# Effect of Acute Cadmium Exposure and Short-Term Depuration on Oxidative Stress and Immune Responses in Meretrix meretrix Gills

## TL;DR

This study examines how cadmium exposure affects gill health in clams and how short-term depuration can partially reverse these effects.

## Contribution

The study demonstrates partial recovery of oxidative and immune impairments in clams after short-term depuration from cadmium exposure.

## Key findings

- Cadmium exposure increased gill Cd content and caused histological damage, which partially recovered after depuration.
- Depuration slightly reduced Cd levels and improved antioxidant and immune enzyme activities.
- Biomarkers like lysozyme and metallothionein transcripts increased with exposure, indicating their potential for monitoring Cd stress.

## Abstract

Cadmium (Cd) is a typical pollutant with strong toxicity even at low concentrations. In the marine environment, Cd is a problem of magnitude and ecological significance due to its high toxicity and accumulation in living organisms. The clam Meretrix meretrix is a useful bioindicator species for evaluating heavy-metal stress. This study investigated the extent of recovery from Cd2+-induced oxidative and immune impairments in M. meretrix gills achieved by short-term depuration. Clams were exposed to 3 mg/L Cd2+ for six days or three days followed by three days of depuration, and the Cd contents, morphological structure, osmoregulation, oxidative stress, and immune responses in the gills were evaluated. The results showed that gill Cd contents increased with exposure, reaching 9.857 ± 0.074 mg·kg−1 on day 3 but decreased slightly to 8.294 ± 0.056 mg·kg−1 after depuration, while reaching 18.665 ± 0.040 mg·kg−1 on day 6 after continuous exposure. Histological lesions, including lamellar fusion, hemolymphatic sinus dilation, and ciliary degeneration, partially recovered after depuration. Reactive oxygen species (ROS) and malondialdehyde (MDA) levels decreased significantly, while DNA-protein crosslinking rate (DPC) and protein carbonyl (PCO) showed minor reductions. Total antioxidant capacity (T-AOC) and the activities of Ca2+/Mg2+-ATPase (CMA), cytochrome c oxidase (COX), succinate dehydrogenase (SDH), and lactate dehydrogenase (LDH) increased by over 10% during depuration, though these changes were not statistically significant. Lysozyme (LZM) activity and MT transcript levels increased progressively with Cd exposure, indicating their suitability as biomarkers of Cd stress. Acid and alkaline phosphatase (ACP, AKP) activities and Hsp70 and Nrf2 mRNA transcripts exhibited inverted U-shaped response consistent with hormetic response. ACP and AKP activity levels rose by more than 20% after depuration, suggesting partial restoration of immune capacity. Overall, Cd exposure induced oxidative damage, metabolic disruption, and immune suppression in M. meretrix gills, yet short-term depuration allowed partial recovery. These findings enhance understanding of Cd toxicity and reversibility in marine bivalves and reinforce the usage of biochemical and molecular markers for monitoring Cd contamination and assessing depuration efficiency in aquaculture environments.