Cnidarians, a diverse group of aquatic invertebrates including jellyfish, sea anemones, and corals, populate marine and freshwater environments globally. A common question arises regarding their capacity for movement and the underlying structures that facilitate it: Do cnidarians possess muscles?
The Presence of Muscle-Like Cells
Cnidarians do not have true mesodermal muscle tissue, unlike more complex animals like vertebrates. Instead, they utilize specialized epitheliomuscular cells that perform muscle-like functions. These cells combine epithelial properties, forming body linings, with contractile elements. Epitheliomuscular cells are the primary contractile cell type across most cnidarian species. This arrangement distinguishes cnidarian movement from that of bilaterians, which rely on distinct muscle tissues.
Epitheliomuscular cells are integrated directly into the two primary tissue layers of cnidarians: the ectoderm (outer layer) and the endoderm (inner layer, also called gastrodermis). While epitheliomuscular cells are the predominant type, some cnidarians, particularly certain medusae, also possess striated epitheliomuscular cells. A few highly specialized cnidarians, such as some parasitic groups, even have smooth muscle cells embedded within their mesoglea, completely separate from the epithelia.
How Cnidarian Muscle Cells Are Structured
Cnidarian epitheliomuscular cells have distinct ends. Myofibrils, the contractile fibers within these cells, extend from the basal side, aligning within the extracellular matrix to provide contractile properties. These myofibrils are organized into sheets. In polyps, such as sea anemones, ectodermal muscles are typically oriented lengthwise along the body and tentacles, while endodermal muscles are usually circular.
This orthogonal arrangement of longitudinal and circular fibers is important for their movement. For instance, in Hydra, ectodermal epitheliomuscular cells contain longitudinally oriented myofibrils, while endodermal cells have circularly oriented ones. In medusae, muscles are ectodermal and primarily found on the subumbrellar surface, organized into both circular and radial tracts. The specific arrangement and type of epitheliomuscular cells can vary significantly across different cnidarian groups, reflecting their diverse forms and lifestyles.
Mechanisms of Cnidarian Movement
Cnidarian movement is driven by the contraction and relaxation of epitheliomuscular cells, working with a hydrostatic skeleton. This skeleton is formed by the water-filled gastrovascular cavity, which acts as an incompressible fluid. When epitheliomuscular cells contract, they exert pressure on this fluid, changing the animal’s shape.
For example, in jellyfish, circular muscle contraction in the bell margin expels water from the subumbrellar cavity, generating a jet-propulsion force. The elasticity of the mesoglea, the jelly-like substance between cell layers, helps the bell return to its original shape, drawing in water for the next propulsion cycle. Sea anemones use similar principles; longitudinal muscle contraction shortens the body, while circular muscle contraction against the hydrostatic skeleton causes elongation.
The Purpose of Cnidarian Movement
Movement in cnidarians serves several biological functions, enabling their survival and reproduction. Locomotion is one purpose, particularly for free-swimming medusae like jellyfish. Their bell pulsations allow them to move through the water column, albeit often slowly, in search of food or to escape predators. Some jellyfish can adjust their buoyancy to rise or sink, further aiding their navigation.
Feeding is another primary driver of movement. Cnidarians use their contractile tentacles, armed with stinging cnidocytes, to capture prey. The epitheliomuscular cells allow tentacles to extend, contract, and bend, bringing captured food to the mouth. Defense also relies on movement, as seen when sea anemones rapidly retract their tentacles or entire bodies in response to a threat. Some can even detach and move by pedal locomotion or eject defensive structures. Finally, movement plays a role in reproduction, particularly in the dispersal of gametes into the water column for external fertilization, or for larvae to settle in suitable habitats.