Abstract
Palaemon carteri (Gordon, 1935) and Palaemon ivonicus (Holthuis, 1950) are morphologically similar species of South American freshwater shrimps. Past studies have questioned the taxonomic status of both species, which are supposed to have partially sympatric geographic distributions in the Amazon basin. We analyzed a 550 bp fragment of the mitochondrial 16S rRNA gene from these Amazonian Palaemon species as well as from 11 palaemonids as the outgroup. Additionally, we checked diagnostic characters of the genus and family as well as other morphological characters that have been little explored before. Palaemon carteri and Palaemon ivonicus are allocated in two sister lineages, with wide genetic divergence and little morphological differentiation. The divergence time between these lineages was estimated as approximately 10 million years ago. Both molecular and morphological data support the taxonomic validity of both Palaemon carteri and Palaemon ivonicus, refuting the hypothesis of synonymy. In addition, a new species, Palaemon yuna sp. n., closely related to Palaemon ivonicus, is described. Our findings indicate that these species can be differentiated using the projection of the anterolateral margin and anterolateral spine of the first antennular segment, shape of the rostrum, and relative size of the appendix masculina.
Highlights
The genus Palaemon Weber, 1795 comprises 84 marine, estuarine and freshwater species in tropical and subtropical regions, including Palaemonetes Heller, 1869, which was recently considered to be a junior synonym of Palaemon by De Grave and Ashelby (2013), and the new species described here
Holthuis (1952) stated that Palaemon ivonicus has the lower margin of the rostrum with two or three teeth, the rostrum rather high and straight, and the branchiostegal tooth removed a considerable distance from the anterior margin of the carapace, with its tip failing to reach beyond this margin
P. carteri has the lower margin of the rostrum with four to seven teeth; the rostrum slender, often curved upwards; and the branchiostegal tooth removed a short distance from the anterior margin of the carapace
Summary
The genus Palaemon Weber, 1795 comprises 84 marine, estuarine and freshwater species in tropical and subtropical regions, including Palaemonetes Heller, 1869, which was recently considered to be a junior synonym of Palaemon by De Grave and Ashelby (2013), and the new species described here. Palaemon carteri and P. ivonicus are freshwater species with abbreviated larval development (see Magalhães 1986 for larval description of P. ivonicus) and are supposed to occur sympatrically in the Amazon basin as well as being very similar morphologically. They have been distinguished primarily based on rostral characters and on the position of the branchiostegal tooth. Holthuis (1952) stated that Palaemon ivonicus has the lower margin of the rostrum with two or three teeth, the rostrum rather high and straight, and the branchiostegal tooth removed a considerable distance from the anterior margin of the carapace, with its tip failing to reach beyond this margin. The high interspecific morphological similarity and intraspecific variability found in these species raised doubts as to whether these two nominal species represent distinct biological entities (Gomes-Corrêa 1977, Odinetz-Collart and Enriconi 1993, García-Dávila and Magalhães 2003, García-Dávila et al 2005)
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