# Comparative Analysis of Eye Traits and Visual Resolution Among Three Hatchery-Bred Giant Clams (Tridacna crocea, T. squamosa, T. maxima)

## TL;DR

This study compares eye traits and visual resolution in three giant clam species, finding differences that may reflect adaptations to their habitats.

## Contribution

The study provides new insights into the adaptive evolution of simple visual systems in giant clams based on ecological niche differences.

## Key findings

- Tridacna maxima had the highest number of eyes, while Tridacna squamosa had the largest eyes.
- Tridacna crocea exhibited the highest visual resolution among the three species.
- All species have lensless pinhole eyes, with structural differences potentially linked to their ecological adaptations.

## Abstract

Giant clams are marine mollusks renowned for their immense size and colorful mantles. However, scientists have limited knowledge about how these eyes differ between species or how clearly they can see. This study investigated three common species: Tridacna crocea, Tridacna squamosa, and Tridacna maxima. We measured fundamental traits such as eye count and diameter, and determined how clearly they can see by observing their reactions to striped grating patterns. We found significant differences: Tridacna maxima possessed the highest number of eyes, whereas Tridacna squamosa had the largest eyes. Tridacna crocea exhibited the highest visual resolution. All three species possess simple “pinhole” eyes that lack lenses. We suggest that these specific ocular variations evolved to help each species survive in its particular habitat, influenced by environmental factors like water depth. These findings are valuable as they provide new insights into how simple visual systems adapt to different ecological niches.

Bivalves possess a diverse array of photoreceptive organs that are significant for their evolutionary success and systematic classification. Giant clams are the largest bivalve mollusks, with mantle tissue permanently extended in nature to maintain symbiosis with zooxanthellae and perceive environmental cues. Eyes serve as critical sensory organs for these organisms, yet the structural and functional characteristics of tridacnine eyes remain inadequately understood. This study systematically investigated the ocular traits and visual resolution of three ecologically distinct giant clam species (Tridacna crocea, T. squamosa, T. maxima) using morphometric analysis, hematoxylin-eosin (HE) staining, transmission electron microscopy (TEM), and grating stimulation assays. Significant interspecific differences were observed in eye count, diameter, and pupil-to-eye ratio (PER): T. maxima exhibited the highest mean eye count (221 ± 8), T. squamosa the largest mean eye diameter (0.490 ± 0.082 mm), and T. crocea the highest mean PER (0.363 ± 0.041). Eyes were numerically symmetric on the left and right mantles but positionally asymmetric, showing random distribution patterns along the mantle margin without fixed corresponding locations across species. All three species possessed typical pinhole eyes lacking lenses and retinas, primarily composed of filler cells, receptor cells, and sparse neurons, with symbiotic zooxanthellae distributed in the surrounding mantle tissue. Grating stimulation assays revealed resolvable stripe periods of 5.82–11.64° (T. crocea), 8.62–13.16° (T. squamosa), and 10.15–12.26° (T. maxima), confirming T. crocea as the species with the highest visual resolution. These ocular variations are inferred to reflect adaptive evolution driven by ecological niches and habitat-specific factors (water depth or light intensity), while the simplified pinhole morphology is consistent with their sedentary lifestyle and metabolic dependence on symbiotic zooxanthellae. These ocular variations provide potential morphological markers for the systematic classification of Tridacninae and offer valuable insights for researchers studying the evolutionary plasticity of bivalve visual systems.