Hydra is a freshwater organism often found attached to submerged objects like rocks or plants in ponds and slow-moving streams. These small, tubular animals, usually measuring between 2 to 20 millimeters when extended, might be mistaken for tiny plant life due to their sedentary nature. However, Hydra are animals with unique biological features that have captivated scientists.
Hydra’s Place in the Tree of Life
In biological classification, Hydra belongs to the Kingdom Animalia. This broad category encompasses all multicellular, eukaryotic organisms that obtain nutrients by consuming others. Below the kingdom level, organisms are grouped into phyla based on fundamental body plans. Hydra is classified under the Phylum Cnidaria.
Defining Traits of Cnidarians
Hydra is placed within the Phylum Cnidaria because it exhibits several defining characteristics common to this group. Cnidarians are aquatic invertebrates, with most living in marine environments, though Hydra is one of the few found in freshwater. A distinguishing feature of Cnidarians is their radial symmetry, meaning their body parts are arranged around a central axis, like spokes on a wheel.
Another hallmark of Cnidarians is the presence of specialized stinging cells called cnidocytes, which contain nematocysts. These cells are located on their tentacles and are used for capturing prey and defense by injecting toxins. Cnidarians are diploblastic, meaning their bodies develop from two primary embryonic layers: an outer ectoderm (epidermis) and an inner endoderm (gastrodermis), separated by a jelly-like layer called the mesoglea.
Cnidarians exhibit two basic body forms: the sessile polyp and the free-swimming medusa. Hydra primarily exists in the polyp form, which is cylindrical with a mouth surrounded by tentacles at one end and an adhesive foot at the other. Their nervous system is simple, organized as a decentralized nerve net distributed throughout the body, rather than a centralized brain. This nerve net allows for responses to stimuli and coordinated movements.
Biology of Hydra
Beyond its classification, Hydra possesses several biological attributes. One feature is its ability to regenerate lost body parts. If a Hydra is cut into pieces, each segment can regenerate into a complete, new organism within a few days. This regenerative capacity stems from its presence of active stem cells throughout its body column.
This cell renewal contributes to Hydra’s biological immortality. Unlike most animals that show signs of aging, Hydra, when maintained in optimal laboratory conditions, shows no senescence or decline with age. This characteristic makes Hydra a model organism for studying the mechanisms of aging and regeneration.
Hydra’s feeding mechanism is also notable. It extends its tentacles, which are covered in cnidocytes, to capture small aquatic invertebrates. Upon contact, the nematocysts discharge, paralyzing or killing the prey, which is then brought to the mouth and ingested into the gastrovascular cavity for digestion. The mouth is not a permanent structure; cells surrounding the mouth fuse when not feeding and re-open when prey is detected.