Salmonids have a relatively primitive appearance amongteleost fish, with thepelvic fins being placed far back, and anadipose fin towards the rear of the back. They have slender bodies with roundedscales and forkedtail fins, and theirmouths contain a single row of sharpteeth.[2] Although the smallest salmonid species is just 13 cm (5.1 in) long for adults, most salmonids are much larger, with the largest reaching 2 m (6 ft 7 in).[3]
The order Salmoniformes first appeared during theSantonian andCampanian stages of theLate Cretaceous,[4][5] and is most closely related topike andmudminnows in the orderEsociformes, to the extent that some authors have grouped the Esociformes within the Salmoniformes.[6] Although it is assumed that salmon and pike diverged from one another during the Cretaceous, no definitive salmonids appear before theEocene.[7] The Salmonidae first appear in thefossil record in theEarly Eocene[8] withEosalmo driftwoodensis, a stem-salmonine, which was first described from fossils found atDriftwood Creek, centralBritish Columbia,[7] and has been recovered from most sites in theEocene Okanagan Highlands.[9][10][11] This genus shares traits found in all three subfamily lineages. Hence,E. driftwoodensis is an archaic salmonid, representing an important stage in salmonid evolution.[1] Fossil scales ofcoregonines are known from the Late Eocene or Early Oligocene of California.[12]
A gap appears in the salmonine fossil record afterE. driftwoodensis until about 7 million years ago (mya), in theLate Miocene, when trout-like fossils appear inIdaho, in theClarkia Lake beds.[13] Several of these species appear to beOncorhynchus — the current genus for Pacific salmon and Pacific trout. The presence of these species so far inland established thatOncorhynchus was not only present in the Pacific drainages before the beginning of thePliocene (~5–6 mya), but also thatrainbow andcutthroat trout, and Pacific salmon lineages had diverged before the beginning of the Pliocene. Consequently, the split betweenOncorhynchus andSalmo (Atlantic salmon and European trout) must have occurred well before the Pliocene. Suggestions have gone back as far as theEarly Miocene (about 20 mya).[1][14]
Based on the most current evidence, salmonids diverged from the rest ofteleost fish no later than 88 million years ago, during the lateCretaceous. This divergence was marked by awhole-genome duplication event in the ancestral salmonid, where the diploid ancestor becametetraploid.[15][16] This duplication is the fourth of its kind to happen in the evolutionary lineage of the salmonids, with two having occurred commonly to all bony vertebrates, and another specifically in the teleost fishes.[16]
Extant salmonids all show evidence of partial tetraploidy, as studies show the genome has undergone selection to regain a diploid state. Work done in therainbow trout (Onchorhynchus mykiss) has shown that the genome is still partially-tetraploid. Around half of the duplicated protein-coding genes have been deleted, but all apparentmiRNA sequences still show full duplication, with potential to influence regulation of the rainbow trout's genome. This pattern of partial tetraploidy is thought to be reflected in the rest of extant salmonids.[17]
The first fossil species representing a true salmonid fish (E. driftwoodensis) does not appear until the middle Eocene.[18] This fossil already displays traits associated with extant salmonids, but as the genome of E.driftwoodensis cannot be sequenced, it cannot be confirmed if polyploidy was present in this animal at this point in time. This fossil is also significantly younger than the proposed salmonid divergence from the rest of the teleost fishes, and is the earliest confirmed salmonid currently known. This means that the salmonids have a ghost lineage of approximately 33 million years.
Given a lack of earlier transition fossils, and the inability to extract genomic data from specimens other than extant species, the dating of the whole-genome duplication event in salmonids was historically a very broad categorization of times, ranging from 25 to 100 million years in age.[17] New advances incalibrated relaxed molecular clock analyses have allowed for a closer examination of the salmonid genome, and has allowed for a more precise dating of the whole-genome duplication of the group, that places the latest possible date for the event at 88 million years ago.[16]
This more precise dating and examination of the salmonid whole-genome duplication event has allowed more speculation on the radiation of species within the group. Historically, the whole-genome duplication event was thought to be the reason for the variation within Salmonidae. Current evidence done with molecular clock analyses revealed that much of the speciation of the group occurred during periods of intense climate change associated with the last ice ages, with especially high speciation rates being observed in salmonids that developed an anadromous lifestyle.[16]
The only extant family within Salmoniformes, Salmonidae, is divided into threesubfamilies and around 10genera containing about 220species. The concepts of the number of species recognised vary among researchers and authorities; the numbers presented below represent the higher estimates of diversity:[3]
^abcdMcPhail, J.D.; Strouder, D.J. (1997). "Pacific Salmon and Their Ecosystems: Status and Future Options".The Origin and Speciation ofOncorhynchus. New York, New York:Chapman & Hall.
^abMcDowell, Robert M. (1998). Paxton, J.R.; Eschmeyer, W.N. (eds.).Encyclopedia of Fishes. San Diego:Academic Press. pp. 114–116.ISBN978-0-12-547665-2.
Behnke, Robert J.Trout and Salmon of North America, Illustrated by Joseph R. Tomelleri. 1st Chanticleer Press ed. New York: The Free Press, 2002.ISBN0-7432-2220-2
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