The term “tiny shrimp” broadly describes various small, swimming crustaceans found everywhere from ocean depths to desktop aquariums. This common name highlights the diversity within the subphylum Crustacea, where many small arthropods have evolved similar, shrimp-like body shapes. These organisms belong to distinct taxonomic groups, each with unique biological characteristics and ecological roles. The confusion arises because “shrimp” is a common descriptor rather than a precise biological classification.
The Planktonic Powerhouses
The open ocean contains immense populations of small crustaceans that form the base of marine food webs. Many people searching for “tiny shrimp” are thinking of organisms typically seen in nature documentaries, such as krill and copepods.
Krill (order Euphausiacea) are famous oceanic powerhouses. These shrimp-like animals grow up to six centimeters long and form massive, dense schools called swarms, sometimes reaching 30,000 individuals per cubic meter. Antarctic krill (Euphausia superba) have one of the largest total biomasses of any single animal species on Earth, estimated at hundreds of millions of tonnes. Krill are a foundational food source, sustaining whales, seals, penguins, and many fish species. They are characterized by exposed, branching gills, which are not covered by a carapace as they are in true shrimp.
Copepods are a tiny and ubiquitous group of crustaceans, often measuring between 0.5 and 5 millimeters. They are considered the most numerous multicellular animals on the planet, inhabiting both freshwater and marine environments. These organisms are a dominant component of the zooplankton, acting as a crucial link by consuming microscopic algae (phytoplankton) and transferring energy to larger planktivores. Copepods typically have a teardrop-shaped body and large antennae used for swimming and sensing the environment.
The Aquarium and Hobby Favorites
In domestic and commercial settings, the term “tiny shrimp” often refers to species used as live food or novelty pets in the aquarium trade. Two groups frequently encountered in this context are brine shrimp and mysid shrimp.
Brine shrimp (Artemia) are famous for producing dormant eggs called cysts. These cysts tolerate extreme conditions, surviving desiccation, hypersalinity, and extreme temperatures before hatching when placed in water. This survival mechanism makes them convenient for storage and hatching on demand, making them a staple live or frozen food in aquaculture and the aquarium hobby. The newly hatched larvae, or nauplii, are valued as a highly nutritious first food for the young of many fish and invertebrate species.
Mysid shrimp, often called “opossum shrimp,” are small, translucent crustaceans (order Mysida). Females possess a specialized brood pouch, or marsupium, where they carry and rear their young. Mysids are highly nutritious and are cultivated as a popular food source for many carnivorous fish and invertebrates. Most species measure between 5 and 25 millimeters in length, and they are frequently used as bioindicators because of their sensitivity to water pollution.
Distinguishing Features and Taxonomy
All the organisms commonly called “tiny shrimp”—krill, copepods, brine shrimp, and mysids—are members of the subphylum Crustacea, a broad group within the phylum Arthropoda. However, the designation of a “true” shrimp is reserved for a specific infraorder, Caridea, which resides within the order Decapoda, the “ten-legged” crustaceans.
The classification of a true shrimp is based on distinct morphological features that separate them from their look-alikes. The structure of the gills, which are lamellar or plate-like, is one primary feature. Another distinction is the arrangement of the abdominal segments: in true shrimp (Caridea), the second segment overlaps both the first and the third segments.
Krill (Euphausiacea) and mysids (Mysida) are distinct from true shrimp because they belong to different orders. Mysids, for example, have specialized balance organs called statocysts located on their tail fan, a feature absent in true shrimp. Copepods are taxonomically distant, being much smaller and lacking the obvious segmentation and complex body structure of larger malacostracan crustaceans like true shrimp and krill. True shrimp also typically have two pairs of claws (chelae) on their first two pairs of walking legs, which differentiates them from many other small, shrimp-like groups.