The Pacific Purple Sea Urchin, a common and ecologically significant marine invertebrate, inhabits the dynamic coastlines of the Pacific Ocean. It is widespread along the North American Pacific coast, from British Columbia to Baja California, where its presence influences marine ecosystems.
Characteristics and Habitat
The Pacific Purple Sea Urchin, Strongylocentrotus purpuratus, is known for its deep purple coloration, though some are reddish or black. Its spherical body, called a test, can grow up to 10 centimeters (4 inches) in diameter. Numerous spines cover the test, providing defense against predators and aiding movement.
Tube feet with suction cups allow it to grip surfaces and move along the seafloor, also assisting in respiration. It thrives in shallow, subtidal areas, particularly rocky intertidal zones and kelp forests, found from the low intertidal down to 160 meters (520 feet).
Ecological Role in Marine Ecosystems
The Pacific Purple Sea Urchin is a herbivorous grazer. It feeds on kelp and other algae, regulating seaweed populations. The urchin uses a mouth structure called Aristotle’s Lantern, equipped with five bony teeth, to scrape algae off rocks and chew on macroalgae.
Its role as a grazer influences the marine food web. When urchin populations are unchecked, their grazing can lead to “urchin barrens.” These are seafloor areas where kelp and other macroalgae are consumed, leaving a landscape dominated by sea urchins and encrusting coralline algae. This transformation reduces biodiversity and alters the underwater environment.
Population Dynamics and Ecosystem Impact
The population dynamics of Pacific Purple Sea Urchins are influenced by their natural predators. Sea otters and sea stars, such as the sunflower sea star (Pycnopodia helianthoides), are important consumers of these urchins, helping to maintain balance within kelp forest ecosystems. Sea otters, for instance, can consume large quantities of urchins, sometimes causing their bones and teeth to turn purple from the diet.
However, the Sea Star Wasting Disease, exacerbated by marine heatwaves, has led to a sharp decline in sea star populations, with some areas seeing over 90% mortality. This reduction in predation has allowed purple sea urchin populations to increase rapidly, leading to widespread degradation of kelp forests. In some regions, kelp canopy cover has declined by as much as 95% due to unchecked urchin grazing, transforming once-thriving kelp forests into barren landscapes. The recovery of predator populations can help restore this balance, as sea otters increase their urchin consumption in areas with outbreaks, helping to maintain remaining kelp patches.
Human Interactions and Scientific Research
Humans interact with Pacific Purple Sea Urchins in several ways. Their roe, or gonads, is considered a delicacy in some culinary traditions, particularly in Japanese cuisine where it is known as “uni”. While there isn’t a large commercial fishery specifically for purple sea urchins in some areas like Oregon, their collection for this purpose could potentially aid in kelp forest restoration efforts by reducing urchin populations in barren areas.
Beyond their culinary use, Pacific Purple Sea Urchins are also highly valued in scientific research. They serve as a model organism, particularly in developmental biology studies, due to their transparent embryos and well-documented developmental stages. Their genome was the first echinoderm genome to be sequenced, further enhancing their utility in genetic and biological research. Conservation efforts and management strategies are also being explored, including “urchin ranching” where urchins are removed from barrens, fattened, and then sold, aiming to both restore kelp forests and create economic opportunities.