# Skeletal Adaptations to Locomotion and Feeding in Mediterranean Batoids (Raja asterias, Myliobatis aquila) and the Teleost Sparus aurata: A Comparative Study

## TL;DR

This study compares the skeletal structures of two cartilaginous fish and one bony fish to understand how their skeletons adapt to movement and feeding habits.

## Contribution

The discovery of two distinct chewing systems in Myliobatis aquila and shared mineral strategies in skeletal strengthening across species.

## Key findings

- The pectoral fin structures of the three species differ in flexibility, adapting to pelagic or benthic locomotion.
- Myliobatis aquila has two separate chewing systems, possibly allowing a broader diet or reflecting evolutionary remnants.
- Cartilaginous and bony fishes use the same mineral (Ca3(PO4)2) to strengthen skeletal elements, suggesting a shared evolutionary strategy.

## Abstract

This study explored how the skeleton of the pectoral fins and feeding structures has evolved in two cartilaginous fishes (Raja asterias and Myliobatis aquila) and one bony fish (Sparus aurata). Our goal was to understand how these skeletal parts develop and adapt to different swimming styles and feeding needs. We found that the three species have distinct fin structures, which provide varying levels of flexibility suited to either open-water or bottom-dwelling movement. In M. aquila, we discovered two separate chewing systems: one external, using rows of teeth, and another internal, using special plates adapted for crushing hard prey (a feeding strategy known as durophagy). This dual system may allow the species to exploit a broader diet, or it could represent a remnant of an earlier evolutionary stage. Chemical analyses also showed that the minerals used to strengthen skeletal elements are the same in both cartilaginous and bony fishes, pointing to a shared evolutionary strategy. These findings provide new insights into how fish adapt their bodies to their environment and diet, offering a clearer picture of the evolutionary processes that shape survival in aquatic life.

In the Chondrichthyes Raja asterias and Myliobatis aquila and in the Teleost Sparus aurata, the appendicular skeleton of the pectoral fins (including the calcified structures of the mouth in M. aquila) was investigated to find out how the specific skeletal segments were formed and stiffened over the course of evolution, not only with regard to the adaptation of the ontogenesis of the cartilage “anlagen” to the mechanical requirements of locomotion in the water column, but also to the specific feeding habits (durophagy) of M. aquila. The morphology of the pectoral fins of the three species showed a different layout, characterized by the geometry of the basic units (aligned tesserae and calcified radial columns), which provide varied flexibility of the pectoral fins, suggesting an adaptation to the “pelagic” and “benthic” locomotion patterns in the environment where the species live. The morphology of the calcified structures in the mouth of M. aquila showed the presence of two different masticatory systems: the first (external) with the rows of teeth resting on the maxillary and mandibular arches, and the second (internal, in the oral cavity) with the symphyseal plates specialized for durophagy. Chemical–physical analyses revealed that the calcified cartilage matrix of the Chondrichthyes fin rays, teeth and durophagy plates is stiffened by the same Ca3(PO4)2 mineral phase deposed in the organic matrix of the Teleost S. aurata fins (with the characteristic SEM morphological texture of calcified bone matrix). The hitherto unknown presence of two different chewing systems in M. aquila documents an evolutionary adaptation to nutritional requirements that can be explained by two hypotheses: the coexistence of two functioning systems in current specimens, allowing for the ingestion of harder and softer prey (or plant food), or the persistence of a rudimentary dentition that is no longer used (vestigial dentition). Furthermore, the texture of the calcified matrix in teleost fishes, as observed by scanning electron microscopy, may indicate a bone-like organic matrix substrate, similar to that found in endochondral ossification.