SDI Open Water Scuba Diver Practice Exam 2026 – Complete Test Prep

The stinging cells found in some aquatic animals, particularly those within the phylum Cnidaria, such as jellyfish, anemones, and corals, are known as nematocysts. These specialized cells contain a tiny, coiled thread that can be rapidly expelled to inject venom into predators or prey, serving both for defense and to aid in capturing food.

Nematocysts are crucial for the survival of these organisms, as they allow them to subdue prey and deter potential threats. The mechanism of action is fascinating; when an animal comes into contact with the triggers on the nematocyst, it discharges, ejecting the thread and venom in a fraction of a second, effectively immobilizing small fish and invertebrates.

In contrast, stomata are pores on the surfaces of plants that facilitate gas exchange, neurons are the fundamental units of the nervous system responsible for transmitting signals, and chloroplasts are the organelles within plant cells that conduct photosynthesis. Each of these serves different biological functions entirely unrelated to the stinging capabilities of aquatic animals.