The taxonomic status ofRhabdolichops troscheli (Kaup, 1856), and speculations on gymnotiform evolution

Abstract

SynopsisAnatomical and ecological studies of the gymnotiform Rhabdolichops troscheli (Kaup 1856), one of the weakly electric fish of South America, indicate it to be unique among gymnotiforms in possessing certain characteristics found in characins. These include post-temporal fossae, well developed gillrakers, a planktivorous feeding mode, and diurnal feeding activity . We use these findings to speculate about the evolution of gymnotiform fishes, by assuming that these character states in this most primitive gymnotiform convey information about the group's evolutionary history . We suggest that the electric organs in this group of ostaryophysans have evolved primarily as a means of locating and capturing prey in river habitats of South America whose turbid waters result in low light penetration, ruling out visual sensing systems as an effective means of receiving information . Other aspects of morphology and behavior such as the absence or reduction of scales, and the nocturnal feeding activities characteristic of other gymnotiform, can be explained as consequences of food resource availability and predation pressures in these riverine environments .

Phylogenetic analysis of the South American electric fishes (order Gymnotiformes) and the evolution of their electrogenic system: a synthesis based on morphology, electrophysiology, and mitochondrial sequence data.

“…No intrafamily relationships were described by Gayet et al ( 1992). The four morphotypes defined by Mago-Leccia (1976) are preserved as monophyletic groups in Triques' s and Gayet et al' s hypotheses (number in circles), but noticeable discrepancies remain between these authors between and within the family level. The genus Parupygus, appearing in the family Rhamphichthyidae in B corresponds to Hypopomus artedi, a genus of the family Hypopomidae according to Mago-Leccia ( 1994).…”

“…The monophyly of the family has not been questioned by subsequent studies ( Mago-Leccia 1978;Mago-Leccia and Zaret 1978;Fink and Fink 198 1;Lundberg and Mago-Leccia 1986;Gayet et al 1992;Triques 1993 ). Until the last couple of years, the monophyly of Sternopygidae had been supported by only two morphological characters: villiform teeth in the mandible and the complete and well-developed bones of the infraorbital series ( Mago-Leccia 1976;Zaret 1978: Lundberg andMago-Leccia 1986). Fink and Fink ( 198 1) had pointed only to the enlarged infraorbitals as the unique synapomorphy (shared derived characters) between Sternop~y+s and the other genera in the family.…”

“…Previously, authors have suggested that the characiforms and gymnotiforms shared an immediate common ancestor (Rosen and Greenwood 1970;Mago-Leccia and Zaret 1978;Mago-Leccia 1994 The molecular phylogeny obtained in the present study was compared to current evolutionary hypotheses for the group based on morphology, and we also examined our results in conjunction with some neurophysiological and neuroanatomical data of the EES. By taking this approach we were able to generate the first phylogenetic hypothesis for the order Gymnotiformes based on cladistic analysis of molecular data.…”

“…Unfortunately, no consensus about gymnotiform phylogeny has been achieved. With exception of Hopkins and Heiligenberg ( 1978)) who have suggested an evolutionary history for the group based on the physiological aspects of their EODs, all other studies proposing taxonomic or phylogenetic relationships within the gymnotiforms are based on morphological characters (Ellis 19 13;MagoLeccia 1976MagoLeccia , 1978MagoLeccia , 1994Mago-Leccia and Zaret 1978;Fink and Fink 198 1;Lundberg and Mago-Leccia 1986;Gayet et al 1992;Triques 1993). These studies did not provide a solid phylogenetic hypothesis for the order for various reasons: for instance, those that have employed a cladistic approach reached conflicting conclusions (see fig.…”

“…2). Previous studies also lacked an explicit phylogenetic perspective (Ellis 19 13;Mago-Leccia 1978, 1994, dealt only with a subset of the order (Fink and Fink 198 1;Lundberg and Mago-Leccia 1986), or both (Hopkins and Heiligenberg 1978;Mago-Leccia and Zaret 1978).…”