Atherinopsidae Fitzinger 1873

Betancourt-Resendes, Isai, Perez-Rodríguez, Rodolfo, Barriga-Sosa, Irene De Los Angeles, Piller, Kyle R. & Domínguez-Domínguez, Omar, 2020, Phylogeographic patterns and species delimitation in the endangered silverside “ humboldtianum ” clade (Pisces: Atherinopsidae) in central Mexico: understanding their evolutionary history, Organisms Diversity & Evolution (New York, N. Y.) 20 (2), pp. 313-330 : 326

Family Atherinopsidae Fitzinger 1873

Subfamily Menidiinae Schultz 1948

Tribe Menidiini Schultz 1948

Genera Chirostoma Swaison 1839

Chirostoma sphyraena Boulenger 1900

Chirostoma estor Jordan 1879 (includes Chirostoma grandocule Steindachner 1894 and Chirostoma patzcuaro Meek 1902 ) Chirostoma copandaro (species level of C. estor copandaro

De Buen 1945)

Chirostoma chapalae Jordan & Snyder 1899 (includes Chirostoma consocium Jordan & Hubbs 1919 , Chirostoma lucius Boulenger 1900 and Chirostoma promelas Jordan & Snyder 1899 )

Chirostoma humboldtianum Valenciennes 1835 .

Lake Chapala contains the highest species richness of Chirostoma ( Barbour 1973b) , including five species of the “ humboldtianum ” clade ( Barbour 1973b; Echelle and Echelle 1984). However, the genetic results presented herein do not support recognition of the five species. Instead, the data support the occurrence of two well-differentiated groups. The first corresponds to specimens identified as C. sphyraena , which do not share haplotypes with any other species or genetic groups found in Lake Chapala. Furthermore, C. sphyraena is the most genetically and morphologically differentiated species, with no overlap in diagnostic characters with respect to other species within the “ humboldtianum ” clade ( Barbour 1973b; Barbour and Chernoff 1984). The genetic data presented herein and previous morphological analyses support the validity of this species. The second genetic group found in Chapala includes four species ( C. chapalae , C. consocium , C. promelas , and C. lucius ), as well as individuals of C. humboldtianum from the Lerma (Tepuxtepec and Juanacatlán dam) and San Pedro Lagunillas dam. The genetic results presented herein support the existence of one species within this group, with C. chapalae the valid name in accordance with the principle of priority ( Table 7).

These results reject the validity of the nine morphological species, confirming that, rather than distinct taxa, several morphs correspond to intra-specific polymorphisms, a possibility that was previously suggested as plausible ( Echelle and Echelle 1984; Barbour and Chernoff 1984). One of the explanations proposed for the high morphological polymorphism within Chirostoma is hybridization ( Alaye 1993, 1996). This process may predispose populations to the adaptive radiation ( Seehausen and Wagner 2014) that occurs after colonization events. However, the low colonization capacity of silversides, due to the strong association of those species with lacustrine ecosystems, could be a barrier for dispersal following cladogenetic events ( Betancourt-Resendes et al. 2018). A lack of resolution in the molecular markers used is less likely, since a previous study demonstrated the utility of these markers for elucidating recent speciation events in C. attenuatum and C. zirahuen ( Betancourt-Resendes et al. 2018) . The discrepancies between molecular markers and morphological characters in the “humboldtianum ” clade could therefore be related to high phenotypic plasticity, mainly related to habitat pressures, as has been demonstrated for C. humboldtianum ( Alarcón-Duran et al. 2017) and to rapid adaptive divergences, as proposed for the related C. grandocule from Lake Pátzcuaro ( Barriga-Sosa et al. 2004) and the inland silverside Menidia beryllina population in adjacent environmental regimes ( Fluker et al. 2011). This has strong taxonomic implications, since it decreases the number of species within the genus Chirostoma (see Table 7).