### Identification To the naked eye, *Acartia longiremis* appears as a mere crystalline speck in a jar of seawater, but under a lens, its elegance is revealed. This calanoid copepod is characterized by a sleek, cigar-shaped body (prosome) typically measuring 0.8 to 1.3 mm. Its most defining field mark—and the source of its name—is its exceptionally long first antennae, which extend well beyond the end of its body. Look for the distinct, sharp spines on the posterior corners of the last thoracic segment; these distinguish it from its cousin *A. tonsa*. While mostly transparent, they often catch the light with a faint, pearlescent blue or amber tint.
### Habitat & Range *A. longiremis* is a cold-water specialist of the Northern Hemisphere. It thrives in the neritic (coastal) waters of the North Atlantic, North Pacific, and the frigid Arctic Ocean. Unlike some copepods that prefer the open deep, this species hugs the continental shelves and estuaries. You’ll find them most abundant in the upper 50 meters of the water column, where the sunlight fuels the microscopic "pastures" they call home.
### Behaviour Observation reveals a staccato, "hop-and-sink" swimming style. They are masters of the Diel Vertical Migration, rising toward the surface under the cover of darkness to feed and retreating to the depths at dawn to avoid visual predators like larval fish. They are also incredibly sensitive to vibrations; at the slightest pressure wave, they execute a lightning-fast "escape jump," propelled by their powerful thoracic legs.
### Diet These are selective suspension feeders, acting as the ocean’s discerning grazers. While they consume vast quantities of diatoms and dinoflagellates, they are not mere sieves. They use their feathery mouthparts to create tiny currents, sensing the chemical signatures of their prey and snatching individual ciliates or phytoplankton cells out of the water column with surgical precision.
### Fascinating Fact Despite being barely a millimeter long, *Acartia longiremis* is one of the fastest "sprinters" on Earth relative to its size. During an escape jump, it can accelerate at a rate that would subject a human to over 100 Gs of force—making it effectively the "supercar" of the plankton world!