Arthropods

Arthropods are organisms belonging to the kingdom Animalia and the phylum Arthropoda. They are a very diverse group of animals that includes but is far from limited to insects, crustaceans, spiders, scorpions, and centipedes. Arthropods make up the largest phylum in the world, with more numbers and species diversity than most other phyla. With more than 800,000 known species of arthropods, it is no wonder that they dominate the land and sea.

Characteristics of Arthropods

All Arthropods

• Jointed legs: Jointed legs allow arthropods to travel quickly regardless of their method of transportation. Whether swimming or scurrying across the ground, arthropods are speedy because of their jointed legs.
• A segmented body: An arthropod's body can be divided into one, two, or three main sections. If they have one section, it is called a trunk. If they have two sections, these are called the cephalothorax and abdomen. If they have three sections, the third section is the head.
• A hard exoskeleton: The exoskeleton of an arthropod is made of a strong polysaccharide called chitin. This hard shell protects the animal, retains moisture, and sometimes even plays a role in reproduction.
• Compound eyes: Compound eyes allow arthropods to take in their environment in a variety of ways. Arthropods can see through a very wide lens and use their compound eyes to detect the slightest of motions and perceive any depth.

Additional features make certain species of arthropods better suited for their specific habitat.

Terrestrial Arthropods

Land dwelling arthropods have a number of features that enable them to succeed in their environment.

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• Stinger: The stinger allows terrestrial arthropods to inject their prey with poison and paralyze, injure, or dissolve it into an edible liquid.
• Book Lungs/Trachea: In order to respirate air, terrestrial arthropods need a special set of lungs and/or trachea. Book lungs are layered organs that expand to take air in and contract to absorb it.
• Spinnerets: Terrestrial arthropods like spiders use spinnerets to produce webs. These can be used for shelter, prey entrapment, courtship, etc.

Aquatic Arthropods

Like land-dwelling arthropods, aquatic arthropods require adaptations that make living wholly or partially underwater possible.

• Gills: Just as book lungs allow for terrestrial respiration, gills allow for aquatic respiration. Marine arthropods use their gills to take in water and absorb its oxygen into their bloodstream.
• Cement Glands: Cement glands are unique adaptations that allow barnacles to adhere to nearly any surface. The adhesive secreted helps barnacles cling to rocks, ships, and other organisms and is so strong that scientists study its properties as inspiration for new materials.
• Swimmerets: Swimmerets allow some species of aquatic arthropods to swim, a movement that closely resembles running quickly through the water. In some species, a pair of swimmerets is used to inseminate mates.

Habitat and Distribution

Arthropods can survive in nearly any habitat. Different species can be found on dry land, water, or a combination of both. Aquatic arthropods are often found in coastal habitats like sandy beaches and intertidal areas but can even live comfortably in the deep sea. Horseshoe crabs are one of the oldest known species of marine arthropods. They have been known to inhabit both deep ocean waters and coastal sands. With as many species of arthropods as are living on the earth, it is far more difficult to find an environment or ecosystem where arthropods are not present than to find one where they are.

Reproduction

Arthropods usually reproduce sexually through external fertilization or, more uncommonly, asexually in cases where both male and female reproductive organs are present in one organism. External fertilization occurs when a male arthropod encases its sperm in a pouch that is deposited directly into a female arthropod or sent free to be taken up by a female.

The offspring of most species of arthropods begin as eggs, then hatch from these and enter a larval stage. In many arthropods, such as crabs, you can see these eggs attached to the hard abdomen. The larvae undergo metamorphosis, sometimes emerging from a cocoon during the pupal stage, to progress into adulthood. Water presents interesting challenges to the offspring of aquatic arthropods. Throughout this process of metamorphosis, young marine arthropods drift through the sea and can cover great distances in this manner. They have no control over where they end up before they reach adulthood.

Examples of Marine Arthropods

Examples of marine arthropods include:

• Lobsters
• Crabs (e.g., green crab, spider crab, hermit crab)
• Horseshoe crabs
• Sea spiders
• Barnacles
• Copepod
• Isopods
• Amphipods
• Skeleton shrimp
• Barnacles
• Krill

Sources

• “Arthropods.” Biology, Libretexts, 15 June 2019.
• “Arthropods: Underwater Knights in Shining Armor.” The Wonders of the Seas, Oceanic Research Group.
• Fleury, Bruce E. “Lab 5 - Arthropods.” Diversity, Tulane University.