The Hydra is a small freshwater invertebrate belonging to the phylum Cnidaria. This organism has a tubular body structure, typically ranging from 2 to 30 millimeters when fully extended, and is characterized by a ring of delicate tentacles surrounding a single opening that serves as both the mouth and the anus. Hydra is a predator that remains attached to submerged surfaces like rocks or vegetation in ponds and slow-moving streams. Its carnivorous diet consists entirely of live, small aquatic prey that it captures from the water column.
The Hunting Mechanism: Tentacles and Stinging Cells
The capture of prey begins when the Hydra extends its thin, mobile tentacles, which can stretch four to five times the length of its body. These tentacles are densely covered with specialized stinging cells called cnidocytes, which house miniature harpoon-like structures known as nematocysts.
When a small organism brushes against a tentacle, it triggers the explosive discharge of the nematocysts. One type, the stenotele, fires a dart-like thread that injects neurotoxins into the prey, causing immediate paralysis. Other types, like the desmonemes, discharge threads that coil around and entangle the victim, securing it to the tentacle.
The prey is quickly immobilized and secured. Within seconds of contact, the remaining tentacles join the attack, and the captured food is then maneuvered toward the mouth aperture, which opens to receive the meal. The prey is engulfed whole into the body cavity, and the process from initial contact to ingestion typically takes only a few minutes.
Specific Prey: Micro-Crustaceans and Larvae
The primary diet of the Hydra is composed of small, live aquatic invertebrates. Micro-crustaceans form a substantial part of its food intake, including common organisms like Daphnia (water fleas) and Cyclops (copepods). These tiny crustaceans are abundant in the Hydra’s habitat and are easily subdued by its neurotoxic sting.
The diet also includes various insect larvae, such as those from mosquitoes, and small aquatic worms like rotifers. The size of the prey is limited by what the Hydra can fit through its single mouth opening. However, the organism has a remarkable ability to stretch its body wall, allowing it to engulf and digest prey that is more than twice its own size.
The ingested food is held in the gastrovascular cavity, where digestion begins, a process that can take several days. Once nutrients are absorbed, indigestible remains are expelled back out through the mouth aperture via body contractions. The reliance on these small, motile organisms highlights the Hydra’s role as a specialized predator of the zooplankton community.
Dietary Flexibility and Selectivity
Despite its potent hunting mechanism, the Hydra exhibits selectivity in its feeding behavior. A key factor in stimulating a feeding response is the presence of chemical cues, specifically the release of reduced glutathione from the injured tissues of a captured organism. This chemical signal triggers the mouth to open and initiates the ingestion process.
This requirement for chemical stimulation means that Hydra rarely consumes dead or completely immobile prey, as such food sources do not typically release the necessary feeding-reflex activator. Beyond the chemical trigger, the physical characteristics of the prey, such as a hard carapace, can also influence whether a Hydra successfully captures and eats it. Some larger species of micro-crustaceans with hard outer shells may be rejected even if they are within a manageable size range.
The Hydra can also survive extended periods of starvation, demonstrating a high degree of metabolic flexibility. However, active growth and reproduction require the regular consumption of live prey. While size is a factor, the effectiveness of the Hydra’s stinging cells and the prey’s swimming efficiency are equally important in determining a successful meal.