Scale patterns, maturational status and otolith microchemistry (strontium to calcium ratios) were analysed in sympatric anadromous and non-anadromous rainbow trout Oncorhynchus mykiss in the Santa Cruz River (Patagonia, Argentina) to investigate the life-history differences of anadromous and non-anadromous lifestyles and the association between maternal origin and progeny life history. The analyses revealed that both forms can give rise to one another, indicating a single population with alternative phenotypes. Anadromous fish smolted at ages 2 and 3 years, matured after 1 to 2 years in the ocean, and survived up to 11 years, spawning up to eight times. Non-anadromous fish survived up to 6 years, spawning up to three times. The extended reproductive life span associated with anadromy in this river suggests that increased energetic and physiological demands associated with ocean migration may not necessarily result in reduced postspawning survival, as has been suggested for salmonids in general. Alternatively, reduction in parity may be regarded as the evolutionary outcome of reproductive traits resulting from the adoption of anadromy (i.e. augmented reproductive investment) coupled with longrange migrations to and from the ocean. The life-history patterns of Santa Cruz River rainbow trout provide a natural experiment for investigating the evolutionary transition and maintenance of anadromy and non-anadromy within salmonid populations.

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.