Pugheadedness in Fishes

Abstract This review summarizes the current state of knowledge of pugheadedness in fish. Records in the scientific literature range from detailed descriptions to brief notes and mere remarks. In total, at least 164 species from 60 families were identified to exhibit pugheadedness, with records published over a span of 465 years (1555 − 2020). The main osteological feature behind pugheadedness appears to be shortening or deformation of the parasphenoid bone, which leads to additional deformations of the ethmovomer- and frontal region. Several other deformations and abnormalities of other cranial bones, eyes, and tongue are occasionally observed, depending on the severity of the pugheadedness. Possible cases in elasmobranchs are also encountered, although the developmental causation may differ from actinopterygians, since their crania have a different organization. Natural cases of pugheadedness are found world-wide, covering a wide range of environments and lifestyles (freshwater-, brackish- and marine environments; benthic, neritic and pelagic species). Cases are found in all life-stages, from embryo to mature adults, suggesting that it does not necessarily lead to early-life mortality. There is some evidence for natural selection acting against pugheaded individuals, likely because of e.g. inappropriately functioning mouth parts, sense organs, and possibly brain deformation. High numbers of pugheads are mainly found in aquaculture, but moderate numbers have been found at some localities also in the wild. Abnormally high occurrence in the wild is commonly attributed to pollution, non-normal water chemistry parameters, or temperature. The causation, however, it typically speculated upon. Based on the reviewed literature, there is support for several causative factors, including genetic mutation and embryonic environmental conditions (toxic and non-toxic) affecting development. Pugheadedness, as the term has been used in the literature, is not a single well-defined pathology, but rather a suite of pathological conditions with similar phenotypic expression.