# Alimentary Tract Anatomy and Morphology in Early Adult Mediterranean Killifish Aphanius fasciatus (Valenciennes, 1821)

## TL;DR

This paper describes the anatomy of the digestive system in Mediterranean killifish, revealing a specialized intestinal chamber and sexual dimorphism in the anal region.

## Contribution

The first detailed anatomical study of the digestive tract in Aphanius fasciatus, including a novel description of a food-processing intestinal chamber and sex-based structural differences.

## Key findings

- A specialized anterior intestinal chamber functions for food storage and preparation without gastric glands.
- Sexual dimorphism is observed in the anal region, with males having a more toned and folded structure compared to females.
- The oropharyngeal tract and esophagus lead to a dilated intestinal region with mucosal folds and mucus-secreting epithelium.

## Abstract

The Mediterranean killifish, Aphanius fasciatus, is a small fish that is able to survive in variable salinity and temperature, making it an important bioindicator in coastal ecosystems. Despite its ecological relevance, little is known about its anatomy. This study provides the first detailed examination of its digestive tract, revealing a specialized anterior intestinal chamber that stores and processes food before digestion, and differences between males and females in the anal region. These findings offer valuable insight into the biology of this species and support its use as a reliable experimental model for studies in anatomy, physiology, and environmental monitoring.

Background/Aims: The Mediterranean killifish, Aphanius fasciatus (Valenciennes, 1821), is a small euryhaline and eurytherm cyprinodont. While its ecology and role as a bioindicator are well known, its anatomy remains poorly understood. This study aimed to provide the first detailed description of the digestive tract of A. fasciatus, from the oropharyngeal cavity to the rectum. Methods: An anatomical and morphological approach supported by light microscopy was applied to examine oral, pharyngeal, and intestinal structures in adult specimens. Results: The jaws bear tricuspids incisiform teeth, whereas the pharynx has caniniform teeth. The tongue forms a muscular thickening of the oral floor and aids swallowing. The oropharyngeal tract and esophagus lead to a dilated anterior intestinal region resembling a primitive stomach, with mucosal folds and mucus-secreting epithelium, but lacking gastric glands. This chamber functions mainly for food storage, absorption, and preparation rather than true gastric digestion. The intestine extends to the rectum, showing sexual dimorphism: females have a relaxed anus, males a more toned and folded structure. Conclusions: These findings provide essential baseline information for comparison with other experimental models and reinforce the suitability of A. fasciatus as a reliable model for anatomical and functional studies.