249 Ichthyol. Explor. Freshwaters, Vol. 14, No. 3, pp. 249-272, 7 figs., 11 tabs., October 2003 © 2003 by Verlag Dr. Friedrich Pfeil, München, Germany – ISSN 0936-9902 Corydoras diphyes (Siluriformes: Callichthyidae) and Otocinclus mimulus (Siluriformes: Loricariidae), two new species of catfishes from Paraguay, a case of mimetic association Thomas E. Axenrot* and Sven O. Kullander** Corydoras diphyes and O. mimulus, new species, are found in association in tributaries to the Río Monday, a right bank tributary of the Río Paraná, and are conspicuously similar to each other in color pattern. Otocinclus mimulus has been confused with O. flexilis up till now. Otocinclus mimulus mimics O. diphyes. The mimesis is unusual because the two species occupy different microhabitats and it is hypothesized to operate with a primarily visual predator moving between the microhabitats, tentatively identified as the cichlid Crenicichla lepidota. The mimetic association expressed by C. diphyes and O. mimulus extends to other species pairs of the same genera in southern South America with similarly impressive agreement in coloration, but the operating mechanism is not known in these pairs. In a cladistic analysis of Otocinclus using morphological characters, O. mimulus is included in a clade consisting also of O. flexilis and O. affinis. Addition of mimetic similarity results in a single most parsimonious tree with O. mimulus, O. flexilis, O. affinis and O. xakriaba forming a monophyletic group. Introduction purpose of this paper is to describe the new taxa and to discuss their mimetic association. In 1998, one of us (SOK) sampled several locali- Otocinclus is a genus of small hypoptopoma- ties in the vicinity of Ciudad del Este and ob- tine loricariid catfishes living in groups or schools. served the frequent co-occurrence of a species of They are diurnal and generally found in smaller- the callichthyid catfish genus Corydoras La Ce- sized streams or along the margins of larger riv- pède and a species of the loricariid catfish genus ers, clinging to substrates using the mouth as a Otocinclus Cope. The species were of about the sucker, and feeding from algae or aufwuchs on same size and had about the same color pattern. roots, stones, macrophytes, and broad-leaved Both species turned out to be undescribed and grasses. Otocinclus species are small-sized; the the closer analysis of the putative mimicry yield- largest species reach about 45 mm SL (Schaefer, ed conclusions of a more general nature. The 1997). The genus is distributed east of the Andes, * University of Linköping, Department of Biology, SE-58183 Linköping, Sweden. Current address: Department of Systems Ecology, Stockholm University, SE-106 91 Stockholm, Sweden. E-mail: [email protected] ** Department of Vertebrate Zoology, Swedish Museum of Natural History, POB 50007, SE-104 05 Stockholm, Sweden. E-mail: [email protected]. Address for reprint requests. Ichthyol. Explor. Freshwaters, Vol. 14, No. 3 250 throughout the lowlands from Venezuela to north- information on two catfish species to test for the ern Argentina, but is generally absent from the probability of a real, coevolved mimetic resem- Amazon and Orinoco lowlands (Schaefer, 1997). blance. Schaefer (1997) revised the genus, providing de- scriptions, illustrations and diagnoses for all spe- cies, as well as a phylogenetic analysis. He re- Material and methods ported 13 species in total, and three species from Paraguay, viz., O. flexilis Cope, 1894, O. vestitus Measurements were taken with digital calipers Cope, 1894, and O. vittatus Regan, 1904. Britto & recording to 0.01 mm precision, rounded to the Moreira (2002) most recently added O. tapirape nearest 0.1 mm. Measurements were normally from central Brazil, and analysed it using a mod- taken on the left side of the fish. Landmarks were ification of Schaefer’s character matrix. chosen according to previous studies of Otocin- Material representing the new Otocinclus spe- clus (Schaefer, 1997) and Corydoras (Strauss, 1985; cies described below was identified by Schaefer Nijssen & Isbrücker, 1986; Reis, 1998b), and the (1997) as the widespread species O. flexilis, but terminology should be self-explanatory. All meas- comparison with O. flexilis from Uruguay and urements were taken point to point. Standard southern Brazil does not confirm that identifica- length (SL) is taken from the tip of the snout to tion. the base of the caudal fin. Caudal peduncle length The family Callichthyidae, or armored cat- is taken from immediately posterior to the base fishes, includes about 160 species in 8 genera of the last anal-fin ray to the middle of the cau- (Reis, 1998b). Callichthyids have a heavy body dal-fin base. armor, but lack suction devices. The callichthyid All meristic features were counted on the left genus Corydoras includes approximately 142 spe- side of the fish, except for canal-bearing plates in cies (Reis, 1998a), which makes it by far the most the median series, which were counted on both species-rich genus of Neotropical fishes. Corydo- left and right sides in Otocinclus. Premaxillary ras species are all small-sized, ranging from 25 to and mandibular teeth were counted on one side 120 mm maximum standard length. Species of (usually the left side) in Otocinclus. Corydoras are distributed east of the Andes to the In Otocinclus lateral plates were counted in Atlantic coast, from Trinidad to the La Plata drain- the lateral median series, from the first plate pos- age in northern Argentina (Strauss, 1985; Reis, terior to the pterotic to the base of the caudal fin. 1998b). They are generally found in smaller-sized Canal-bearing plates were counted separately. streams, along the margins of larger rivers, in The plates of the posterior part of the median marshes and ponds. Most of the species are bot- series are duplicated in, e.g., O. mimulus and O. fle- tom-dwellers, foraging in sand, gravel or detri- xilis, so that two plates occupy the position nor- tus. They are often seen in shoals (pers. obs.). mally occupied by one plate. Such duplicated Unlike most catfishes, which are nocturnal, Cory- plates were counted as one plate. In Corydoras doras species are active during day-time (Nijssen dorsal scutes were counted from the first scute & Isbrücker, 1980, 1986; Strauss, 1985). Six spe- posterior to the head to the base of the caudal fin. cies have been reported from Paraguay (Nijssen Ventral scutes were counted from the first scute & Isbrücker, 1979). posterior to the coracoid to the base of the caudal There are numerous records of non-arbitrary fin. Pre-adipose scutes were counted except for resemblance between different species of animals, the adipose spine (which is a modified mid-dor- as well as between plants and animals (e.g., Ei- sal scute according to Nijssen & Isbrücker, 1980). genmann & Allen, 1942; Wickler, 1968; Gilbert, Vertebrae were counted on radiographs and 1983; Pough, 1988; Thompson, 1994; Goldschmidt, are reported as abdominal, precaudal and total 1996). External resemblance between different fish vertebrae. Vertebrae of the Weberian complex are species is frequently observed in syntopic fish excluded from vertebral counts. Color characters assemblages in tropical freshwaters. A limited were observed on specimens preserved in etha- number of obvious cases of mimesis have been nol. Internal characters were observed from radi- described from South America. Sazima (1977, ographs and from specimens cleared and stained 1983) described cases of scale-eating fishes (Cha- (C&S) with alcian blue and alizarin red following racidae) and Sands (1994) described mimesis in the method described by Taylor & Van Dyke Corydoras. In this paper we examine the available (1985). Axenrot & Kullander: Corydoras and Otocinclus mimetic association 251 a b Fig. 1. Otocinclus mimulus, Paraguay; Estancia María Belén; a, holotype, MNHNP uncat (ex NRM 43480), 34.1 mm SL; showing a continuous midlateral stripe; b, paratype, NRM 43557, 33.7 mm SL; showing a row of blotches along middle of side. Statistics were calculated using SYSTAT ver- Otocinclus mimulus, new species sion 10 (SPSS, 2000). Phylogenetic analysis of (Fig. 1) Otocinclus was performed with PAUP* 4.0 (Swof- ford, 2002), using the character matrix of Schaefer Holotype. MNHNP uncat. (ex NRM 43480), (1997) with the addition of corresponding data 34.1 mm SL; Paraguay: Departamento Caaguazú: for O. mimulus. The branch-and-bound exhaus- río Paraná drainage, small stream (arroyo) at Es- tive search algorithm was used to find the most tancia María Belén, 8 km fom Colonel Patricio parsimonious trees. CI=consistency index, RI= Colman; 25°40'13"S 55°5'52"W; 24 Feb 1998; S. O. retention index, RC=rescaled consistency index. Kullander, M. Medina & W. Gill M. Material is deposited in: MCP, Laboratório de Paratypes. 49 specimens, 16.5-49.8 mm SL. All Ictiología, Museu de Ciências, Pontifícia Univer- from Paraguay, río Paraná drainage. Departamen- sidade Católica do Rio Grande do Sul, Porto to Caaguazú: MHNG 2531.072, 7, 16.5-32.7 mm Alegre; MHNG, Muséum d’Histoire Naturelle, SL; río Güyrauguá, small tributary 3 km E of D. Genève; MNHNP, Museo Nacional de Historia Juan M. Frutos; 7 Nov 1990; C. Dlouhy et al. – Natural del Paraguay, San Lorenzo; NRM, Swed- MHNG 2353.027, 1, 49.8 mm SL; río Güyrauguá, ish Museum of Natural History, Stockholm. Hol- small tributary 3 km E of D. Juan M. Frutos; 16 otypes and part of paratype material currently Apr 1985; F. Baud et al. – NRM 41800, 3, 32.7- catalogued at NRM will eventually be deposited 37.1 mm SL; a few km E of Juan M. Frutos, Ar- at MNHNP. royo Zanja Pé; 25°22'24"S 55°47'44"W; 12 Mar 1998; E. Åhlander et al. – NRM 41924, 9, 34.7- 42.7 mm SL; riverbed/swamp crossing at about 7 km on road Caaguazú-Repatriación; 25°30'17"S 56°0'8"W; 15 Mar 1998; E.