Abstract
The phenomenon of a massive vertebral deformity was recorded in the radiating Labeobarbus assemblage from the middle reaches of the Genale River (south-eastern Ethiopia, East Africa). Within this sympatric assemblage, five trophic morphs—generalized, lipped, piscivorous and two scraping feeders—were reported between 1993 and 2019. In 2009, a new morph with prevalence of ~10% was discovered. The new morph, termed ‘short’, had an abnormally shortened vertebral column and a significantly deeper body. This type of deformity is common in farmed Atlantic salmon and other artificially reared fish, but is rare in nature. In the Genale Labeobarbus assemblage, the deformity was present exclusively within the generalized and lipped morphs. The short morph had between seven and 36 deformed (compressed and/or fused) vertebrae. Their body depth was positively correlated with number of deformed vertebrae. In another collection in 2019, the short morph was still present at a frequency of 11%. Various environmental and genetic factors could contribute to the development of this deformity in the Genale Labeobarbus, but based on the available data, it is impossible to confidently identify the key factor(s). Whether the result of genetics, the environment, or both, this deep-bodied phenotype is assumed to be an anti-predator adaptation, as there is evidence of its selective advantage in the generalized morph. The Genale monstrosity is the first reported case of a massive deformity of the vertebral column in a natural population of African fishes.
Highlights
The emergence and establishment of morphological novelties in a population is central to morphological evolution, this process remains poorly understood
In the field, the short morph was identified based on altered body proportions: these individuals had relatively deeper and shorter bodies compared to the normal individuals (Fig 1)
Taking into account all of the above, we suggest that genetic factors are the most parsimonious explanation of the emergence and 10 year persistence of the SH morph in the middle Genale Labeobarbus assemblage
Summary
The emergence and establishment of morphological novelties in a population is central to morphological evolution, this process remains poorly understood. As initially highlighted by Darwin [1], there is no clear discrimination between morphological abnormalities (monstrosities) and regular variation. Morphological abnormalities that are caused by genetic factors—and may be shaped by natural selection—are interesting to evolutionary biologists as potential novelties. Ray-finned fishes form one of the major vertebrate groups, comprised of more than 35,000 species with extreme variations in morphology and body plans [2]. The ocean sunfishes (Molidae, Tetraodontiformes) exhibit one of the most impressive evolutionary transformations of the axial skeleton among ray-finned fishes [3, 4]. It has been suggested that the enigmatic loss of caudal fin and caudal part of the vertebral column is related to the existence of a number of malformed adult tetraodontiforms without caudal fins [5, 6]
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