Monsters with a shortened vertebral column: A population phenomenon in radiating fish Labeobarbus (Cyprinidae)

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 heightened 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 height 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 high-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. "We have also what are called monstrosities; but they graduate into varieties. By a monstrosity I presume is meant some considerable deviation of structure in one part, either injurious to or not useful to the species, and not generally propagated. If it could be shown that monstrosities were even propagated for a succession of generations in a state of nature, modifications might be effected (with the aid of natural selection) more abruptly than I am inclined to believe they are." Darwin (1860, pp. 46, 426).