Krill are small crustaceans found throughout the world’s oceans, playing a fundamental role in marine ecosystems. These tiny, shrimp-like creatures form a significant link in the aquatic food web, consuming microscopic plants and serving as a primary food source for many larger marine animals. Whales, seals, penguins, and various fish species rely on krill for sustenance. Their large biomass, especially in the Southern Ocean, highlights their importance in nutrient cycling and supporting diverse marine life.
The Common Appearance of Krill
Krill are often recognized by their small size, generally ranging from less than half an inch to about 2.5 inches in length. While many picture them as reddish or pink, a large number of krill species are quite transparent. Their bodies resemble miniature shrimp or prawns, featuring prominent compound eyes and two pairs of antennae.
Despite their transparent bodies, many krill species can display a reddish or pinkish tinge, particularly on their shells. This coloration is not uniform, and its intensity can vary. Their translucent skin often reveals their digestive system, which may appear green depending on their recent diet.
What Influences Krill’s Color
The coloration of krill is influenced by their diet, specifically the consumption of phytoplankton containing astaxanthin, a red-orange pigment. As filter feeders, krill use their specialized appendages to strain these microscopic plants from the water. When consumed, astaxanthin is absorbed and stored in their tissues, particularly in specialized pigment cells called chromatophores.
The concentration of astaxanthin directly affects the shade of red or pink they exhibit. A diet rich in astaxanthin-producing algae leads to a more vibrant red hue, while a less pigmented diet results in a paler, more transparent appearance. Krill species have natural variations in baseline coloration, and even within a species, color can change throughout life stages and seasons. For example, larval krill may be more transparent than adults, and astaxanthin levels in adults can be higher in summer than in winter.
The Role of Color in Krill’s Life
Krill coloration, or its absence, serves important functional purposes for survival. Their transparency provides camouflage, allowing them to blend into the surrounding water and become less visible to predators in well-lit surface waters. This blending is enhanced by their bioluminescent organs, photophores, which emit yellow-green or blue light downwards to counter-illuminate their silhouette.
Reddish or pinkish pigments, primarily astaxanthin, also offer protection from ultraviolet (UV) radiation. When krill are exposed to intense sunlight, especially during daily vertical migrations closer to the surface, these pigments disperse within their chromatophores to cover larger body areas, acting as a shield. This ability to change the intensity and spread of their red spots helps them balance UV protection with maintaining transparency for camouflage. In deeper, darker waters where blue light is filtered out, reddish hues also provide camouflage, as red light is absorbed at shallower depths, making red objects appear black or invisible.