Brine shrimp, belonging to the genus Artemia, are small crustaceans that thrive in extremely salty environments like salt lakes and brine ponds. Their unique appearance is a direct result of their adaptation to these harsh habitats.
Adult Physical Characteristics
The mature brine shrimp is a slender, segmented animal typically measuring between 10 and 12 millimeters in length, though some individuals may reach up to 15 millimeters. The body is divided into a head, a thorax, and an abdomen, covered by a thin, flexible exoskeleton that is shed periodically as the animal grows. The color of the adult can vary significantly, ranging from pale white or transparent to pink, reddish-orange, or green.
This coloration is not genetic but is influenced by its diet and the oxygen levels in the water. When oxygen is scarce, the shrimp produces more hemoglobin, a blood pigment that gives it a reddish hue. The head features two prominent, stalked compound eyes that provide the adult with its visual sense.
The thorax bears eleven pairs of leaf-like appendages called phyllopods or swimming legs. These feathery limbs beat rhythmically, serving a dual purpose: they propel the animal through the water and function in respiration by transferring oxygen across tiny gill plates. Unlike many other crustaceans, the brine shrimp lacks a hard, protective outer shell, or carapace, which contributes to its soft, elongated appearance.
Life Stages and Visual Differences
The brine shrimp life cycle features two forms that look dramatically different from the mature adult: the cysts and the nauplii. Cysts, often incorrectly called eggs, are metabolically dormant embryos encased in a tough, brown or tan shell. These small, spherical objects measure about 0.2 to 0.3 millimeters in diameter and are designed for long-term survival in dry, harsh conditions.
When environmental conditions improve, the cysts hatch into the first larval stage, known as the nauplius (plural nauplii). The nauplius is a tiny, free-swimming larva, initially measuring about 0.4 to 0.5 millimeters in length, appearing brownish-orange due to stored yolk material. Unlike the multi-legged adult, the nauplius starts with only three pairs of simple appendages used for swimming and feeding.
A major visual distinction is the eye structure; the nauplius possesses a single, simple eye, known as the naupliar eye, which senses the presence and direction of light. As the larva grows, it molts repeatedly, and the trunk elongates, gradually developing the paddle-like thoracic limbs seen in the adult. The paired, stalked compound eyes eventually develop alongside the original simple eye, resulting in the three-eyed structure of the adult.
Distinguishing Features and Locomotion
A characteristic that identifies a brine shrimp is its unique swimming behavior. The animal often swims upside down, or on its back, a movement pattern linked to its attraction to light.
The continuous, synchronized beating of its leaf-like legs creates a current that propels the shrimp forward while simultaneously drawing food particles toward its mouth. This constant, fluttering movement is distinctly different from the jerky motion of many other small aquatic organisms, like copepods or water fleas. The lack of a hard carapace places it within the classification of “fairy shrimp.”