?That's really pretty disturbing. Since cowries and fighting conch are gastropods, they do not filter phytoplankton in the water as do bivalves..so unlikely from a toxic microalgae...and cowries are carnivores and fighting conch are herbivore/detritivores, so it's very unlikely they were "poisoned" from consumption of similar food items.

To me, the most likely scenario would be hypoxia - low dissolved oxygen- that could be caused by a high "biological oxygen demand" as bacteria decompose any dead phytoplankton in the water (perhaps that's the yellow color you're seeing in the water)...as the bacteria multiply their respiration can rapidly consume what dissolved [...]