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Animals have different types of muscle fibres, which enable them to move. Some animals, such as jellyfish, have muscles but move slowly, while others, like cheetahs, move at a much faster pace. Muscle contractions are the basis of movement for many species, but not all. Some animals, like sea sponges, can move without muscles as they evolved from a different branch.
| Characteristics | Values |
|---|---|
| Do animals have muscles? | Muscle contractions are the basis of movement in many, but not all, species. Some animal groups don't have any muscles at all, as they branched off from the evolutionary path before muscle cells evolved. |
| Which animals don't have muscles? | Sea sponges are an example of animals that don't have muscles but can still move. |
| How do animals without muscles move? | It is not yet known which cells in sponges are contracting to allow movement. However, there is evidence that sponge epithelial cells and the muscle cells of other animals share a common contractile cellular predecessor. |
| Do all animals with muscles have the same type? | No, animals have different muscle fibre types: slow fibres with a low maximum velocity of shortening (Vmax) and fast fibres with a high Vmax. |
| How do different muscle fibre types benefit animals? | Slow fibres are used during slow locomotion to generate peak mechanical power and efficiency, while fast fibres are used to power maximal movements. |
| Do animals with muscles use different fibres for different movements? | It is not known whether animals use different fibres at shortening velocities that are optimal for mechanical power production and efficiency. |
| Do animals with exoskeletons have muscles? | Yes, for example, jellyfish have muscles. |
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What You'll Learn
Some animals move without muscles
Muscle contractions are the basis of movement in many, but not all, species. Some animal groups don't have any muscles at all, as they branched off from the evolutionary path before muscle cells evolved. These include sea sponges, which are able to contract without muscles.
Sponges have long been known to be able to contract, with Aristotle describing this ability as far back as 350 BC. However, the cells responsible for these contractions have been debated for over a hundred years. Far spindle-shaped cells in the tissue of sponges, as well as epithelial cells, were thought to be possible candidates.
In 2011, a group of scientists led by Dr. Michael Nickel of Friedrich Schiller University Jena in Germany used 3D volumetric analysis to identify the cells responsible for sponge contractions. They found that the inner and outer surfaces of sponges—and therefore the epithelial cells, or pinacozytes—are responsible for the strong body contractions of the sponges.
Nickel and his team also found evidence that sponge epithelial cells and the muscle cells of other animals share a common contractile cellular predecessor. In the future, they hope to test this hypothesis using genome and gene expression-related data.
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Muscle fibres and contraction
Muscle contractions are the basis of movement in many, but not all, species. Some animals, such as sea sponges, can contract their bodies without the use of muscles as they branched off from the evolutionary path before muscle cells evolved.
The physiological concept of muscle contraction is based on two variables: length and tension. Muscle shortening and muscle contraction are not synonymous. Tension within a muscle can be produced without changes in its length, for example, when holding a dumbbell in the same position.
Mammals have three types of muscles: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and give the body structure and strength. Cardiac muscles comprise the walls of the heart, allowing blood to be pumped through the vasculature.
The striated muscles in our bodies are made up of many individual muscle fibres. Inside these muscle fibres are smaller units called myofibrils, which are the basic functional organelles in the skeletal muscle system. Myofibrils are made of parallel thin and thick filaments that are arranged longitudinally in small units known as sarcomeres, which give the muscle a striated appearance under microscopy. The thick filaments are made from the protein myosin, which has one pair of heavy chains and two pairs of light chains. The two heavy chains of myosin twist around each other to make the helical tail of the myosin, whereas the light chains interact with the heavy chains to form the two heads of the myosin at the other end. The thin filament is mainly composed of three proteins: actin, tropomyosin, and troponin.
Actin is a globular protein that combines with other actin globules to form two intertwined strands with positive and negative ends. The double-stranded actin filaments are covered by tropomyosin, which blocks the interaction between myosin and actin when the muscle is inactive. Troponin is a three-protein complex located along the actin filaments next to tropomyosin. Troponin I serves the same purpose as tropomyosin in stopping the actin-myosin interaction by blocking the myosin-binding sites. Troponin C binds calcium to initiate muscle contraction.
The complex process leading to muscle contraction, called excitation-contraction coupling, begins when an action potential causes depolarization in the myocyte membrane. The depolarization is spread via the transverse (T) tubules, which help spread depolarization signals to the entire muscle fibre. Depolarization of the T tubules causes a conformational change in the dihydropyridine receptors, which causes the opening of nearby ryanodine receptors on the sarcoplasmic reticulum (SR), the storage site for calcium within muscle cells.
Cross-bridge cycling begins when ATP binds to an ATP-binding domain on the myosin head. Myosin dissociates from the actin, breaking the cross-bridge. ATP is then hydrolyzed into ADP and P, which causes the myosin heads to change conformation and move toward the positive end of the actin, cocking the myosin head. The phosphate is released, and the ADP-bound myosin binds to a new location on the actin filament. ADP is then released, which causes the myosin to return to its original position, pulling on the actin filament and causing the sarcomere (and, therefore, the muscle fibre) to contract. These cycles continue until calcium levels in the myocyte fall, causing tropomyosin to cover the actin filaments' myosin-binding sites.
In invertebrates such as annelids, mollusks, and nematodes, obliquely striated muscles contain bands of thick and thin filaments that are arranged helically rather than transversely, like in vertebrate skeletal or cardiac muscles. In bivalves, these muscles can maintain tension over long periods without using too much energy, allowing them to keep their shells closed.
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Muscle structure in molluscs
Molluscs are a phylum of protostomic invertebrate animals that exhibit great morphological diversity. They are the second-largest animal phylum, with around 76,000 recognised extant species. The body of a mollusc has a ventral muscular foot, which is adapted for different purposes, including locomotion, grasping, burrowing, and feeding. The foot is composed of muscle fibres running in all directions, and it is used to propel the mollusc forward through waves of muscular contraction. In gastropods, the foot is flat and used for crawling, while in bivalves, it is bladelike and pointed for digging.
The shell of a mollusc is also moved by muscles. The shell muscle of the abalone Haliotis, for example, connects the domed shell to the mollusc's adhesive foot. When the muscle shortens, the shell is pulled down over the animal for protection. When the muscle lengthens, the shell is raised, allowing respiratory water currents to circulate.
In addition to the foot and shell muscles, molluscs also have complex digestive systems that utilise muscle-powered "hairs" called cilia. These cilia play important roles in the mollusc's feeding system. Molluscs also have nerve cords served by ganglia, which are bundles of nerves. The visceral cords serve the internal organs, while the pedal cords serve the foot.
While muscle contractions are the basis of movement in many animal species, there are some groups, such as sea sponges, that do not have muscles but can still move through cell contractions.
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Muscle fibre types
Slow oxidative fibres, also known as slow-twitch fibres, contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They produce low-power contractions over long periods and are slow to fatigue. The legs and thighs of a turkey contain slow oxidative fibres, which is why the meat in these parts is dark.
Fast oxidative fibres, or fast-twitch fibres, have relatively fast contractions and primarily use aerobic respiration to generate ATP. They produce higher-tension contractions than slow oxidative fibres.
Fast glycolytic fibres, also known as fast-twitch fibres, have relatively fast contractions and primarily use anaerobic glycolysis to generate ATP. They have a large diameter and possess large volumes of glycogen, which is used to generate ATP quickly. Due to their reliance on anaerobic metabolism, these fibres do not possess a substantial number of mitochondria, resulting in a limited capillary supply and a white coloration for muscles containing large numbers of these fibres. Fast glycolytic fibres fatigue quickly, allowing them to be used only for short periods. However, during these short periods, the fibres can produce rapid, forceful contractions associated with quick, powerful movements.
The number of slow and fast-twitch fibres in the body varies between individuals and is determined by genetics. Muscle fibres can adapt to changing demands by changing size or fibre type composition. This plasticity serves as the basis for physical therapy interventions designed to increase a patient's force development or endurance. For example, endurance training can modify slow fibres to make them more efficient by producing more mitochondria to enable more aerobic metabolism and ATP production.
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Muscles in soft-bodied animals
Soft-bodied animals typically do not have a skeleton, and thus their movement is not produced by lever action. However, they do have muscles. For example, the simple worm-like animal has longitudinal muscle fibres that run lengthwise along the body, and circular fibres that encircle it. The body contents are liquids or tissues that can be deformed into different shapes, but they maintain a constant volume. If the longitudinal muscles contract and the body shortens, it must widen to accommodate its volume. If the circular muscles contract and the body thins, it must lengthen.
The principle involved is sometimes called the principle of the hydrostatic skeleton. This principle can apply to individual muscles as well if their fibres run in several directions. For example, a muscle that has some fibres running longitudinally and others running circularly and/or radially will become shorter and fatter when the longitudinal fibres shorten and will become longer and thinner when the circular and radial fibres shorten. There are many examples of muscle structure like this in the molluscs. One such example is the shell muscle of the abalone Haliotis, which connects the domed shell of the animal to its adhesive foot. When the muscle shortens, with the foot attached to a rock, the shell is pulled down over the animal to protect it.
Some soft-bodied animals do have a functional skeleton maintained by body fluid hydrostatics, known as a hydroskeleton, such as earthworms, jellyfish, tapeworms, squids, and several invertebrates. Many of these have hardened teeth that allow them to chew, bite and burrow despite their soft bodies. The heaviest soft-bodied organisms are likely the giant squids, with a maximum weight of 275 kilograms. The longest animal on record is also thought to be a soft-bodied organism, a 55-metre-long thread-like bootlace worm, Lineus longissimus, found on a Scottish beach in 1864.
Hydras, jellyfishes, and sea anemones are part of the phylum Cnidaria. They have two main body forms: the cylindrical tentacled polyp and the bell-shaped (or inverted saucer-shaped) medusa. Hydras are some of the simplest multicellular animals to have muscle. They are hollow, cylindrical, freshwater creatures about 10 mm long. One end attaches to a plant or some other support, and the other end is free and has a mouth surrounded by tentacles. The body wall consists of two layers of cells with a middle gelatinous layer called mesoglea. In hydras and other two-layered animals, one kind of cell serves as both muscle and epithelial cells.
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Frequently asked questions
Yes, animals have muscles. Muscle contractions are the basis of movement in many, but not all, species.
Muscle fibres can run longitudinally, circularly, or radially. When longitudinal fibres shorten, the muscle becomes shorter and fatter. When circular and radial fibres shorten, the muscle becomes longer and thinner.
No, some animal groups, such as sea sponges, do not have muscles as they branched off from the evolutionary path before muscle cells evolved.
Sea sponges, for example, are able to contract their bodies without the use of muscles. However, the specific cells responsible for this movement are unknown.
Yes, animals have different muscle fibre types. Some have slow fibres with a low maximum velocity of shortening (Vmax), while others have fast fibres with a high Vmax.
