# Preliminary Study of the Antimicrobial Capacity of the Cutaneous Mucus and Smear Cytology of the Epidermis in a Population of European eels (Anguilla anguilla, Linnaeus 1758)

## TL;DR

This study explores non-invasive methods to assess the health of European eels by analyzing their skin mucus and cells, revealing antimicrobial properties and normal cell composition.

## Contribution

The first application of non-invasive microbiological and cytological methods to assess health in free-living European eels.

## Key findings

- Eel cutaneous mucus showed antibacterial activity against Aeromonas hydrophila and Aeromonas salmonicida.
- Skin scraping cytology identified epidermal and goblet cells without pathological elements.
- Non-invasive methods proved effective for evaluating eel health and could aid conservation efforts.

## Abstract

The European eel (Anguilla anguilla, Linnaeus 1758), with its peculiar life cycle, represents a fish species of great interest. From a naturalistic perspective, through restocking campaigns to encourage migration of eels, financed projects within the EU LIFE program have supported the purpose of species conservation. Nevertheless, many factors continue contributing to the decline of eel populations, and among them, environmental conditions and health issues play an important role. The skin and the cutaneous mucus represent the first line of defense of the host against infections. The development and application of non-invasive methods to study fish secretions, including the cutaneous mucus and admixed exfoliated cells, can significantly contribute to obtain an overall picture of the fish’s health status, thus contributing to safeguarding this endangered fish species. With this preliminary investigation, two non-invasive methods were applied for the first time to a free-living European eel population. The antibacterial capacity of the mucus and the cellular components obtained through skin scrapings were investigated. Both methods were revealed to be effective, providing additional contributions to the eels’ health status.

The skin and its products, such as the mucus, represent an important defense mechanism against infection by pathogens. Various environmental conditions can alter mucus composition and/or cells embedded in this matrix. The analysis of skin mucus is considered a useful method to evaluate the biological response of fish to stimuli. The mucus and cells can be considered suitable non-invasive biomarkers. With this preliminary study, an antibacterial capacity method was applied to cutaneous mucus samples of a European eel population. This standardized method highlighted the ability of the mucus to significantly counteract the proliferation of two bacteria, the opportunistic pathogen Aeromonas hydrophila and the primary pathogen Aeromonas salmonicida. The cytological method using skin scraping has proven to be a useful non-invasive tool, having allowed the highlighting of the two most represented cellular populations of the cutaneous covering, the epidermal cells and the goblet cells, in combination with mucous strands. Cytological findings did not disclose pathologic elements in the mucus. A broader use of these two non-invasive microbiological and cytological methods can provide useful information on fish health, indirectly contributing to the conservation of the species.