Haptocysts

Haptocysts are specialized stinging cells found in certain marine invertebrates, notably in some species of cnidarians and ctenophores. These cells are a variation of the more general nematocysts, which are characteristic of the phylum Cnidaria. While nematocysts typically discharge a harpoon-like structure to inject venom or entangle prey, haptocysts are adapted for adhesion rather than envenomation. They contain a coiled thread that, upon contact, everts and adheres to the surface it touches, effectively acting as a sticky trap. This adhesive function is crucial for securing prey, particularly small planktonic organisms, which are then drawn towards the predator's mouth. Haptocysts are thought to be more primitive than venom-injecting nematocysts and are found in a variety of cnidarian groups, including some hydroids and medusae. Their presence indicates a feeding strategy focused on passive capture through physical attachment. The precise mechanisms of haptocyst discharge and adhesion are still areas of active research, but it is understood to be a rapid, biomechanical process triggered by external stimuli.