Crustaceans are a massive and ancient subphylum of the animal kingdom, belonging to the phylum Arthropoda, which also includes insects and spiders. This group is remarkably diverse, inhabiting nearly every environment on Earth where water is present, from the deepest ocean trenches to high-altitude lakes. Understanding the number of crustacean species allows us to grasp the magnitude of life’s diversity and the challenges of cataloging it all.
Defining the Crustacean Subphylum
Crustaceans are a distinctive subphylum within the Arthropoda, characterized by shared physical features that separate them from insects and other groups. A defining trait is the hard, external skeleton, or exoskeleton, which provides protection and support for the body. This structure must be periodically shed and regrown in a process known as molting to allow for growth.
The head region is revealing, as all crustaceans possess two pairs of antennae: the antennules and the antennae. Unlike other arthropods, they possess specialized, two-branched appendages, known as biramous limbs, which can be modified for functions like walking, swimming, or feeding. The earliest larval form is the nauplius, a stage with a simple, unsegmented body and only three pairs of appendages, often used by scientists to confirm a species’ classification.
The Current Estimate of Described Species
The current scientific consensus places the number of formally described crustacean species at approximately 67,000 to 68,000 worldwide. This figure is constantly being updated as new species are identified and cataloged by taxonomists. This count, however, represents only the species that science has formally documented and named.
Researchers agree that the actual total number of extant crustacean species is a significant underestimate, potentially only a fraction of the true global diversity. Estimates suggest the true number may be anywhere from ten to a hundred times greater than the described count. The discovery rate of new species shows no sign of slowing down, meaning the cumulative number continues to rise steadily.
Quantifying this group is challenging due to the constant discovery of microscopic forms, collectively known as meiofauna. Many of these tiny, undescribed organisms inhabit the interstitial spaces between sand grains on the ocean floor or live in unexplored deep-sea environments. The sheer variety of habitats, from subterranean caves to vast pelagic zones, ensures that large portions of the world’s crustacean fauna remain inaccessible to modern science.
Mapping the Major Crustacean Classes
The described species are organized into several major taxonomic classes, each representing a distinct evolutionary pathway and body plan. The largest class is Malacostraca, which accounts for over 40,000 species, making it the dominant group. This class includes the well-known decapods (crabs, lobsters, shrimp, and crayfish), alongside numerous groups like the terrestrial woodlice and the diverse marine amphipods and isopods.
Another diverse group is the class Maxillopoda, which encompasses a range of smaller and often microscopic forms. This class includes the copepods, which are tiny, free-swimming organisms that are globally abundant, and the sessile barnacles, which spend their adult lives attached to surfaces. Maxillopoda also contains the Ostracods, tiny creatures encased in a bivalve-like shell.
The Branchiopoda is a third significant class, characterized by leaf-like appendages used for both movement and filter-feeding. Familiar examples include the water fleas, such as Daphnia, common in freshwater ponds, and the brine shrimp, or “sea monkeys,” found in highly saline environments. These classes demonstrate the vast morphological differences within the subphylum, ranging from the hard-shelled crab to the minute, almost transparent copepod.
Ecological Abundance and Global Habitats
Crustaceans are overwhelmingly aquatic, with the vast majority of species dominating marine and freshwater ecosystems globally. They occupy every depth, from the intertidal zone to the deepest abyssal plains, and are key components of the food web in nearly all environments. Their collective biomass is immense, with small planktonic species forming the foundation of many aquatic food chains.
Copepods and krill are primary consumers that graze on phytoplankton and serve as the main food source for countless fish, seabirds, and marine mammals, including the largest whales. Larger crustaceans, such as crabs and lobsters, function as detritivores and scavengers, playing a significant role in nutrient recycling by breaking down dead organic matter on the seafloor.
While most species are aquatic, a few groups have successfully colonized terrestrial environments, though they still require high humidity. The most successful land-dwelling crustaceans are the isopods, commonly known as woodlice or pill bugs, which are detritivores found in damp soil and leaf litter. Other terrestrial forms include land crabs that migrate to the sea only to breed.