The short answer: some animals already do

In the deep pastures of the sea, where daylight fizzles out and bodies must find each other by touch and signal, a group of cartilaginous fishes called chimaeras (also known as ghost sharks, ratfish, or rabbitfish) carry a remarkable adaptation: a retractable, rod-like organ on the forehead, studded with tiny tooth-like spines. Males use it as a grasping tool during mating. Biologists call it the cephalic tenaculum or cephalic clasper. It is, in fact, a literal answer to the provocative question: why not have a rod of teeth on your forehead for sex reasons?

This isn’t a creative writing exercise from evolution’s cutting-room floor; it’s a working solution that has persisted for hundreds of millions of years in the lineage Holocephali, the sister group to sharks and rays.

Meet the owners: chimaeras, the other cartilaginous fishes

Chimaeras belong to the class Chondrichthyes, which also includes sharks and rays (the elasmobranchs). While sharks and rays share many familiar traits—cartilaginous skeletons, placoid scales, internal fertilization—chimaeras have taken a more idiosyncratic path. They have large, curious eyes adapted to dim light, a single gill opening on each side covered by a soft flap, venomous dorsal spines in some species, and a taste for benthic fare like crustaceans and mollusks.

Their reproductive toolkit is similarly specialized. Like sharks, male chimaeras possess paired pelvic claspers used to transfer sperm. Unlike sharks, they also have one (and in some families, additional) grasping organ on the head: the cephalic tenaculum. This trio of holdfasts—head and hips—lets males stabilize and align with females in a three-dimensional, often low-visibility environment.

What is the “rod of teeth,” exactly?

The cephalic tenaculum is a cartilaginous, retractable structure that protrudes from the forehead of mature males. Its surface is armed with denticles: microscopic or macroscopic spines built from the same tissues that form shark “skin teeth” (placoid scales). These denticles point backward, improving grip while minimizing tearing, much like the barbs on a fishhook. When not in use, the organ can tuck back into a groove or fold of skin; when deployed, it projects as a short rod or paddle bristling with spines.

A crucial clarification: these are not jaw-teeth misplaced onto the forehead. They’re tooth-like armor elements—part of the dermal skeleton—akin to the gritty sandpaper feel of a shark’s skin. In some species, the tenaculum’s tip broadens into a pad; in others, it resembles a stake bristling with hooks. Its musculature and ligaments allow precise control, and blood supply supports growth and seasonal maintenance as males mature.

How it’s used during mating

Observations of spotted ratfish (Hydrolagus colliei) and other chimaeras show a ritual that is at once gentle and practical. A male approaches a female, often following or circling. At close quarters, he deploys the cephalic tenaculum to seize the edge of her pectoral fin or the skin near her head. With this secure point of contact, he can position his body alongside hers and bring one of his pelvic claspers into alignment with her cloaca for internal fertilization.

The forehead grip does at least three important things:

• Stabilization: In surge, current, or near the seafloor, the grip reduces drift and misalignment during copulation.
• Precision: It frees the male’s mouth from having to bite and hold, which can be risky and imprecise, and gives finer control than pelvic claspers alone.
• Communication: Tactile contact likely serves as both restraint and a cue, coordinating timing between partners.

Does it hurt the female? Typically, no more than a firm Velcro hold—though researchers do note occasional scrapes or small scars. As in sharks, mating in chondrichthyans can leave superficial marks, a trade-off of internal fertilization in a fluid medium.

Engineering notes: form, materials, and maintenance

The tenaculum’s outer armature is built from dentine and enameloid-like tissues, which are tough, wear-resistant, and replaceable across a fish’s life. Beneath that, cartilage forms the core, supported by connective tissues and muscles that allow protraction and retraction. The geometry—fine, posteriorly directed spines on a slightly flexible beam—delivers high friction when pulled and low resistance when released, a biomimetic principle familiar from one-way barbs and gecko-inspired adhesives.

Because denticles emerge from the skin rather than from the jawbone, they can be renewed more readily; their development taps into conserved genetic pathways (including signaling systems used in tooth and scale formation) that vertebrates have repurposed again and again.

Why evolution invented a forehead clasp

Three pressures likely converged to favor this oddity:

• Internal fertilization requires contact. In open water, drifting apart during copulation wastes energy and opportunity. Additional grips pay dividends.
• Low-light habitats reward tactile solutions. If you can’t always see each other, you benefit from an organ that ensures coupling once contact is made.
• Sexual selection can embellish functional traits. Once a rudimentary clasp exists, variation in grip strength and reliability may affect male success, promoting elaboration.

Sharks, which face similar constraints, converged on different solutions: strong jaws for holding and robust pelvic claspers alone. Chimaeras, by contrast, split the holding task across mouth, hips, and head, reducing the need for biting and allowing more precise alignment.

Species sampler

• Hydrolagus colliei (spotted ratfish): Coastal northeastern Pacific. Males use a spined forehead tenaculum; mating has been filmed by divers, offering rare documentation.
• Callorhinchus milii (Australian ghost shark or elephant fish): Features a distinctive, leaf-shaped cephalic pad; a model organism in evolutionary genomics due to its compact genome.
• Chimaera monstrosa (rabbitfish): Eastern Atlantic/Mediterranean; deepwater species with a robust tenaculum and ornate pelvic claspers.

In some lineages, males also carry small prepelvic tenacula—additional spiny pads near the pelvic region that act like auxiliary grips. The overall theme is redundancy: multiple safe points of contact for a crucial behavior.

“Teeth” that aren’t teeth

Calling the spines “teeth” makes for a punchy headline, but anatomically they’re closer to fortified skin. Placoid denticles share developmental logic with teeth—similar signaling genes and tissue types—but arise from the dermis and do not sit in sockets or cycle like jaw teeth. This distinction matters because it shows how evolution can retool a common building block for new purposes: armor becomes grip; scales become hooks; a forehead becomes a handhold.

Why don’t land animals have one?

Natural selection is opportunistic, not prescient. Terrestrial animals solve mating stability with limbs, tails, prehensile structures, or behavioral choreography. Insects have genital claspers; birds rely on feet and balance; mammals use limbs and body mass. A forehead clasp would add drag, impede locomotion, and risk injury in crowded, obstacle-rich environments. The ocean’s 3D, low-friction world—plus the chondrichthyan toolkit of denticles and cartilage—made the tenaculum viable where it would be maladaptive on land.

Behavior, eggs, and the long game

Chimaeras are oviparous: females lay large, leathery egg cases—sleek, spindle-shaped capsules often secured to the seafloor. Embryos develop slowly, sometimes over many months. With low fecundity, long lifespans, and late maturity, every successful mating matters. A specialized clasp that nudges success rates up even a little can be strongly favored over evolutionary time.

Conservation: protecting the strange

Deep-sea trawling and bycatch threaten many chimaera species. Their life histories make them slow to rebound once populations are depleted. Protecting essential habitats and regulating deepwater fisheries preserves not just species but also the peculiar solutions they embody—like a forehead clasp that turns skin-teeth into a mating tool.

So, why not?

From an evolutionary standpoint, “why not” questions are invitations to look for organisms that already took the leap. The chimaera’s cephalic tenaculum shows how existing materials and developmental programs can be reassembled into something that solves a specific mechanical problem. It’s weird, yes. But it’s also elegant: a retractable, self-renewing, nonlethal, hydrodynamic clamp that makes mating work in the dark.

We don’t need one. Chimaeras do. And that’s the real joy of this trait: it reminds us that biology is not a ladder to a single best design, but a library of context-dependent hacks that prosper where and when they’re useful. In a quiet lane of the ocean, a forehead full of tiny spines became not a monster’s crown, but a careful handhold.