Fish with teeth on its head : Scientists describe a toothed head organ in adult male spotted ratfish ( Hydrolagus colliei ) observed in the northeastern Pacific. The structure, called a tenaculum, emerges between the eyes as a white, hook-like protrusion and is covered by several rows of true teeth. The discovery, documented using microscopy, genetics, and microtomography, provides evidence to revise hypotheses about the origin and distribution of vertebrate dentition.
What they found and why it matters
Unlike the dermal denticles typical of sharks and rays, the tenaculum elements are derived from the dental lamina, the same tissue that gives rise to teeth inside the mouth. Therefore, the tooth-headed fish represents a functional and developmental exception: it displays extraoral dentition with histology and markers compatible with teeth, not modified scales. In reproductive terms, the tenaculum serves a dual role. On the one hand, it facilitates the male's attachment to the female's pectoral fin during mating. On the other hand, it serves as a deterrent signal to rivals, as described by the authors based on behaviors observed in mature specimens.
The variability is notable. Seven or eight main rows were recorded in adult males, with periodic replacement. Organ size does not correlate with total body length, but rather with sexual maturity and the development of other reproductive traits, such as pelvic claspers. This suggests endocrine and genetic control linked to the reproductive cycle rather than general somatic growth.
Study methods and comparative context
The team used micro-CT to visualize the internal architecture of the tenaculum, histology to identify mineralized tissues, and gene expression analysis associated with odontogenesis. The results converged: dental lamina signal was present in the tenaculum, while dermal denticle patterns were absent. Furthermore, comparison with fossils of chimaera and other chondrichthyans provided an evolutionary framework in which extraoral toothed structures may have been more common than previously thought.
In parallel, the toothy fish is inserted into a lineage with specialized reproductive traits. Chimeras (or ratfish) retain primitive characteristics while exhibiting unique adaptations. This contrast allows us to explore how a dental development program, originally restricted to the oral cavity, can be activated in the cephalic region under specific local and hormonal signals.
Evolutionary implications
The map of homologies between teeth, denticles, and other mineralized structures becomes more complex with this case. If the dental lamina can be activated outside the mouth, then the "rule" limiting dentition to the oral territory needs to be nuanced. In macroevolutionary terms, the tooth-headed fish reopens questions about the plasticity of odontogenic tissues, the embryological limits of developmental fields, and the conditions that favor the retention of dental programs in alternative locations.
The authors propose three lines of research . First, to track the exact moment in embryos when the dental lamina is specified in the cephalic region, in order to compare the chronologies with oral dentition. Second, to evaluate whether inductive signals shared with other reproductive appendages exist and whether activation depends on hormonal thresholds. Third, to expand the sampling to closely related species and both sexes, since females and juveniles showed early signs of the structure, although they did not develop the organ on the surface.
Function, replacement and biological cost
The tenaculum combines mechanical grasping and display. The presence of true teeth improves traction on the female's pectoral fin without causing deep cuts, consistent with small, multiple, and replaceable teeth. Periodic replacement would entail a moderate metabolic cost, offset by the reproductive benefit. In shallow waters of the northeast Pacific, where the species was observed, competition for mates and the need to secure copulation may have favored the establishment of this morphofunctional solution.
What's next
The next step is to extend the search for extraoral tooth structures to other chondrichthyans and teleosts. If signatures are detected in non-oral regions, this will increase evidence that the boundaries of the dentition are more permeable than previously thought. Likewise, experimental models could manipulate classic signaling pathways of odontogenesis (e.g., Wnt, Shh, BMP) to verify whether ectopic activation reproduces the tenaculum pattern.
In short, the toothed fish provides a verifiable case of extraoral dentition with reproductive function, histological support, and a genetic basis consistent with oral teeth. This result calls for refining definitions and revisiting evolutionary scenarios in which the dental lamina maintains the ability to emerge outside the mouth under specific biological contexts.
