Hydra are small, freshwater invertebrates from the phylum Cnidaria, a group that also includes jellyfish and corals. Their survival depends on microscopic stinging structures housed in specialized cells on the animal’s tentacles. These organelles, called nematocysts, are used for capturing food and defending against threats. The simple, radially symmetric body of a Hydra has a foot-like basal disc for attachment and a mouth surrounded by tentacles armed with these stinging organelles.
Anatomy of Hydra’s Stinging Cells
The stinging capability of a Hydra originates from specialized cells called cnidocytes, which are most abundant on its tentacles. Each cnidocyte acts as a housing unit for a single nematocyst. These cells develop from stem cells in the Hydra’s body column and migrate to the tentacles, where they are arranged in clusters called batteries.
Inside every cnidocyte is the nematocyst capsule, a bulb-shaped structure made of collagen-like proteins. The capsule contains a long, coiled, and hollow thread that, in many cases, is lined with sharp barbs or spines. The entire apparatus is sealed by a small lid called an operculum. Protruding from the surface of the cnidocyte is a hair-like sensory trigger known as a cnidocil, which acts as the firing mechanism.
The Firing Mechanism of Nematocysts
The discharge of a nematocyst is a remarkably fast physical process initiated when the cnidocil receives a stimulus. This trigger is both mechanical and chemical; it requires physical contact, but the response is often sensitized by chemicals released by nearby prey, which lowers the firing threshold. This dual-sensing mechanism prevents the Hydra from wasting its single-use nematocysts on inanimate objects.
Once triggered, the process relies on a rapid change in osmotic pressure. The nematocyst capsule wall is permeable to water, and its contents are held at an extremely high internal pressure. The firing signal causes a massive influx of water into the capsule, driven by the release of calcium ions from the capsule into the cnidocyte’s cytoplasm, creating a steep concentration gradient.
This surge of water increases the intracapsular pressure, forcing the operculum to open. The stored potential energy is released, propelling the coiled thread outwards with an acceleration reaching up to 40,000 times the force of gravity. The thread everts, or turns inside out, as it fires in a process that takes only a few microseconds, making it one of the fastest known cellular events.
Functions of Nematocysts in Hydra’s Life
The primary purpose of nematocysts is to secure food and provide defense. As carnivores, Hydra prey on small aquatic organisms like water fleas (Daphnia) or brine shrimp (Artemia). When prey brushes against a tentacle, nematocysts fire to pierce its outer layer and inject paralyzing neurotoxins, which quickly immobilize the victim.
Other types of nematocysts are not for piercing but for ensnaring. Their threads are sticky or function like a corkscrew, wrapping around the prey’s appendages to hold it fast. Once subdued, the Hydra uses its tentacles to maneuver the food into its mouth for digestion.
Beyond predation, nematocysts serve as a defense mechanism. When threatened by a potential predator, a Hydra can discharge its stinging cells to deter the attacker. The injected toxins can cause pain or irritation, convincing most predators to leave it alone. A secondary function for some nematocyst types is temporary anchorage, using sticky threads to help the Hydra adhere to surfaces as it moves.
Different Types of Nematocysts in Hydra
A Hydra possesses a specialized toolkit of at least four different types of nematocysts, each with a structure tailored to a specific function. This diversity allows the animal to respond effectively to different situations. The main types found in Hydra are stenoteles, desmonemes, and two kinds of isorhizas, which are organized into specific patterns within the tentacle batteries.
- Stenoteles: The largest and most complex type, they function as the primary weapon for piercing prey. They feature a prominent, barbed stylet at the base of the thread designed to puncture the tough exoskeletons of small crustaceans. Once the stylet penetrates, the rest of the hollow tubule follows, delivering potent neurotoxins to paralyze the victim.
- Desmonemes: Smaller and lacking a piercing stylet, their thread is a short, thick, coiled filament that, upon discharge, wraps tightly around the bristles or appendages of prey, acting as a lasso. This entangling action is purely mechanical and is used to hold the struggling organism in place.
- Holotrichous isorhizas: These have a thread armed with a dense spiral of small spines, likely used in both defense and adhering to prey.
- Atrichous isorhizas: This type lacks spines and secretes a sticky substance, which helps the Hydra anchor itself to surfaces during movement.