Pacific Purple Sea Urchin

The Pacific purple sea urchin (Strongylocentrotus purpuratus) is a prominent marine invertebrate inhabiting the rocky coastlines of the eastern Pacific Ocean. Noted for its vivid violet hue and spherical shape covered in spines, this species is an integral component of temperate marine ecosystems. It functions as both a key herbivore in kelp forest habitats and a vital model organism in biological research. In recent decades, the Pacific purple sea urchin has become emblematic of ecological change along the west coast of North America due to its expanding populations and their effect on kelp forest degradation.

Taxonomy and Classification

The Pacific purple sea urchin belongs to the phylum Echinodermata, a group characterised by radial symmetry, spiny exteriors, and water vascular systems. Its taxonomic hierarchy is as follows:

• Kingdom: Animalia
• Phylum: Echinodermata
• Class: Echinoidea
• Order: Echinoida
• Family: Strongylocentrotidae
• Genus: Strongylocentrotus
• Species: Strongylocentrotus purpuratus

Closely related species include the red sea urchin (Mesocentrotus franciscanus) and the green sea urchin (Strongylocentrotus droebachiensis), both of which share overlapping habitats and similar ecological functions.

Physical Description

The Pacific purple sea urchin is easily identified by its deep purple to violet coloration and spherical body, known as the test, composed of interlocking calcium carbonate plates. Adults typically reach 6 to 10 centimetres in diameter, although size varies depending on habitat and food availability.
Key physical features include:

• Numerous rigid, movable spines used for defence, movement, and burrowing.
• A complex feeding apparatus called Aristotle’s lantern, consisting of five calcareous teeth used to scrape algae from rocks.
• Tube feet operated by hydraulic pressure, which aid in locomotion, respiration, and food handling.
• A pentamerous radial symmetry, typical of echinoderms, reflecting their unique evolutionary lineage.

Distribution and Habitat

The Pacific purple sea urchin inhabits the eastern Pacific coastline, ranging from Baja California, Mexico, to British Columbia, Canada, with the densest populations found along the California coast. It thrives in intertidal and subtidal zones to depths of approximately 160 metres, favouring rocky substrates and kelp-dominated habitats.
These urchins are commonly found in regions influenced by the California Current, which brings cold, nutrient-rich waters conducive to algal growth. Their ability to withstand exposure during low tide allows them to occupy tide pools as well as deeper subtidal regions.

Feeding and Behaviour

The Pacific purple sea urchin is a primarily herbivorous grazer, feeding on a variety of macroalgae, especially giant kelp (Macrocystis pyrifera) and bull kelp (Nereocystis luetkeana). Using Aristotle’s lantern, the urchin scrapes algal tissue from hard surfaces.
Feeding behaviour is strongly influenced by environmental conditions and predator abundance. In healthy ecosystems, predators such as sea otters, sunflower sea stars, and certain fish species regulate urchin populations, maintaining a balance between kelp and herbivores. However, when predators decline—such as following the sea star wasting disease epidemic in the 2010s—urchin populations can explode, overgrazing kelp forests and creating “urchin barrens”, where little vegetation or biodiversity remains.

Reproduction and Life Cycle

Reproduction occurs via external fertilisation, typical of echinoderms. Spawning usually takes place in late winter or early spring, when males and females release gametes into the water column. Fertilised eggs develop into planktonic larvae (echinoplutei) that drift with ocean currents for several weeks before settling and metamorphosing into juveniles.
The species can live for 20 years or more, and its reproductive output is closely tied to environmental conditions such as food supply, temperature, and ocean acidity.

Ecological Role and Interactions

As a keystone herbivore, the Pacific purple sea urchin profoundly influences kelp forest structure and productivity. Moderate grazing supports algal turnover and habitat heterogeneity, but excessive populations can devastate kelp ecosystems.
Predators that control its numbers include:

• Sea otters (Enhydra lutris), which consume urchins and maintain kelp forest health.
• Sunflower sea stars (Pycnopodia helianthoides), which prey upon both juvenile and adult urchins.
• Sheephead fish and crabs, which target smaller individuals.

When these predators decline, the resulting trophic imbalance contributes to widespread ecological degradation and loss of carbon-sequestering kelp habitats.

Economic and Cultural Importance

The Pacific purple sea urchin holds modest but growing economic value in coastal fisheries. Its gonads (roe), known as uni, are a culinary delicacy in sushi cuisine and are harvested commercially in California and Baja California. However, overabundant populations are now viewed more as an ecological threat than a fishery asset in some regions.
In response, conservation and restoration projects have begun commercially harvesting urchins from barren zones, both to supply local markets and to aid kelp forest recovery.
Culturally, the species also features in indigenous coastal traditions, where sea urchins were historically harvested for food and used in coastal trade.

Scientific Significance

The Pacific purple sea urchin is one of the most studied marine invertebrates in modern biology. Since the early 20th century, it has been a model organism in developmental biology, embryology, and genetics, primarily due to its transparent embryos and external fertilisation process, which facilitate observation and experimentation.
The sequencing of its genome in 2006 provided insights into the evolutionary relationship between echinoderms and vertebrates. Researchers discovered that the species shares many gene families associated with immunity, sensory perception, and developmental processes in higher animals, making it invaluable for evolutionary and biomedical studies.
Furthermore, its sensitivity to ocean acidification and temperature change makes it a useful indicator species for monitoring the impacts of climate change on marine ecosystems.

Environmental Challenges

Pacific purple sea urchin populations face several anthropogenic and environmental threats:

• Ocean warming affects reproductive timing and larval development.
• Ocean acidification reduces calcification rates, weakening shells and spines.
• Habitat degradation from pollution and sedimentation impacts feeding and recruitment.
• Ecological imbalance from predator loss leads to unsustainable grazing and ecosystem collapse.

Simultaneously, population booms can harm biodiversity and fisheries by transforming lush kelp forests into unproductive barrens. Thus, the species is both vulnerable to and responsible for major ecological change.