What can cause shellfish toxins?

The correct answer centers on the fact that shellfish are filter feeders and obtain food by consuming microscopic algae, some of which can produce harmful toxins. These toxic algae blooms are known as harmful algal blooms (HABs), and when shellfish ingest these algae, they accumulate the toxins in their tissues without being affected by them. This means that by the time the shellfish are harvested and consumed by humans, they can pose serious health risks such as paralytic shellfish poisoning. Since the primary source of these toxins in shellfish comes specifically from the algae they consume, this option accurately identifies the root cause of shellfish toxins.

In contrast, while infected seafood handling or a contaminated water supply might contribute to other forms of foodborne illness, they are not the primary causes of the specific toxins associated with shellfish. Similarly, improper cooking of shellfish does not eliminate the toxins that have already accumulated in the tissue; rather, cooking may only kill pathogens but not remove the already present toxins if the shellfish were previously contaminated through algae consumption.