Marine toxins from harmful algal blooms (HABs) present a growing challenge for aquatic life—and for the humans who depend on healthy oceans and thriving aquaculture. These toxins can enter aquatic animals through their food or be absorbed directly from the water, disrupting vital physiological processes and, in some cases, leading to mass die-offs. This chapter unravels the toxicity pathways of marine toxins from harmful algal blooms in aquatic animals, as well as the potential defense mechanisms employed by these animals, which lead to varying degrees of successful toxin elimination. We advocate studying low-dose exposure, which may stimulate an adaptive (hormetic) response in exposed organisms, be they competitors or predators. This adaptive response may include trained innate immunity. Understanding this response could provide insight into how organisms adapt or resist exposure and facilitate the selection of less susceptible strains for aquaculture. Although marine toxin poisoning has been monitored in wild and farmed populations for decades, key aspects of the story remain elusive. How do toxins influence gene regulation over time? Do they disrupt or reshape the gut microbiome? Initial findings suggest that epigenetic changes may play a role, but whether these changes help or hinder remains an open question. Likewise, we know almost nothing about how gut microbes might protect—or fail to protect—the host from toxin exposure. This gap limits the development of effective feed-based solutions, such as photobiotics, prebiotics, or probiotics, which are widely used in aquaculture for disease prevention. Until we know more, the most effective strategy remains prevention: monitoring blooms, improving farm design, and reducing exposure to contaminated waters. But as our understanding of the complex interactions between toxins, hosts, and microbial allies grows, new strategies may emerge to support the resilience of aquatic animals in an increasingly toxic world.