Palaemon mercedae (Pereira, 1986)

Carvalho, Fabrício L., Magalhães, Célio & Mantelatto, Fernando L., 2019, A molecular and morphological approach on the taxonomic status of the Brazilian species of Palaemon (Decapoda, Palaemonidae), Zoologica Scripta 49 (1), pp. 1-16 : 9-10

4.1.1 | Palaemon mercedae

Among the species of the “ Palaemon ” lineage, which present the projection of the anterolateral margin of the first segment of antennular peduncle not reaching the middle of the second segment, P. mercedae can be easily distinguished from the remaining Palaemon s.s. by bearing four–nine pair of plumose setae and two pairs of short stout setae on the distal margin of the telson, with inner pair of stout setae not overreaching the tip of the telson, and three teeth on the incisor of the left mandible.

Although efforts have been made to obtain sequences of individuals from all locations with morphological samples, the amplification of the regions of interest could be performed only for one sample of P. mercedae obtained during fieldworks. Due to the absence of more sequences of the species, we could not analyse the intraspecific genetic variability. However, the sequence obtained indicates a clear distinction in relation to the other Palaemon species from Brazil. In addition, there is no evidence for the proposition of any species closely related to P. mercedae , since no consistent morphological differences were found within the characters analysed between the Brazilian specimens and those from the type locality (Atabapo River, Chamuxina, Amazonas, Venezuela). Further collecting efforts to obtain specimens suitable for molecular analyses along the species distribution are necessary to evaluate the existence of cryptic species and to re‐evaluate small variations in the external morphology.

Palaemon mercedae may present a distribution pattern similar to P. ivonicus + P. yuna ( Carvalho et al., 2014 ), since it occurs both in the Amazon basin and in the Orinoco basin. However, the divergence time between P. mercedae and Palaemon s.s. (estimated at approximately 21 My) is older than the estimates of divergence among the other Amazonian species. Although this value has a wide degree of uncertainty, we suggest that this Amazonian species diverged from Palaemon s.s. in an event different from that associated with the other species of this region.

Furthermore, the pattern of larval development between P. mercedae and the other species of the “ Palaemon ” lineage is significantly different. Whereas the land‐locked P. mercedae exhibits an extremely abbreviated postembryonic development (Magalhães, 1988), the marine or estuarine P. argentinus , P. floridanus , P. northropi and P. paivai have extended larval development (Abrunhosa, Costa, Moura, & Pires,2009; Knowlton & Vargo, 2004; Menu‐Marque, 1973; Moura, Abrunhosa, & Pereira Da Costa,1990, respectively). Although the evolution of larval development patterns in palaemonid shrimps does not reflect phylogenetic traits ( Botello & Alvarez, 2013; Magalhães & Walker, 1988; Murphy & Austin, 2005), the high degree of “freshwaterization” shown by P. mercedae may indicate an early invasion of the continental aquatic habitats and consequently divergence from the other species of Palaemon s.s.

As suggested by Carvalho et al. (2017), P. mercedae should be allocated in a new genus, so far monospecific. The main diagnostic characteristics for the distinction between P. mercedae and the Brazilian species of Palaemon s.s will also be employed in distinguishing the new genus. Thus, although P. mercedae is still treated here as a species of Palaemon , future comparative studies with the species should consider the phylogenetic status outside Palaemon s.s.