Curiosity Sparks Joy - Learning about Sea Urchins

I joined a sea urchin harvesting adventure at Salt Point out of curiosity. I wanted to taste uni from our ocean. It was there that I learned how purple sea urchins, native to this coast, have grown so numerous that they are beginning to unsettle the balance of the reef.

Sea urchins are not invasive. They have always been part of our waters. For a long time, their numbers were kept in balance by sea otters moving through the kelp and by abalone sharing the slow work of grazing. Over time, abalone populations collapsed and otters disappeared from much of the Bay Area. What remained was an absence, and into that absence the urchins multiplied. They began to overgraze the reefs, stripping kelp faster than it can recover and turning once lush forests into what are now called urchin barrens.

Kelp forests hold ecosystems together by offering shelter and food to countless species, softening waves before they reach shore, and quietly regulating the chemistry of the water itself. As kelp disappears, the imbalance spreads and the water holds less life.

That afternoon, I gathered sea urchins from the rocks, a small but joyous act in support of the ecosystem. As night fell, tired from climbing rocks and picking urchins, we opened them and ate the uni, creamy and briny, tasting the ocean they came from.

After we had eaten, I boiled the shells, removed their spines, and washed them clean. The deep purples faded, revealing soft greens beneath, their true mineral bound color uncovered once the living layer was gone.

I hope this shell sparks your curiosity and with it a small new understanding of our ocean.

Kelp

1. How kelp supports biodiversity

Kelp forests are three-dimensional habitats. Their long fronds create shelter, nursery space, and feeding grounds for fish, invertebrates, seabirds, and marine mammals. Juvenile fish hide among the blades; invertebrates cling to the holdfasts; predators follow the life that gathers there. Where kelp thrives, life layers itself.

2. How kelp protects coastlines

Dense kelp forests slow waves and absorb their energy before they reach shore. This reduces erosion and helps stabilize coastal sediments. In storms, kelp acts like a living buffer—flexible, rooted, and resilient—softening the force of the ocean against the land.

3. How kelp helps regulate ocean health

Kelp grows quickly and draws large amounts of carbon dioxide from the water, helping moderate ocean acidity and supporting carbon cycling. Through photosynthesis, it releases oxygen and helps maintain healthier local water chemistry. When kelp forests disappear, the water often becomes warmer, more acidic, and less capable of supporting diverse life.

Why the sea urchin shells became green

The color change you noticed is not caused by bleaching or washing in the sense of “adding” color. Instead, cleaning the urchins revealed the shell’s true, underlying color.

1. The purple color belongs to the living animal

In a live purple sea urchin, the deep purple comes mostly from pigments in the skin, spines, and thin organic layersthat cover the shell. These pigments help protect the animal from sunlight and predators, but they are not part of the shell itself.

2. The shell (test) has its own mineral-bound color

Beneath the living tissue is the test—the rigid shell made of calcium carbonate.This shell is formed slowly during the urchin’s life and contains mineral-bound pigments that are:

• Laid down during growth

• Species-specific

• Naturally green, olive, or celadon in many purple sea urchins

This color exists before the urchin is harvested.

3. Cleaning removes what was masking the color

When you cleaned or washed the urchins:

• Soft tissue, membranes, and remaining pigments were removed

• The purple living layer disappeared

• The mineral surface of the test was exposed

Cleaning does not change the shell’s chemistry or create new color—it simply uncovers what was already there.

4. Why the green becomes more noticeable over time

After cleaning, the green can appear stronger because:

• Dark pigments that once masked it are gone

• Light reflects differently off a clean mineral surface

• Exposure to air and water allows subtle oxidation of trace minerals

• Urchins from kelp-poor environments often have paler, greener tests

What creates different shades of green in sea urchin shells

1. Species and genetics

Even within “purple” sea urchins, shell color varies slightly by:

• Genetics

• Growth rate

• Age

Some tests are naturally sage, olive, celadon, or yellow-green, even among the same species.

2. Age and growth layers

Sea urchin shells grow in layers, similar to tree rings:

• Older urchins often show deeper, more complex greens

• Younger ones may appear lighter or more uniform

• Subtle banding or tonal shifts reflect changes over time

3. Diet during life

What an urchin eats strongly influences shell tone:

• Urchins with access to healthy kelp forests tend to develop richer, warmer greens

• Urchins living in kelp-poor barrens often have paler, chalkier, or cooler green shells

Kelp contains pigments and trace minerals that become incorporated into the shell during growth.