Sea urchins are marine invertebrates belonging to the phylum Echinodermata, a group that also includes starfish and sea cucumbers. They are defined by their globe-shaped bodies encased in a hard shell, or test, which is covered in spines. Sea urchins exhibit a wide spectrum of colors across the world’s oceans. This variation in coloration is tied to survival mechanisms, genetics, diet, and habitat.
The Diverse Palette of Sea Urchin Colors
Sea urchin species display a wide palette of colors. This palette ranges from the deep, matte black of the long-spined urchins (Diadema genus) found in tropical waters to the bright purple of the Pacific purple sea urchin (Strongylocentrotus purpuratus). Other species exhibit reddish-browns, olive greens, and even bleached whites.
The edible sea urchin, for instance, can be pinky-purple, red, green, or yellow. The green sea urchin (Strongylocentrotus droebachiensis) is typically green with purple-tipped spines, while the red sea urchin (Strongylocentrotus franciscanus) is known for its deep, brick-red coloration. The sea potato, a burrowing species, is colored brown to blend into the sandy sediment.
The Role of Color in Urchin Survival
Coloration plays an important role in sea urchin survival, primarily through camouflage. Species inhabiting rocky reefs often possess darker or mottled colors that allow them to blend seamlessly with the complex substrate. For those living on sandy bottoms, like the sea potato, a muted brown or beige coloration helps them remain inconspicuous to predators.
Coloration also provides protection against intense solar radiation, particularly in shallow-water environments. Darker pigments in the spines and test absorb and dissipate ultraviolet (UV) light, shielding internal tissues from damage. Some species enhance this defense by using their tube feet to hold pieces of shell or algae over their bodies, which also serves as added camouflage. Brightly colored urchins may use their striking hues as a warning signal (aposematism), indicating to predators that they possess venomous spines or are unpalatable.
Factors Influencing Urchin Pigmentation
Sea urchins produce unique pigments called echinochromes and spinochromes, which are naphthoquinone compounds that contribute to their color. These pigments are genetically determined and are also involved in immune function, offering protection against certain microbes.
A significant portion of a sea urchin’s color is derived directly from its diet, as they cannot synthesize certain pigments on their own. By grazing on various species of algae and kelp, they ingest carotenoids, which are incorporated into their tissues, including their gonads and tests. The consumption of different macroalgae can lead to variations in the saturation and specific hue of coloration. Environmental conditions, such as water depth, light exposure, and seasonal changes, also influence how these pigments are expressed.