Hyperostosis refers to a hyperossification, an overgrowth of bone that increases bone volume and or compactness in vertebrate skeletons from fish to mammals, including humans. It can arise pathologically, but in many cases it is a natural, species-specific phenomenon, particularly in tropical and subtropical marine acanthopterygian fishes, the most species-rich and evolutionarily advanced group of teleosts. These fishes are characterized by acellular, anosteocytic bone, and their hyperostotic bones typically develop through two processes: excessive periosteal osteogenesis followed by resorption, which produces a spongy internal structure around the pre-existing acellular bone, generally enclosed by a thin sheath of compact bone. Because hypertrophic bone shows high radiographic densities, such structures were sometimes erroneously interpreted as osteomas, bony tumors, in earlier literature.

The Vienna skeletons span a remarkable phylogenetic range. Among them are the spotted sicklefish Drepane punctata from Senegal and Java, the Atlantic spadefish Chaetodipterus faber from Suriname, the Peruvian banded croaker Paralonchurus peruanus from Callao, the crevalle jack Caranx hippos from Rio de Janeiro, the Senegal jack Caranx fischeri from Dakar, the silver scabbardfish Lepidopus caudatus from Tenerife, and five specimens of the John Dory Zeus faber from Europe and Trieste. All can be traced back to the rich scientific life of Franz Steindachner, curator of the ichthyological and herpetological collections and head of the k.k. naturhistorisches Hof-Museum, the forerunner of the Natural History Museum Vienna.

Among the most historically significant objects are three hyperostotic first anal-fin pterygiophores known as Ossa wormiana. The Danish scholar Ole Worm described and illustrated such a bone in 1655, confessing that he did not know from which animal or which part of the body it came. He wrote of a strange bone resembling a mouse in shape, with a thick spherical belly, two moveable pointed beaks, and a long narrow tail. Later generations named the bone Os wormianum in his honor, and seventeenth-century scholars speculated wildly about its origin; Olearius even suggested in 1674 that such bones might have grown in the ground. It took more than a century before the surgeon William Bell, in 1793, realized that the bone belonged to a fish and that the swellings were natural rather than a disease. The moveable pointed peaks that mystified Worm are actually the first and second anal-fin spines, moveably articulated to the base of the pterygiophore, whose distal part is greatly gall-like expanded and roundish.

The John Dory Zeus faber stands out as the exception that defines the second type of hyperostosis. While all other skeletons in the study show the typical spongy internal structure surrounded by a thin compact sheet, as demonstrated in the first anal-fin pterygiophore of Chaetodipterus faber and in a rib of Caranx hippos, the hyperostotic bones of Zeus faber are characterized by compact, laminar bone. MicroCT imaging of the supracleithrum, a dorsal buckler scale, and a dorsal-fin spine reveals this dense pseudolamellar tissue, which earlier histological work by Meunier and Béarez showed carries annual wide and narrow growth marks corresponding to spring, summer, autumn, and winter body growth. Although Zeus faber also possesses acellular bone, its hyperostotic structures are all externally visible, giving the living fish a distinctly spiny impression, and they appear to serve at least a protective function, unlike the spongy hyperostoses whose functional basis remains unknown.

In Zeus faber, hyperostosis affects the posterior half of the lacrimal, the frontal in the shape of a triangular narrow helmet, the ventral half of the preopercle, the supracleithrum, the spur of the cleithrum, and the massive wing-like enlarged bases of the third to eighth spines of the first dorsal fin. Large roundish bulky bony scutes, the so-called bucklers or buckler scales, extend along the bases of the second dorsal fin and the anal fin, with those of the dorsal row distinctly larger than those of the ventral row. A skeleton of nearly the same size as the largest hyperostotic specimen showed no signs of bone overgrowth at all, underscoring how variable the phenomenon can be within a species.

The crevalle jack Caranx hippos, at 83 centimeters the most impressive skeleton in the collection, shows hyperostosis confined to the postcranial skeleton. Affected are the ribs of vertebrae six to nine, the first pterygiophore of the first dorsal fin, the tenth and twelfth pterygiophores of the second dorsal fin, the first anal-fin pterygiophore with its pulvinate central part and concave anterior rim, the fifth to sixteenth neural spines, the fourth and fifth haemal spines, and the basipterygia. The neural spines of the five last precaudal vertebrae are gall-like in shape, yet the overgrowth does not affect the volume of the neural canal, and the hyperostosis occurs only in the ventral half of these spines, leaving the dorsal-fin pterygiophores undistorted. The haemal spines, by contrast, are wing-like enlarged, and the hyperostotic ribs are inflated and cylindrical over almost their entire length, tapering ventrally.

Perhaps the most striking result concerns the timing of hyperostosis onset. Based on juvenile skeletons and radiographs, the study demonstrates that the overgrowth of bone does not begin simultaneously across all affected bones but sequentially. In a juvenile Drepane punctata of 145 millimeters standard length, only the frontal bone shows first signs of hyperossification, appearing whitish against the darker non-hyperostotic bones, while the supraoccipital and the fin pterygiophores, heavily hyperostotic in adults, remain unaffected. In Paralonchurus peruanus, radiographs of three syntypes of increasing size show the progression clearly: a 150-millimeter specimen with no hyperostosis, a 230-millimeter specimen with beginning hyperostosis, and a 320-millimeter specimen with many bones fully affected. In the adult croaker, the supraneurals two and three are even fused into a single hyperostotic mass, and partly different vertebrae are affected than in juveniles, showing that the pattern itself changes during growth.

The study also resolved a case of mistaken identity through hyperostosis itself. A skeleton long labeled as Caranx hippos and displayed in the public exhibition of the museum was reidentified as Caranx fischeri, the Senegal jack, because hyperostosis in carangids follows a species-specific pattern that enables species identification from the skeleton alone. Although hyperostosis is well documented in Caranx hippos, the posttemporal bone was never affected in that species, whereas hyperostotic posttemporals are known from Caranx caninus, Caranx crysos, and Caranx fischeri. The Pacific distribution of Caranx caninus rules it out for a specimen collected off Dakar, and Caranx crysos lacks hyperostotic ribs and differs in fin lobe size and anal-fin ray count, leaving Caranx fischeri as the only consistent match.

What emerges from the Vienna collection is a picture of hyperostosis as a phenomenon with two distinct structural expressions, one spongy and one compact, both growing with age, and an ontogeny that unfolds bone by bone rather than all at once. The whitish coloration of newly hyperostotic frontal bone in juvenile Drepane may even reflect remodeling of acellular bone tissue to cellular bone tissue, as hyperostotic bone of acanthopterygian fishes shows spatial juxtaposition of these two tissues. The authors note that a clear picture of the onset and ontogenetic development of hyperostosis across species will require further series of different size classes, but the museum skeletons, some collected more than a century ago, have already shown that the swollen bones that puzzled Ole Worm in the seventeenth century still hold surprises for modern imaging and comparative anatomy.

Subject of Research: Hyperostosis in fish skeletons

Article Title: Fish skeletons with hyperostosis in the Ichthyological Collection of the Natural History Museum Vienna (Austria)

Article References: Ahnelt, H., Klein, J. S., & Winkler, V. (2025). Fish skeletons with hyperostosis in the Ichthyological Collection of the Natural History Museum Vienna (Austria). Annals of the Natural History Museum Vienna, 126, 5-25. https://doi.org/10.3897/anhmw.170482

Image Credits: AI Generated

DOI: 10.3897/anhmw.170482

Keywords: hyperostosis, fish skeleton, Natural History Museum Vienna, Zeus faber, Caranx hippos, Ole Worm, acellular bone, microCT, ichthyology, bone overgrowth, Fish, skeletons