Proceedings of the Biological Society of Washington
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PROC. BIOL. SOC. WASH. 101(2), 1988, pp. 444-465 MINIATURIZATION IN SOUTH AMERICAN FRESHWATER FISHES; AN OVERVIEW AND DISCUSSION Stanley H. Weitzman and Richard P. Vari Abstract.— MiniatuTQ species within the South American freshwater fish fau- na are hsted and their phylogenetic and ecological distribution is analyzed. Criteria used in identifying miniature South American freshwater fishes are briefly discussed and eighty-five such fishes of five ordinal groups, Clupei- formes, Characiformes, Siluriformes, Cyprinodontiformes and Perciformes, are listed. Approximately eighty-eight percent of these miniatures are characiforms and siluriforms, all primary division ostariophysan fishes. Phylogenetic infor- mation on the relationships of these miniatures indicates that such species have arisen in the freshwaters of South America about sixteen times within the Characiformes, twelve in Siluriformes, four in the Cyprinodontiformes and once each in the Clupeiformes and Perciformes. Miniatures of most groups occur in lentic or slow-flowing waters, but the one pimelodid and some tricho- mycterid catfish miniatures are cryptobenthic in swift-flowing streams. Resumo.—Feixes miniatura de agua doce da America do Sul sao definidos e uma lista contendo 85 especies miniatura, pertencentes a cinco diferentes ordens de teleosteos, e fornecida. Peixes miniatura de agua doce sul americanos sao arbitrariamente definidos como aqueles que, ate onde se sabe, nao excedem 26 milimetros de comprimento padrao, sendo que a maioria atinge a matu- ridade sexual com menos de 20 milimetros de comprimento padrao. As especies incluidas entre estes limites tem em comum varios caracteres pedomorficos (redutivos) quando comparadas com taxons relacionados. Duas especies mi- niatura sao conhecidas dentre os Clupeiformes, 49 dentre os Characiformes, 26 dentre os Siluriformes, cinco dentre os Cyprinodontiformes e tres dentre os Perciformes. Aproximadamente 88% destes peixes miniatura sao Ostariophysi da divisao primaria. A informagao disponivel sobre as relacoes filogeneticas da maioria dos peixes miniatura e de qualidade questionavel; no entanto, nos provisoriamente esti- mamos que especies miniatura surgiram independentemente pelo menos 34 vezes na fauna de peixes de aqua doce da America do Sul; 1 6 dentre os Cha- raciformes, 12 dentre os Siluriformes, quatro dentre os Cyprinodontiformes e uma unica vez dentre os Clupeiformes e dentre os Perciformes. A informa^ao sobre a ecologia de peixes miniatura e tambem escassa mas, como seria de se experar no caso de peixes diminutos, a maioria ocorre em aguas lenticas ou de fluxo lento. No entanto, um pimelodideo miniatura e alguns tricomicterideos miniatura sao criptobenticos em riachos de fluxo ra- pido. VOLUME 101, NUMBER 2 445 The South American freshwater fish fau- species known from South American drain- na is one of the most diverse in the world, ages. with numerous species unknown to science being continually discovered, and a large Miniature Fishes in South percentage of the fauna still undescribed American Freshwaters (Bohlke et al. 1978). The early collectors and researchers engaged in studies of the The range in body sizes among fishes, ichthyofauna of that continent failed to ap- bony and cartilaginous, is remarkable, cov- preciate the degree of this diversity, partic- ering the spectrum from diminutive fishes ularly among smaller species, and as a con- under 26 mm standard length (SL), such as sequence often focused on the more obvious those discussed in this paper, to species such species of larger adult body size (Myers 1 947: as the whale shark which achieves lengths 13-14). Subsequent research by various au- of over 12 m and possibly up to 21.4 m thors has resulted in a better appreciation (Campagno 1981). Whereas the larger of the large number of species of smaller species of fishes are typical of open waters, body size, but it is only in the last few de- particularly in marine environments, small- cades that a number of species of very small er species, especially miniatures, are usu- size as adults have been described. The pace ally, although not exclusively, associated of description of such miniature species has with more complex substrates and ecosys- quickened in the last decade and recent col- tems. This correlation is reflected in the lecting efforts in various regions of South ichthyofaunas of South American streams America revealed the existence of many in- and rivers, and of Indo-Pacific coral reefs, teresting miniature species (Stewart 1985, which are the most diverse in the freshwater 1 986; M.Weitzmanl 985; Weitzman& Fink and marine environments respectively. 1985; Gery & Uj 1986; Weitzman 1986, Correlated with that high species diversity 1987; Fernandez & Weitzman 1987; Weitz- is the presence in those habitats of a large man & Vari 1 987). The comparative studies number of species of small fishes of ap- associated with the description of some of proximately 25 to 70 or 80 mm SL as adults, those species (Weitzman & Vari 1987) dis- together with many miniature species. In closed that the number of miniature species Table 1 we list 85 nominal species not known is much greater than commonly recognized. to exceed 26 mm SL, many of which are Indeed, during the twelve decades that have known to mature at under 20 mm SL. These passed since the Thayer Expedition about are considered to be miniature fishes in this 85 freshwater South American species (Ta- paper. Although the term "miniature" im- ble 1) that are miniature as adults, or not plies a very small body size, our primary known to exceed 25 to 26 mm in standard criterion for distinguishing miniature fishes length in nature, have been described from is not solely size related. Rather it involves the freshwaters of South America. the presence in such miniatures of a large In the present paper we discuss the phy- number of apparently paedomorphic mor- logenetic distribution of miniatures in the phological features including the degree of Neotropical freshwater ichthyofauna, and development of the laterosensory canal sys- provide an estimate of the number of min- tem of the head and body, reductions in the iaturization events within this fauna. The number of fin rays and body scales, and a available information on the ecology of diminution of the sculpturing on the surface South American miniature fishes is sum- bones of the head. 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