Salmonids have a relatively primitive appearance among teleost fish, with the pelvic fins being placed far back, and an adipose fin towards the rear of the back. They have slender bodies with rounded scales and forked tail fins, and their mouths contain a single row of sharp teeth.[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).[1]
The Salmonidae first appear in the fossil record in the Middle Eocene with Eosalmo driftwoodensis, which was first described from fossils found at Driftwood Creek, central British Columbia. This genus shares traits found in all three subfamily lineages. Hence, E. driftwoodensis is an archaic salmonid, representing an important stage in salmonid evolution.[3]
A gap appears in the salmonine fossil record after E. driftwoodensis until about 7 million years ago (mya), in the Late Miocene, when trout-like fossils appear in Idaho, in the Clarkia Lake beds.[4] Several of these species appear to be Oncorhynchus — the current genus for Pacific salmon and Pacific trout. The presence of these species so far inland established that Oncorhynchus was not only present in the Pacific drainages before the beginning of the Pliocene (~5–6 mya), but also that rainbow and cutthroat trout, and Pacific salmon lineages had diverged before the beginning of the Pliocene. Consequently, the split between Oncorhynchus and Salmo (Atlantic salmon and European trout) must have occurred well before the Pliocene. Suggestions have gone back as far as the Early Miocene (about 20 mya).[3][5]
Based on the most current evidence, salmonids diverged from the rest of teleost fish no later than 88 million years ago, during the late Cretaceous. This divergence was marked by a whole-genome duplication event in the ancestral salmonid, where the diploid ancestor became tetraploid.[6][7] 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.[7]
Extant salmonids all show evidence of partial tetraploidy, as studies show the genome has undergone selection to regain a diploid state. Work done in the rainbow 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 apparent miRNA 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.[8]
The first fossil species representing a true salmonid fish (E. driftwoodensis) does not appear until the middle Eocene.[9] 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.[8] New advances in calibrated 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.[7]
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.[7]
The only extant family within Salmoniformes, Salmonidae, is divided into three subfamilies and around 10 genera containing about 220 species. The concepts of the number of species recognised vary among researchers and authorities; the numbers presented below represent the higher estimates of diversity:[1]
The following table shows results of hybrid crossbreeding combination in Salmonidae.[12]
Crossbreeding
male
Salvelinus
Oncorhynchus
Salmo
leucomaenis (white-spotted char)
fontinalis (Brook trout)
mykiss (Rainbow trout)
masou masou (masu salmon)
masou ishikawae (Amago Salmon)
gorbuscha (pink salmon)
nerka (Sockeye salmon)
keta (chum salmon)
kisutsh (coho salmon)
tshawytscha (chinook salmon)
trutta (Brown trout)
salar (Atlantic Salmon)
female
(Salvelinus)
leucomaenis (white-spotted char)
-
O
X
O
O
X
X
O
fontinalis (Brook trout)
O
-
X
O
O
X
X
O
X
X
(Oncorhynchus)
mykiss (Rainbow trout)
O
O
-
O
O
O
X
X
X
X
X
masou masou (masu salmon)
O
X
X
-
O
X
X
O
O
X
masou ishikawae (Amago Salmon)
O
O
X
O
-
X
O
gorbuscha (pink salmon)
X
-
O
O
O
nerka (Sockeye salmon)
X
X
X
X
X
O
-
O
O
O
X
keta (chum salmon)
X
X
X
X
O
O
-
O
X
X
kisutsh (coho salmon)
X
X
O
O
X
-
O
X
X
tshawytscha (chinook salmon)
O
O
O
X
O
-
Salmo
trutta (Brown trout)
O
O
X
O
O
X
X
-
O
salar (Atlantic Salmon)
O
X
X
X
O
-
note :- : The identical kind, O : (survivability), X : (Fatality)
References
^ abcFroese, Rainer, and Daniel Pauly, eds. (2008). "Salmonidae" in FishBase. December 2008 version.
^ 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.
^ abcMcPhail, J.D.; Strouder, D.J. (1997). "Pacific Salmon and Their Ecosystems: Status and Future Options". The Origin and Speciation of Oncorhynchus. New York, New York: Chapman & Hall.
Wikimedia Commons has media related to Salmonidae.
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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