# A New Research Tool for Use in Sharks and Rays: Relevance of Reproductive Hormone Levels in the Skin of Small-Spotted Catshark (Scyliorhinus canicula)

## TL;DR

This study explores using shark skin to measure reproductive hormones, offering a non-invasive alternative to blood sampling for studying shark reproduction.

## Contribution

The study validates skin biopsies as a non-invasive method to measure reproductive hormones in sharks, potentially reducing harm in wildlife research.

## Key findings

- Skin hormone levels differ between male and female small-spotted catsharks.
- The enzyme immunoassay reliably measures progesterone, 17β-estradiol, and testosterone in shark skin.
- Mature and immature males show distinct hormone levels in skin samples.

## Abstract

Given the conservation concerns surrounding many chondrichthyan species and the growing importance of reducing the use of animals in research and minimizing their suffering, we should prioritize the use of methods that avoid harming or killing the animals. Although blood is the “gold standard”, applying standard blood collection protocols is not always feasible for chondrichthyans living in the wild, especially because these methods require chase, capture, and restraint. The present study was performed to evaluate whether sex steroid hormones detected in shark skin, a sample that can be collected remotely, provide relevant information about their reproductive state. The analytical methodology was first validated, demonstrating that the method can reliably quantify sex steroid hormones in shark skin. After ensuring that reproductive hormones are accurately measured in this sample type, we compared hormone levels between males (mature and immature) and females. Although further validation is required, differences detected between sexes suggest that skin may be a promising alternative approach for obtaining reproductive data in free-ranging sharks.

The present study marks a significant step forward in validating the use of skin biopsies for measuring reproductive steroid hormones in sharks. Given the conservation concerns surrounding many chondrichthyan species and the growing emphasis on adhering to the 3R principles in wildlife research, we should prioritize the acquisition of data through non-destructive or minimally invasive sampling techniques collected remotely. Dart skin biopsy samples can be collected remotely, without the need to chase, capture, and restrain the animal. Accordingly, the present study aimed to (i) develop a suitable extraction and analytical technique for reproductive hormone determination in shark skin biopsies and (ii) evaluate whether sex steroid hormones detected in shark skin differ between males and females and depending on males’ maturity. The results of assay validation demonstrated that the commercial enzyme immunoassay used can provide reliable measures of progesterone, 17β-estradiol, and testosterone measured in shark skin extracts. After ensuring that reproductive hormones are accurately measured in this sample type, we compared hormone levels between males (mature and immature) and females to evaluate the variation in reproductive hormones relative to sex and males’ maturity stage. We detected significant differences in skin hormone levels between sexes, suggesting that this technique may be a promising alternative approach for obtaining relevant biological data in free-ranging sharks. By employing this novel technique, we expect to gain a deeper understanding of the reproductive processes of living chondrichthyans, which is essential for formulating effective science-based conservation and management strategies.