# Revealing Phenotypic Differentiation in Ochetobius elongatus from the Middle Yangtze River Through Geometric Morphometrics

## TL;DR

This study uses geometric morphometrics to reveal how a critically endangered fish species adapts to different habitats in the Yangtze River.

## Contribution

The study identifies phenotypic differentiation in Ochetobius elongatus populations using geometric morphometrics in the middle Yangtze River.

## Key findings

- River populations of O. elongatus have elongated bodies and narrower caudal peduncles compared to lake populations.
- Lake individuals show deeper bodies and a shifted pelvic fin origin, indicating habitat-driven morphological adaptation.
- Environmental factors like water temperature and food abundance influence phenotypic variation, but synergistically rather than individually.

## Abstract

Phenotypic differentiation reflected species adaptation to different habitats. Ochetobius elongatus was once an economically important species in the south of the Yangtze River, China. However, due to environmental degradation and human activities, its wild populations have declined drastically, and it is now classified as a critically endangered (CR) species. Following the implementation of the Ten-Year Fishing Ban in the Yangtze River, O. elongatus has reappeared in certain sections of the mainstem, the tributary and the river-connected lakes of the middle Yangtze River, which could provide valuable samples for studying its morphological variation and habitat adaptation. Using geometric morphometrics based on 14 landmarks, this study revealed significant phenotypic differentiation among O. elongatus populations from distinct habitats. These results indicated that phenotypic plasticity might underlie their adaptation to diverse aquatic environments. The findings provided important insights into the survival strategies of this species and could support its conservation and restoration efforts.

Ochetobius elongatus, a critically endangered (CR) fish species of the Yangtze River Basin in China, has experienced a severe decline in its wild population. Understanding its mechanisms of phenotypic variation is essential for developing effective conservation and restoration strategies. Using geometric morphometrics based on 14 landmarks, we examined the phenotypic difference among five populations from the mainstem, the tributary, and the river-connected lakes of the middle Yangtze River. The results showed that significant phenotypic divergence was detected between river and lake populations. River individuals exhibited a more elongated body, smaller head, inferior mouth position, larger operculum, and narrower caudal peduncle, whereas lake individuals showed a deeper body, and anterior shift in the origin of pelvic fin. The first canonical variable effectively distinguished river and lake populations, with the accuracy of both original and cross-validation classification exceeding 90%, indicating that habitat heterogeneity was the primary driver of phenotypic differentiation. No significant correlation was found between morphological distance and geographical distance. Water temperature, flow velocity, water depth, and food abundance significantly influenced phenotypic variation, but their individual effects were limited, which suggested that environmental shaping of morphology depended more on synergistic effects. Our findings provide important insights into the adaptive evolution of this critically endangered species and offer a scientific basis for conservation efforts.