I consider the manuscript to be very timely and thematically important because such work is currently missing in the literature despite its urgent need, and this group of freshwater animals (Unionid bivalves) is an ideal model. The text is well written, the tables and appendices are sufficient and well-organized. I only have a few specific comments on the manuscript:
1. The protocol is very clear, including the detailed diagrams, but the study would benefit either from a specific example of a successful application of this particular protocol, or more examples of more or less successful detections of some mass mortality events and its causes in the past – ideally in different contexts and different parts of the world. I feel that some of these case studies can be important additions. For example, the study by Cmiel et al. 2019. Is Pseudanodonta complanata the most vulnerable of widespread European species of unionids? An intense stress test leading to a massive die‐off. Aquatic Conservation: Marine and Freshwater Ecosystems, 29, 2185-2192, which likely documented a cause of a mass die-off in standing water habitat. Possibly even examples from other groups of animals, such as, for example, Quiroga et al. 2023. Mass mortality of the Critically Endangered naked characin (Gymnocharacinus bergii): Possible causes and impact. Aquatic Conservation: Marine and Freshwater Ecosystems, 33, 1039-1048 could be also used if needed.
2. Lines 115-124: The description of what is and what is not the subject of this ms needs to be more precisely formulated; it is not clear to me. Habitat destruction - I assume it means physical habitat destruction ? - chemical accidents are included. Long-term changes – after all, even a gradual change can result in an important sudden mass mortality. Invasive species - predators are already mentioned, what about invasive parasites - are they not the subject? Please rewrite this to make it clearer.
3. Lines 170-172: In this part of the text, I suggest emphasizing the urgency of rapid sampling. The current text implies that the sampling team must start by obtaining permission – that is clear, but in many contexts, it is not feasible to obtain the necessary samples in a timely manner. At a minimum, for example, water samples can often be collected in many countries without permission. Wouldn't it be good to have a pre-known plan in the given region and, for example, competences – which agency to contact so that sampling can be started/carried out even without some of the permissions due to urgency, which can be obtained later? I understand its complex, but I feel the issue of time urgency is not sufficiently emphasized in the manuscript.
4. Figure 2 is probably damaged - I don't see any mussel icons in the PDF.
5. Line 322: Typo "toxi"
6. Figure 4 – you probably mean the term adductor and not abductor.
7. Discussion – I missed a stronger appeal in the text, in addition to other follow up steps, for the most detailed possible subsequent publication of the recorded MME in the scientific literature.

All my comments have been successfully addressed.