# Synergistic Effects of Ammonia and Hypoxia Stress on the Transcriptomic Responses of the Razor Clam (Sinonovacula constricta)

## TL;DR

This study explores how razor clams survive in environments with high ammonia and low oxygen by using different strategies in their gills and hepatopancreas.

## Contribution

The study reveals tissue-specific molecular strategies in razor clams for coping with ammonia and hypoxia stress.

## Key findings

- The gill uses HIF-2α and COX-6b to maintain respiratory health under stress.
- The hepatopancreas prioritizes ammonia detoxification via the urea cycle over immune signaling.
- These findings offer molecular markers for breeding more resilient clams in aquaculture.

## Abstract

In contemporary intensive aquaculture, the razor clam frequently encounters the dual threat of high ammonia nitrogen and low dissolved oxygen (hypoxia). This study identifies how different tissues in the clam coordinate their molecular “defensive shields” to survive these stressors over 96 h (the 96 h exposure period was selected based on its widespread application in acute stress studies of S. constricta. This duration is a well-established physiological window that allows for the observation of stable metabolic shifts and tissue-specific adaptive responses to ammonia and hypoxia, without reaching critical mortality levels that would compromise the integrity of the omics analysis.). We discovered that the clam does not respond uniformly; instead, the gill and hepatopancreas perform specialized roles. The gill maintains respiratory health by switching its oxygen-sensing machinery—temporarily suppressing HIF-1α to avoid metabolic acidosis and later activating HIF-2α along with the energy-producing gene COX-6b. Simultaneously, the hepatopancreas acts as a detoxification hub, prioritizing the clearance of poisonous ammonia through the urea cycle (via ARG upregulation) at the expense of its immune signaling (NOS suppression). This strategic trade-off reveals how the razor clam balances its limited energy between self-cleansing and survival under environmental pressure. These findings provide critical molecular markers for selecting and breeding more resilient clam strains for sustainable aquaculture.

The co-occurrence of ammonia nitrogen and hypoxia represents a physiologically taxing synergistic challenge for benthic bivalves—as it forces a conflict between the high energy demand for detoxification and the limited energy supply under low oxygen, yet the tissue-specific strategies underlying their resilience remain poorly understood. This study investigated the physiological and transcriptomic responses of the razor clam Sinonovacula constricta to ammonia (AG), hypoxia (HG), and their combination (HAG) over 96 h. Transcriptomic profiling revealed that the gill and hepatopancreas employ distinct, organ-coordinated adaptive strategies rather than a uniform systemic response. The gill prioritized respiratory homeostasis by fine-tuning oxygen sensing: transcriptional suppression of hypoxia-inducible factor 1-α (HIF-1α) (to limit glycolytic acidosis) was followed by a chronic induction of HIF-2α, alongside the specific upregulation of the mitochondrial respiratory gene cytochrome c oxidase-6b (COX-6b). In contrast, the hepatopancreas executed a critical metabolic trade-off centered on arginine metabolism. Under combined stress, arginine flux was redirected toward the urea cycle via a robust upregulation of arginase (ARG) for detoxification, while nitric oxide synthase (NOS) was concurrently suppressed. This reciprocal regulation suggests a strategy to prioritize ammonia clearance and energy conservation at the expense of immune signaling. These findings elucidate how S. constricta navigates the bioenergetic conflict between detoxification and oxygen limitation, providing molecular targets for breeding stress-resistant aquaculture strains.