The phylum Cnidaria includes familiar marine life such as jellyfish, sea anemones, and corals. These animals are defined by a specialized apparatus called the cnidocyte, a cell containing the stinging organelle known as the nematocyst. These microscopic structures are explosive, single-use harpoons that enable these animals to capture prey and defend against predators. The function of the cnidarian stinging cell hinges on a rapid physical mechanism and a potent venom cocktail. This article explores the structure of this cell and the high-speed biomechanics that make it one of the fastest movements in the natural world.
Anatomy of the Cnidocyte
The cnidocyte is the specialized cell that houses the nematocyst, which functions as a miniature pressure vessel. The nematocyst is a bulb-shaped capsule with a strong wall composed of a collagenous polymer, designed to withstand high internal pressure. Inside this capsule, a long, hollow, inverted tubule—the stinging thread—is tightly coiled. In its resting state, the tubule is folded back upon itself, occupying most of the capsule’s volume. A small cap, called the operculum, seals the opening. Extending from the outer surface of the cnidocyte is the cnidocil, a hair-like sensory appendage that acts as the mechano- and chemo-receptor trigger.
The Biomechanics of Discharge
The firing of the nematocyst is initiated when the cnidocil detects mechanical contact and chemical cues, which helps prevent accidental discharge. Activation of the cnidocil triggers a rapid change in the permeability of the capsule wall. This change causes a rapid influx of water into the capsule from the surrounding cell cytoplasm, a process driven by osmotic pressure.
The capsule’s contents, which are highly concentrated with ions and poly-gamma-glutamate polymers, create an internal osmotic potential that generates explosive pressure. This pressure can reach 150 atmospheres (or bar). The sudden surge of pressure forces the operculum open and drives the explosive eversion of the inverted thread tubule.
This eversion process is an instantaneous turning inside out, propelling the thread tube with force. High-speed studies have measured the entire discharge event to occur as fast as 700 nanoseconds. This speed generates an acceleration that can exceed 5,400,000 times the force of gravity, making nematocyst discharge one of the fastest biological processes known in nature.
Specialized Types and Roles
The universal firing mechanism is adapted across Cnidaria to create over 30 distinct types of nematocysts, each serving a specific function. The primary categories are penetrants, volents, and glutinants.
Penetrants (Stenoteles)
The penetrant, or stenotele, is designed to pierce the tough exoskeleton or skin of prey. These are typically the largest nematocysts, featuring barbs and stylets at the base of the thread tube to anchor the filament and inject venom.
Volents (Desmonemes)
The volent, or desmoneme, discharges a short, thick, spineless thread that does not penetrate. Instead, the thread rapidly coils tightly around the bristles or projections of small prey, serving to entangle the victim.
Glutinants
Glutinants secrete a sticky, adhesive substance upon discharge. They are primarily used for anchoring the cnidarian to a substrate or assisting in locomotion, providing temporary adhesion. The morphology of the thread tube in glutinants can vary, but their shared function is to provide a gripping surface.
Venom Delivery and Cell Replacement
The discharge of a penetrant nematocyst results in the injection of venom through the hollow thread into the target organism. Cnidarian venoms are composed of proteins and peptides, including pore-forming toxins and neurotoxins. Neurotoxins interfere with nerve cell function, quickly paralyzing or killing the prey.
Pore-forming toxins create holes in cell membranes, leading to rapid cell death and tissue damage. Once a nematocyst fires, it is rendered useless, making it a single-use biological weapon. The spent cnidocyte is either shed or digested by the organism.
The replacement process relies on multipotent interstitial stem cells, or I-cells, which are concentrated in the body column of many cnidarians. These stem cells continuously divide and differentiate into new cnidocytes. This process can take about 48 hours in some species before the new stinging cells migrate to their operational location, typically the tentacles.