Revalidation of the Argentinian pouched lamprey Geotria macrostoma (Burmeister, 1868) with molecular and morphological evidence

Abstract

Background: The Argentinian pouched lamprey, classified as Petromyzon macrostomus Burmeister, 1868 was first described in 1867 in De La Plata River, in Buenos Aires, Argentina, and subsequently recorded in several rivers from Patagonia. Since its original description, the validity of P. macrostomus was questioned by several ichthyologists and 36 years after its original discovery it was considered a junior synonym of Geotria australis Gray, 1851. For a long time, the taxonomic status of G. australis has been uncertain, largely due to the misinterpretations of the morphological alterations that occur during sexual maturation, including the arrangement of teeth, size and position of fins and cloaca, and the development of an exceptionally large gular pouch in males. In this study, the taxonomic status of Geotria from across the "species" range was evaluated using both molecular analysis and examination of morphological characteristics.

Methodology/principal findings: Phylogenetic and species delimitation analyses based on mitochondrial DNA sequences of Cytochrome b (Cyt b) and Cytochrome C Oxidase Subunit 1 (COI) genes, along with morphological analysis of diagnostic characters reported in the original descriptions of the species were used to assess genetic and morphological variation within Geotria and to determine the specific status of the Argentinian lamprey. These analyses revealed that Geotria from Argentina constitutes a well differentiated lineage from Chilean and Australasian populations. The position of the cloaca and the distance between the second dorsal and caudal fins in sub-adult individuals, and at previous life stages, can be used to distinguish between the two species. In addition, the genetic distance between G. macrostoma and G. australis for the COI and Cyt b mitochondrial genes is higher than both intra- and inter-specific distances reported for other Petromyzontiformes.

Conclusions/significance: Our results indicate that the Argentinian pouched lamprey, found along a broad latitudinal gradient on the south-west Atlantic coast of South America, should be named as Geotria macrostoma (Burmeister, 1868) and not as G. australis Gray 1851, returning to its earliest valid designation in Argentina. Geotria macrostoma can now be considered as the single lamprey species inhabiting Argentinian Patagonia, with distinct local adaptations and evolutionary potential. It is essential that this distinctiveness is recognized in order to guide future conservation and management actions against imminent threats posed by human actions in the major basins of Patagonia.

Fig 1. Map of southern South America showing sampling locations and published records of G. australis in Argentina, Chile, and Uruguay.

Main river basins and the Patagonian Ice Sheets, with extent of contemporary glaciers shown in blue. The black star indicates the type locality of Petromyzon macrostomus Burmeister from de la Plata River, orange circles indicate bibliographic records of G. australis, and black circles indicate our collection sites. References: 1, de la Plata River (Argentina, [–24, 26]); 2, Isla de Flores (Uruguay, [23, 26]); 3, Mar del Plata (Argentina, [25, 38]); 4, Nahuel Huapi Lake (Argentina, [25]); 5, Nuevo Lake (Argentina, [25, 38]); 6, 32, Argentino Lake (Argentina, [25, 26, 29]); 7, Gallegos River (Argentina, [30]); 8, Rubens River (Argentina, [30]); 9, Turbio River (Argentina, [30, 41]); 10, Santa Cruz River (Argentina, [33]); 11, Malvinas (Falkland) Islands (Argentina, [34]); 12, Paraná River (Argentina, [38]); 13, Santiago (Chile, [48]); 14, Canal del Molino (Chile, [48]); 15, Bahía Concepción (Chile, [48]); 16, Angol (Chile, [48]); 17, Puren River (Chile, [48]); 18, Valdivia River (Chile, [48]); 19, Osorno (Chile, [48]); 20, Lake Llanquihue (Chile, [48]); 21, Maullín River (Chile, [48]); 22, San Juan River (Chile, [37]); 23 Plottier, Negro River Argentina, [39]); 24, Choele Choel Island (Argentina, [39]); 25, Trelew City, Chubut River (Argentina, [39]); 26, Grande River (Argentina, [41]); 27, Fuego River (Argentina, [38]); 28, Negro River (Argentina, this study); 29, Chubut River (Argentina, this study); 30, Chico River (Argentina, this study); 31, Piedra Buena City (Argentina, this study); 33, La Leona River (Argentina, [40]).

Fig 2. Argentinian lamprey from the Santa Cruz River examined in this study.

A) Immature upstream migrant. B) Maturing sub-adult. Both individuals were tagged and released back into the river after examination. Scale bar = 5 cm.

Fig 3. Position of the second dorsal and caudal fins in sub-adults of Geotria.

A) Argentinian lamprey (Santa Cruz River). B) Geotria australis (Waikawa River). The red arrow indicates the origin of the caudal fin. Scale bar = 2 cm.

Fig 4. Position of the cloaca in sub-adults of Geotria.

A) Argentinian lamprey (Santa Cruz River). B) Geotria australis (Waikawa River). The red arrow indicates the position of the cloaca. Scale bar = 1 cm. Adult lampreys caught in the Santa Cruz (Riva Rossi et al. unpublished data) and Negro Rivers (by local fishermen) showed dark brown body and fins and a well-developed gular pouch was observed in two mature males. In these fish the second dorsal and caudal fins are contiguous and the cloaca is positioned posterior to the origin of the second dorsal fin (Fig 5).

Fig 5. Adult lamprey from the Santa Cruz River.

A) Mature male individual with its tail missing. B) Detail of the position of the second dorsal and caudal fins and the cloaca. The red arrow shows the origin of caudal fin and the position of the cloaca.

Fig 6. Strict consensus tree obtained from Maximum Parsimony analysis of the two mitochondrial markers.

Strict consensus of 100 most parsimonious trees of 4130 steps. Branch lengths are proportional to parsimony transformations steps. Gaps were considered as fifth state. Name of samples for Argentinian Geotria are indicated by the institutional acronym and location (Province) of each sample. Terminal taxa where COI and Cyt b sequences were concatenated are indicated in bold, terminal taxa represented only by the COI fragment are shown in plain font and taxa represented only by the Cyt b fragment are shown in grey (see S1 Table). Numbers below the nodes indicate parsimony jackknife support. Values in parentheses show the support obtained when samples represented only by the Cyt b were excluded from the analysis (grey samples).

Fig 7

Median joining haplotype network of Geotria obtained with the COI (A) and Cyt b (B) data sets. The circles represent haplotypes, and the circle sizes are proportional to the haplotype frequencies. Mutational steps between haplotypes are indicated along the branches. Circles are colored according to population membership.

Fig 8. Map of main ocean currents showing ice coverage (white areas) during the Pleistocene glaciations (2.5 mya—10,000 ya).