# A Microcosm Experiment Reveals the Temperature-Sensitive Release of Mucochytrium quahogii (=QPX) from Hard Clams and Pallial Fluid as a Stable QPX Reservoir

## TL;DR

This study shows that temperature affects the release of a clam parasite, QPX, and that it resides stably in the clams' body fluid.

## Contribution

The study identifies pallial fluid as a stable QPX reservoir and reveals temperature-dependent release of the parasite.

## Key findings

- QPX is released into seawater only at low temperatures (13 °C).
- Pallial fluid serves as a stable reservoir for QPX in hard clams over time.
- QPX is classified as an opportunistic pathogen with a host-specific relationship.

## Abstract

Mucochytrium quahogii, also known as QPX or Quahog Parasite Unknown, is the causative agent of QPX disease in the hard clam (Mercenaria mercenaria). Host–pathogen–environment interactions between M. quahogii, the hard clam, and temperature were explored in a microcosm experiment. Hard clams were housed in individual tanks with sterile seawater under two temperature regimes: low (13 °C) temperature, which is thought to be optimal for QPX disease development, and high (20 °C) temperature, which has been shown to promote “healing” of QPX-infected clams. Hard clam tissue, pallial fluid, seawater, and shell biofilms were collected and assayed for M. quahogii. The release of M. quahogii from naturally infected live hard clams into seawater was detected only in the low temperature treatment, suggesting that temperature influences the release of potentially infectious cells. M. quahogii was commonly found in hard clam pallial fluid, even after 9 weeks in the lab, suggesting pallial fluid is a stable reservoir of M. quahogii within its primary host and that M. quahogii is not a transient component of the hard clam microbiota. Overall, results support a host-specific relationship and that M. quahogii is a commensal member of the hard clam microbiota, supporting its classification as an opportunistic pathogen.