMB. No. 64, P.O. Box 189003, Coronado, CA. 92178-9003,

(2) Laboratorio de Ictiologia, Facultad de Ciencias Biologicas, Universidad Autonoma de Nuevo Leon, Apdo. Postal 425, San Nicolas de los Garza, 66450, Nuevo Leon, Mexico
Table 1. Descriptive statistical values of 27 standardized
morphological characters plus one meristic (anal rays), with their
respective levels of significance (P), for individuals of blue
catfish from northern and southern, Mexico. Significant characters
are shown in bold.

 Rio Lacantun N=21 Lower Rio Bravo N=13

Variables Ave. SD Ave. SD

SL (mm) 211 43.3 219 44.8
1-2 15.21 0.92 15.28 1.59
1-3# 13.34 1.09 12.35 1.07
2-3 9.46 1.15 9.36 1.39
2-4# 68.25 6.89 76.83 7.64
2-5 35.62 2.88 34.57 1.64
3-4 66.29 5.46 68.75 3.00
3-5 38.24 5.55 38.54 4.78
4-5 47.27 11.07 53.09 10.48
4-6# 17.02 1.15 15.77 1.00
4-7 54.63 4.26 56.63 5.08
5-6 55.37 3.68 57.60 3.04
5-7# 57.48 5.28 52.19 3.45
6-7 44.63 4.5 47.44 3.26
6-8 85.53 6.77 89.57 5.31
6-9# 57.52 3.05 53.45 2.04
7-8# 82.47 5.07 95.17 4.19
7-9# 28.71 2.82 24.26 1.70
8-9# 60.50 3.99 77.45 6.32
8-10# 25.67 1.59 21.98 1.21
8-11# 45.94 1.94 42.90 1.69
9-10# 60.74 3.64 78.64 4.27
10-11# 34.60 3.02 27.35 2.11
Head width# 34.11 3.27 42.67 1.69
Internostril width# 16.24 0.75 14.67 0.65
Head length# 49.99 2.18 54.08 3.11
Interorbital width# 26.15 1.10 28.05 0.92
Eye diameter 9.13 0.56 9.23 0.81
Anal rays# 26 1 31 2

Variables Student's "t" P

SL (mm) 0.517 0.609
1-2 0.163 0.871
1-3# 2.591 0.014#
2-3 0.228 0.821
2-4# 3.386 0.002#
2-5 1.196 0.241
3-4 1.486 0.147
3-5 0.257 0.797
4-5 1.52 0.138
4-6# 3.231 0.003#
4-7 1.236 0.225
5-6 1.83 0.077
5-7# 3.204 0.003#
6-7 1.952 0.060
6-8 1.828 0.077
6-9# 4.247 <0.001#
7-8# 7.562 <0.001#
7-9# 5.125 <0.001#
8-9# 9.62 <0.001#
8-10# 7.166 <0.001#
8-11# 4.656 <0.001#
9-10# 13.045 <0.001#
10-11# 7.567 <0.001#
Head width# 8.711 <0.001#
Internostril width# 6.23 <0.001#
Head length# 4.512 <0.001#
Interorbital width# 5.196 <0.001#
Eye diameter 0.426 0.673
Anal rays# 9.719 <0.001#

SL (mm) = no transformed standard length.

Note: Significant characters are shown in bold is indicated with #.

Table 2. Summary of the discriminate function analysis (forward
stepwise type) applied for standardized measurements of blue catfish
from northern (Lower Rio Bravo basin) and southern (Rio Lacantun
basin). Number of steps: 9; number of variables in the model: 9;
grouping: 2.

Character Wilks Partial F-remove p-level

 0.036161 0.831570 4.86106 0.037299
9-10 0.057616 0.521912 21.98478 0.000092
Head width 0.043603 0.689646 10.80049 0.003113
8-10 0.032274 0.931708 1.75914 0.197216
Internostril width 0.032742 0.918411 2.13209 0.157211
Interorbital width 0.033956 0.885572 3.10113 0.090976
Anal rays 0.032556 0.923661 1.98355 0.171840
2-5 0.032599 0.922422 2.01848 0.168258
7-9 0.031405 0.957506 1.06513 0.312338

Character Toler. 1-Toler.

 0.522359 0.477641
9-10 0.352112 0.647888
Head width 0.363757 0.636243
8-10 0.552287 0.447713
Internostril width 0.709333 0.290667
Interorbital width 0.764098 0.235902
Anal rays 0.882346 0.117654
2-5 0.458300 0.541700
7-9 0.706935 0.293065

Overall Wilks' Lambda: 0.03007. [F.sub.approx, (9, 24)] = 86.014,
p< 0 .0001.

Standardized coefficients for canonical variables

Variable Root 1

9-10 0.57657
8-10 -1.18316
Head width 0.93789
6-7 -0.35705
Internostril width -0.34437
Interorbital width 0.39294
Anal rays 0.29866
2-5 0.41776
Distance 7-9 -0.24895
Eigenval 32.25540
Cum. Prop. 1.00000
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Author:Ruiz-Campos, Gorgonio; Lozano-Vilano, Maria de Lourdes; Garcia-Ramirez, Maria Elena
Publication:Bulletin (Southern California Academy of Sciences)
Article Type:Report
Geographic Code:1MEX
Date:Apr 1, 2009
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