Hydra is a small, freshwater invertebrate belonging to the phylum Cnidaria, which also includes jellyfish and sea anemones. This tiny organism has a simple, radially symmetrical body resembling a translucent, hollow tube, typically measuring up to 30 millimeters when fully extended. The body is anchored by a basal disc, and the opposite end features a single opening that functions as both a mouth and an anus. A ring of slender, mobile tentacles surrounds this opening, central to its lifestyle as a passive predator.
Primary Components of the Natural Diet
Hydra are carnivorous predators that utilize a “sit-and-wait” strategy to capture food floating past them in the water column. Their natural diet consists primarily of small aquatic invertebrates, collectively known as zooplankton. These prey items are generally microscopic or only slightly larger than the Hydra itself. Common prey species include water fleas, such as Daphnia, various copepods like Cyclops, and the larvae of small aquatic insects or crustaceans.
The Tentacles and Stinging Cells
Prey capture is executed by the Hydra’s extensible tentacles, which can stretch to several times the length of the body column. Each tentacle is densely covered with specialized cells called cnidocytes. These cells house a coiled, harpoon-like structure known as a nematocyst.
Upon physical contact with a potential meal, a minute trigger hair on the cnidocyte causes an explosive discharge of the nematocyst. This action is driven by hydrostatic pressure, rapidly injecting a barbed thread containing a neurotoxin that paralyzes or kills the captured organism. Specific types of nematocysts, like stenoteles, pierce the prey, while others, such as desmonemes, coil around and entangle it, securing the meal.
Feeding Requirements and Digestion
Once the prey is immobilized, the tentacles contract and bend inward, maneuvering the food toward the Hydra’s single mouth opening. The organism can stretch its body wall to engulf prey much larger than its resting size. The food then enters the gastrovascular cavity, a sac-like space lined with cells that begin the digestion process.
Digestion occurs in two distinct phases: extracellular and intracellular. Gland cells within the gastrodermis first secrete digestive enzymes into the cavity, partially breaking down the prey. Flagella-bearing cells mix the contents, and the partially digested food particles are engulfed by other cells lining the cavity. Intracellular digestion then occurs inside food vacuoles within these cells, where nutrients are fully broken down and absorbed. Undigested waste materials are expelled back out through the mouth opening.
Feeding Hydra in Captivity
For those culturing Hydra, providing a consistent supply of live food is required to maintain a healthy population. The most common and effective food source is newly hatched brine shrimp (Artemia nauplii), which are easy to hatch and harvest. Other suitable captive foods include live Daphnia or microworms.
Hydra should be fed daily or every few days, depending on density and water temperature. Avoid overfeeding, as uneaten live food can quickly decompose and foul the water quality. To prevent this, remove any uneaten food from the container after about an hour to maintain a clean environment.