A natural hybrid between two tropical fishes: Semaprochilodus insignis x Semaprochilodus taeniurus (Teleostei, Characoidei, Prochilodontidae)

Abstract

lnterespecific hybridization has become a rather widespread phenomenon among fish groups (Hubbs, 1955). The incidence of natural hybridization among freshwater fishes seems to be a function of environment, decreasing gradually from north temperate to tropical zones. It also appears to be in­ versely correlated with diversity and therefore, in the tropical highly diverse fish fauna of the Amazon region, only a low frequency of natural hybrids may be expected. However, it is probable that in this region some hybrids have already been studied and identified as distinct species while others are still to be discovered. This paper concerns ecological and morphological aspects of three Amazonian forms of the genus Semaprochilodus Fowler (Characoid, Prochilodontidae) and provides evidences on this respect. Semaprochi/odus insignis (jaraqui escama grossa) and S. taeniurus (jara­ qui escama fina) are two median size, sympatric species of high abundance in Central Amazonia. During their extensive and complex migrations (Ribeiro, 1983) a third cryptic species (jaraqui-acu) was found in a very low frequency (1 :880 individuaIs) mixed to individuaIs of both species but never in isolated schoo!s. ln the beginning of the annual floods schools of mature males and females of both species of jaraquis were captured in Rio Negro migrating downstream to spawn in the turbid waters of Rio Amazon. Mature males of jaraqui-acu were also found in these schools. Females of jaraqui-acu were never captured in the spawning area nor were they found in state of sexual maturation throughout the year. ln this pape r 54 individuaIs of S. insignis, 48 of S. taeniurus and 42 of jaraqui-acu were used for the morphological study of these three forms. All individuaIs were collected with non-selective beach seines from schools mi­ grating in Rio Negro. All measurements and counts were based in the last review of the genus Semaprochilodus (Mago Leccia, 1972). The amplitudes of variation and means of body proportions and counts are presented and they were analized by the method of overlap of means (Royce, 1957). The diffe­ rences between two means were tested in terms of common standard deviation of samples and the value of three standard deviations was taken as indicative of interspecific differences between two samples. This value corresponded to 13% of overlap between the samples and the percentage of characters distinct at this leveI constituted the interspecific differenciation degree. The following comparisons can be obtained from Table 1: The jaraqui-acu is neither so elongated as S. taeniurus nor so deep as S. insignis. ln this new form, its back is but little more elevated than in this first species but not sharply so as in the former. Post dorsal region not so straight so that upper profile is sinous and forms an obtuse angle at origin of dorsal which is intermediate as compared to both species. Lower profile more or .Iess evenly convexo Therefore, body shape is neither smooth and linear as in S. taeniurus nor rhomboid as in S. insignis. Dorsal, anal, pectoral and pelvic fins are inserted in an intermediate position in jaraqui-acu as com­ pared to S. insignis (behind) and S. taeniurus (in front). Body coloration is similar to that of S. insignis especially in the reddish pectoral and pelvic fins. The number of black bands in each lobe of caudal fin in jaraqui-acu (3-6) is intermediate between that of S. taeniurus (3-5) and S. insignis (3-7). All