For over 400 million years, sharks, rays, and their relatives have ruled the ocean depths. Their evolutionary success story is written not just in their formidable teeth and powerful bodies, but in something far more subtle: their sophisticated reproductive hormones.
For over 400 million years, sharks, rays, and their relatives have ruled the ocean depths, surviving mass extinctions and evolving into some of the sea's most perfect predators. Their evolutionary success story is written not just in their formidable teeth and powerful bodies, but in something far more subtle: their sophisticated reproductive hormones.
34
species studied for reproductive steroid hormones
2.9%
of all known chondrichthyan species
Yet despite half a century of research, scientists have only scratched the surface of this complex endocrine world. A comprehensive review published in 2020 revealed that a mere 34 species of chondrichthyans—just 2.9% of all known species—have been studied for their reproductive steroid hormones 1 . Even more striking, 93% of these investigations have focused on fewer than six hormones, leaving vast territories of endocrine biology unexplored 1 5 . This knowledge gap has profound implications for conservation, particularly as over one-third of chondrichthyan species now face extinction threats.
The emerging science of chondrichthyan endocrinology represents a race against time—an effort to understand these creatures' reproductive secrets before they disappear from our oceans. What researchers are discovering is revolutionizing not only how we protect these ancient animals but also what they can teach us about the evolution of reproductive systems across all vertebrates, including our own.
Chondrichthyans display a staggering array of reproductive strategies that surpass nearly any other vertebrate group. From egg-laying species that anchor their leathery purses to underwater structures to live-bearing sharks that nourish their young through placental connections, this evolutionary creativity has been key to their persistence through millennia.
Egg-laying species like the small-spotted catshark produce characteristic "mermaid's purses" that protect developing embryos for months outside the mother's body.
Live-bearing species give birth to live young, with some like the sharpnose sharks developing placental connections remarkably similar to mammalian pregnancy 1 .
At the heart of these varied reproductive strategies lies an intricate endocrine system centered on the hypothalamus-pituitary-gonadal (HPG) axis 6 . This system functions as a master conductor, coordinating the complex symphony of reproduction through hormonal signals.
The primary estrogen that drives vitellogenin (yolk protein) production in the liver and promotes development of the reproductive tract in females 7 .
Serves as both a hormone in its own right and a precursor for estrogen synthesis, with levels that often parallel estrogen fluctuations during reproductive cycles 7 .
Particularly important in viviparous species, where it helps maintain pregnancy, though its role in oviparous species appears less critical 7 .
A potent androgen in male sharks, though its functions are still being unraveled 7 .
The science of chondrichthyan endocrinology has undergone dramatic transformation since its inception. Early studies relied heavily on lethal methods, with approximately 65.5% of historical analyses requiring animal sacrifice to obtain hormone samples from tissues like gonads and yolk 1 . This approach, while informative, posed obvious limitations for studying threatened species and created ethical dilemmas for researchers.
Direct access to reproductive organs through lethal sampling, but not applicable to living animals and raised conservation concerns.
Animals could be released after sampling, enabling longitudinal studies. However, capture stress could affect readings and there were safety risks for researchers.
No capture or handling required, reducing stress and making the method applicable to endangered species. This new method requires further validation but represents a major advancement.
The 21st century has witnessed a paradigm shift toward non-lethal techniques that prioritize animal welfare and conservation. The new gold standard involves collecting blood samples from live animals that are captured, sampled, and released—a method used successfully in species from blacktip reef sharks to nurse sharks 7 . While revolutionary, even this approach has limitations, as the capture and restraint process can itself induce stress that may alter hormone measurements 9 .
The most recent breakthrough comes from an unexpected source: shark skin. In 2025, a landmark study demonstrated that steroid hormones accumulate in shark dermal tissue at measurable concentrations, opening the door to remote biopsy sampling that requires no capture or handling whatsoever 9 . This method represents the cutting edge of conservation-focused endocrinology, aligning with the "3R principles" (Replacement, Reduction, Refinement) that advocate for minimizing animal use and suffering in research 9 .
In 2025, an international team of researchers set out to answer a provocative question: Could shark skin serve as a reliable matrix for measuring reproductive hormones? The question emerged from both practical and ethical imperatives. With over one-third of chondrichthyan species threatened, and traditional blood sampling becoming increasingly problematic for both animals and researchers, the need for a less invasive alternative had never been more urgent 9 .
The research team focused on the small-spotted catshark (Scyliorhinus canicula), a model species abundant in European waters and well-studied reproductively. Their hypothesis was grounded in the physiological properties of shark skin: its high vascularization and the presence of dermal denticles (tooth-like scales), which they theorized might trap and accumulate steroid hormones from the bloodstream, much as mammalian hair and feathers do in other species 9 .
Skin samples were taken from 41 catsharks obtained as bycatch from local fisheries. Samples were collected caudal to the first dorsal fin—a location easily accessible in free-swimming animals via remote biopsy dart 9 .
The process began with cleaning samples in isopropanol to remove contaminants, followed by drying and mechanical milling to create a fine powder. Hormones were extracted by incubating the powdered skin in methanol overnight, then evaporating the solvent and reconstituting the hormone-rich residue in buffer solution 9 .
The researchers used commercial enzyme immunoassay kits to measure concentrations of progesterone, 17β-estradiol, and testosterone—three steroids with well-established roles in chondrichthyan reproduction 9 .
Crucially, the team rigorously validated their methods, demonstrating that the assays could reliably detect and quantify hormones in skin extracts without significant interference from the skin matrix itself 9 .
| Reagent/Material | Primary Function | Research Application |
|---|---|---|
| Remote Biopsy Dart | Collects skin samples from free-swimming sharks | Enables sampling without animal capture or handling |
| Methanol | Extracts steroid hormones from skin tissue | Isolates hormones from the skin matrix for analysis |
| Enzyme Immunoassay Kits | Quantifies specific steroid hormones | Measures progesterone, 17β-estradiol, and testosterone concentrations |
| Isopropanol | Cleans skin samples of external contaminants | Prevents contamination from seawater or handling |
| Mechanical Ball Mill | Pulverizes dried skin into fine particles | Increases surface area for more efficient hormone extraction |
The findings from the skin biopsy study were nothing short of revolutionary. Researchers detected measurable levels of all three target hormones in every skin sample, but with striking differences between demographic groups:
These patterns weren't just statistically significant—they were biologically meaningful. The hormonal signatures in skin tissue accurately reflected the reproductive status of the sharks, opening an entirely new window into their physiological world.
Perhaps most importantly, the study demonstrated that skin biopsies could be collected remotely using biopsy darts, eliminating the need to capture, restrain, or even significantly disturb the study animals. This addresses one of the most persistent challenges in chondrichthyan research: the fact that capture and handling stress can alter hormone levels and potentially distort research findings 9 .
Modern chondrichthyan endocrinology relies on a sophisticated array of tools and techniques that have evolved significantly over time. The standard toolkit includes:
The historical workhorse of hormone detection, used in 76.3% of all chondrichthyan studies to date 1 . This method uses antibodies and radioactive tags to measure hormone concentrations with exceptional sensitivity.
Increasingly popular non-radioactive alternatives that offer practical advantages for field researchers while maintaining strong sensitivity and specificity 9 .
A cutting-edge technique that can identify and quantify multiple hormones simultaneously, providing unprecedented detail about hormonal profiles.
Custom-designed darts and poles that allow researchers to collect skin samples from free-swimming animals without capture—the same technology used in the groundbreaking 2025 skin hormone study 9 .
What makes these tools particularly powerful is how they're being integrated with complementary approaches, including genetic analysis, visual observation, and satellite tracking. This multi-pronged strategy allows researchers to connect hormonal data with behavior, ecology, and population dynamics—creating a more complete picture of chondrichthyan lives.
The silent language of shark hormones speaks volumes about the health of our oceans. As we learn to interpret this chemical vocabulary, we gain not just knowledge about individual species but wisdom about marine ecosystem integrity. The shift from lethal sampling to innovative techniques like skin biopsy analysis represents more than methodological progress—it signals a fundamental change in our relationship with these ancient creatures.
With over 97% of chondrichthyan species completely unstudied from an endocrine perspective, and dozens of hormones whose functions remain mysterious, the future of this field is bright with possibility.
The implications extend far beyond academic interest. For conservation managers, hormonal data can identify critical breeding habitats, pinpoint sensitive reproductive periods, and monitor the health of populations without sacrificing a single animal. For policymakers, this science provides the evidence needed to establish fishing moratoriums during breeding seasons or create marine protected areas in key reproductive habitats.
Perhaps most inspiring is how much remains undiscovered. With over 97% of chondrichthyan species completely unstudied from an endocrine perspective, and dozens of hormones whose functions remain mysterious, the future of this field is bright with possibility 1 . Each question answered leads to new, more sophisticated questions about how these evolutionary marvels have reproduced successfully for millennia—and how we can ensure they continue to do so in our rapidly changing oceans.
The journey to understand shark hormones mirrors a larger transformation in marine science: from extraction to understanding, from conquest to coexistence. As we continue to decode the endocrine secrets of chondrichthyans, we don't just become better scientists—we become more responsible stewards of our blue planet.