Animals With Exoskeletons: A Fascinating Overview
Exoskeletons, also known as external skeletons, provide support and protection for many invertebrates. Unlike vertebrates that have internal skeletal structures, exoskeletons are on the outside of an organism’s body. Exoskeletons are made up of a tough external layer called the cuticle, which is comprised of chitin, calcium carbonate, or silica. Let’s explore some of the most interesting animals that have exoskeletons and why this external body armor is so beneficial.
Introduction
Exoskeletons serve a variety of crucial functions for the animals that have them. They provide structural support for the body, act as a protective shield against predators and other threats in the environment, allow for muscle attachment to enable locomotion, and prevent water loss in terrestrial organisms. Exoskeletons come in many shapes and sizes, suited to the lifestyles of the diverse animals that bear them.
Some of the major groups of animals with exoskeletons include:
- Arthropods – This phylum includes insects, crustaceans, arachnids, and others. Arthropod exoskeletons contain chitin and are shed through molting as the animal grows.
- Molluscs – Molluscs like snails, clams, and chitons have a hard exoskeleton made of calcium carbonate.
- Echinoderms – Animals like sea stars, sea urchins, and sand dollars have an endoskeleton covered by an exoskeleton composed of calcium carbonate plates.
- Microscopic organisms – Many single-celled organisms, like radiolarians, diatoms, and foraminifera, produce exquisite silica-based exoskeletons.
In this article, we’ll take a closer look at some of the most captivating animals with exoskeletons and explore how this body plan helps them survive and thrive. Understanding exoskeleton form and function provides insight into the diversity of life on Earth.
Arthropod Exoskeletons
Arthropods exhibit the greatest diversity of exoskeleton forms in the animal kingdom. With over 1 million described species, arthropods include insects, arachnids, crustaceans, and other invertebrates united by their segmented bodies and jointed appendages.
The arthropod exoskeleton is composed of chitin, a tough, flexible nitrogen-based polysaccharide. Chitin is secreted by the epidermal cells and hardens to form the cuticle, sometimes strengthened by calcium carbonate. The cuticle is composed of layers including:
- The epicuticle – a thin outer waxy layer that prevents water loss.
- The exocuticle – a thicker, tough layer containing chitin for strength.
- The endocuticle – a softer layer allowing for flexibility.
This multi-layered exoskeleton provides remarkable protection for arthropods. Let’s look at some specific examples:
Insects
Insects, the most diverse group of animals on Earth, have adapted exoskeletons to suit a staggering variety of ecological niches. Their exoskeleton wraps each body segment and joints between segments allow for flexibility. Wings and appendages are also integrated into the exoskeleton.
Some insects like beetles have an especially heavy exoskeleton for defense. Butterflies and moths develop scales attached to the exoskeleton to repel water and for camouflage. The honey bee exoskeleton even includes biological plastic polymers that generate electricity to detect floral resources.
Molting allows insects to grow by shedding their exoskeleton. Some aquatic larval insects have a dual exoskeleton – an outer layer equipped for underwater life that is shed as the terrestrial adult form emerges.
Arachnids
Arachnids like spiders, scorpions, ticks, and mites possess four pairs of walking legs and segmented bodies armored by chitinous plates. The exoskeleton sections overlap for flexibility but provide remarkable toughness.
Scorpions have thick ventral exoskeleton plates that defend against predators. The folded plates on a spider’s abdomen allow for expansion when the spider digests food. Spider silk also emerges from specialized abdominal exoskeleton structures called spinnerets.
For arachnids, molting allows growth and even regeneration of lost legs or appendages. Before molting, new cuticle is secreted underneath to enable a larger exoskeleton to form.
Crustaceans
Crustaceans like lobsters, crabs, shrimp, and barnacles have tough exoskeletons containing 20-70% chitin reinforced by calcium carbonate deposits. This armor protects them from predators and abrasive environmental conditions in ocean, freshwater, and terrestrial habitats.
The crustacean exoskeleton is shed through complex molting, where the animal extracts itself from the old shell. Molting leaves crustaceans vulnerable until their new exoskeleton hardens. Some crustaceans camouflage themselves with debris or hide during this dangerous period.
Locomotion results from contractions of muscles attached to the inner side of the exoskeleton plates. Crustaceans can maneuver impressively well – crabs can walk sideways, shrimp can dart backwards, and barnacles are permanently cemented headfirst to rocks by their exoskeletons.
The diverse environments inhabited by crustaceans are facilitated by their tough, pliable, and adaptable exoskeletons.
Mollusc Shells
Most molluscs like gastropods (snails and slugs), bivalves (clams, oysters), and cephalopods (squid, octopuses) have an exoskeleton called a shell. The mollusc shell is composed of calcium carbonate secreted by the mantle, a specialized organ.
Shells exhibit tremendous diversity in size, shape, color, and features based on habitat and lifestyle. Let’s overview some examples:
Gastropods
Gastropod shells coil in a spiral shape, providing defense for soft snail and slug bodies. Their shells vary widely – some tropical snails have pointed spikes whilesmooth, streamlined shells aid movement through water or sand. Slugs have greatly reduced internal shells.
Terrestrial snails secrete mucus along the shell edge enabling mobility. Buoyant shells allow aquatic snails to float. Some snails have operculum lid structures to seal their shell openings against threats.
Bivalves
Clams, oysters, mussels, and scallops are bivalves, meaning their shells are hinged and can open and close. Their shells are bilaterally symmetrical, with ridges, scales, and fluting for strength.
Bivalve shells defend against predators like starfish, which use their limbs to pry shells apart. Burrowing bivalves have streamlined, vertical shells while non-burrowers are laterally flattened. The giant clam’s heavy shell deters would-be hunters.
Some bivalves can quickly snap their shells shut when threatened, propelled by a strong adductor muscle. Scallops can even jet propel themselves to escape danger by rapidly snapping their valves.
Cephalopods
Octopuses, squid, nautiluses, and cuttlefish have internal shells for buoyancy and structure. Squid shells are long, narrow, and plume-shaped while nautilus shells are coiled and chambered. Cuttlebones provide cuttlefish with buoyancy control.
The ancestral external shells of cephalopods likely hampered their movement. Through evolution, today’s cephalopods have predominantly internal, reduced, or even absent shells, allowing for incredible swimming capabilities and predation.
While cephalopod shells are reduced or internal, these animals can still rapidly change color, texture, and shape for camouflage, defense, and communication – functions the external shell served for their ancestors.
Echinoderm Endoskeletons
While echinoderms like sea stars, sea urchins, and sand dollars appear covered by a hard exterior, they actually have an endoskeleton covered by a thin epidermis. Nonetheless, this endoskeletal armor functions much like an exoskeleton.
The echinoderm endoskeleton is composed of calcite plates and granules suspended in a skin-like membrane. Ossicles (plates), spines, and pedicellariae (pincers) provide protection against predators.
This endoskeletal armor is flexible, supported by hydraulic pressure within the animals. Echinoderms can thereby stiffen or relax their endoskeletons as needed for both defense and locomotion.
Sea urchins can point their mobile spines towards disturbances to deter potential threats. Sand dollars half-bury themselves in sediment, using their flattened shell for camouflage. Brittle stars can contort the flexible joints between their ossicles to squeeze into crevices.
The echinoderm calcite endoskeleton therefore enables both formidable protection and agile movement.
Single-Celled Silica Exoskeletons
Many marine and freshwater single-celled organisms called protists produce exquisite glassy exoskeletons composed of silica. Silica (silicon dioxide) is derived from their aquatic environment. Let’s look at some examples:
Radiolarians
These planktonic protists produce intricately sculpted, porous silica skeletons laced with internal spines. Different radiolarian species generate circular, bulbous, triangular, spiral, and other skeletal shapes, resembling abstract art.
Their hollow skeletal framework floats in the upper ocean, aiding buoyancy while minimizing weight. A central capsule inside the skeleton contains the cell body. The latticed shell likely aids nutrient absorption and protects against some predators.
Diatoms
Diatoms contain chloroplasts and require sunlight, so their glassy silica cell walls are transparent or translucent. There are over 100,000 species, identified by their distinct frustule (cell wall) designs ranging from cylinders to prisms.
When diatoms divide, each new cell retains one part of the silica frustule, secreting a new complimentary portion. Over generations, diatoms can reduce in size, producing progressively smaller but intricately patterned glass exoskeletons.
Foraminifera
These amoeba-like protists produce seashell-like chambers of porous silica that enable buoyancy. New chambers are added as the single-celled foraminifera grow. Their delicate shells can be preserved in marine sediments for millions of years.
Analyzing accumulation rates of foraminifera shells in seafloor sediments provides insights into past marine environments. Their diversity and rapid evolution also make them valuable index fossils for dating geological sediments.
These single-celled organisms create architectural masterpieces, demonstrating how silica skeletons integrate form and function.
Benefits of Exoskeletons
After exploring these examples, the advantages conferred by exoskeletons for growth, defense, and lifestyle are clear. Let’s outline some of their key benefits:
- Structural support – Exoskeletons provide shape and resist compressive forces for organisms with soft bodies.
- Protection – Armor plating, spines, shells, and other exoskeletal features deter predators and abrasive conditions.
- Attachment for locomotion – Muscles anchor to the exoskeleton, enabling coordinated movement.
- Reduced water loss – The waterproof exoskeleton prevents desiccation in air.
- Streamlining – Hydrodynamic exoskeleton shapes allow rapid water movement.
- Camouflage – Exoskeleton patterns, shapes, and colors provide concealment from predators and prey.
- Communication – Color changes to the exoskeleton can signal territoriality, mating readiness, or aggression.
- Buoyancy – Gas-filled or porous exoskeletons allow flotation.
- Environmental tolerance – Exoskeleton materials like silica and calcium carbonate aid survival in different aquatic conditions.
Billions of years of evolution have shaped a vast diversity of exoskeletons that allow animals to thrive in aquatic, aerial, and terrestrial realms.
Exoskeleton Constraints
Despite their advantages, exoskeletons also impose some limitations:
- Growth limitation – Most exoskeletons must be periodically molted to enable growth. Molting leaves the animal vulnerable.
- Metabolic investment – Secreting an exoskeleton requires substantial energy expenditure.
- Reduced flexibility – Thick, inflexible exoskeletons constrain agile movement in some cases. Soft-bodied cephalopods evolved away from external shells for this reason.
- Dehydration – Terrestrial exoskeletons require adaptations to retain water and prevent desiccation.
- Buoyancy – Heavy protective shells make floating difficult for some aquatic organisms.
- Maneuverability – Exoskeleton spines and shells sometimes hamper speed and agility.
Despite these constraints, exoskeletons transform soft-bodied organisms into armored tanks. Their diversity enables survival in realms far beyond what soft bodies could endure alone.
Conclusion
Exoskeletons exemplify the wondrous innovations of natural selection over hundreds of millions of years. Chitin, calcium carbonate, silica, and other materials combine with inspired morphology to provide multifunctional protection, support, and adaptation.
Examining exoskeletons across the tree of life illustrates how evolution balances robust defense with flexibility, buoyancy with ballast, and streamlining with armor plating. From the microscopic symmetries of diatoms to the massive shells of giant clams, exoskeletons provide fascination and utility in equal measure.
Further explorations of exoskeletons across time and environments will continue to reveal nature’s ingenuity. Whether tiny or tremendous, delicate or durable, exoskeletons profoundly expand life’s diversity and adaptability.