Sea Urchin

In balanced ecosystems, they help control algae and support biodiversity, but when overpopulated, they can devastate kelp forests and reduce marine life.

Known for their typically spiny appearance, sea urchins are prolific marine invertebrates that can be found in every ocean and in every climatic zone.

Are All Sea Urchins Spiny?

All sea urchins have spines, but not all of them have the same spiky appearance. The size, shape, and function of these spines vary by species. Some sea urchins have long, sharp spines for defense, while others have shorter, more rounded spines that help with movement. If a sea urchin loses its spines, it can regenerate them over time, much like a starfish regrows lost limbs.

How Do Sea Urchins Contribute to Marine Ecosystems?

Sea urchins primarily feed on algae. In a balanced ecosystem, these spiny invertebrates control algae growth on reefs and rocky surfaces, preventing them from smothering corals and other marine life. Sea urchins also contribute to maintaining healthy food webs by serving as a food source for predators like sea otters, fish, crayfish, and lobsters.

Where Can Sea Urchins Be Found?

There are over 950 species of sea urchins found in oceans worldwide, from shallow coastal waters to deep-sea environments. About 70 species are found in New Zealand, including the common kina (Evechinus chloroticus) and the sub-tropical long-spined sea urchin (Centrostephanus rodgersii).

Why Are Sea Urchins So Widespread?

Sea urchins are highly adaptable. They can thrive in diverse marine environments due to several factors:

Efficient Reproduction: Many species release thousands of eggs and sperm into the water, and this increases their chances of survival.
Varied Diet: Sea urchins can consume a wide range of plant material, from algae to kelp, allowing them to survive in different habitats.
Strong Defense Mechanisms: Their spines and hard shells play a role in their survival by protecting them from many predators.
Resilience: Sea urchins can tolerate changing water conditions, including variations in temperature and salinity.

How Does Climate Change Contribute to the Spread of Sea Urchins?

Warming ocean temperatures have allowed certain sea urchin species to spread into new areas where they previously couldn’t survive. At the same time, climate change has led to a decline in natural predators like large fish and lobsters, and this has made it easier for sea urchin populations to grow unchecked.

What Happens When There Are Too Many Sea Urchins?

When sea urchin populations become too large, they overgraze on kelp forests and turn these once-thriving ecosystems into barren, lifeless areas called “urchin barrens.” This leads to:

Loss of marine habitat: Kelp forests support fish, shellfish, and other marine life, and their destruction reduces biodiversity.
Reduced fish populations: Many fish species rely on kelp forests for food and shelter.
Weakened coastal protection: Kelp forests help buffer shorelines from waves and storms, and losing them can exacerbate erosion risks.

How Can Sea Urchins Populations Be Monitored?

There are several ways to monitor sea urchin populations and their impact on marine ecosystems. These include:

Diver surveys are conducted by marine scientists and conservation divers to manually count sea urchins and assess their density in affected areas.
Underwater drones and remote cameras can provide real-time imaging of sea urchin populations in hard-to-reach areas.
Satellite and aerial imaging can help track large-scale changes in kelp forest coverage, which can indicate urchin overpopulation.
Studying urchin DNA can also provide insights into their movement, reproduction, and population dynamics.

Researchers in New Zealand are able to use new and emerging technologies to monitor sea urchin populations. One example of a current initiative is Dr. Arie Spyksma’s effort to track the rapid expansion of Centrostephanus using Seascape technology. His research involves capturing extremely high-resolution photos of the seafloor and transforming these into engaging 3D models, which then provide a detailed, long-term view of how sea urchins impact marine habitats. His work has helped track the loss of kelp forests in places like Cape Brett and the Poor Knights Islands and has supported efforts to restore these vital ecosystems. Through this technology, conservationists can make better decisions on managing sea urchin populations and protecting marine biodiversity.

What Interventions Can Be Done to Keep the Sea Urchin Population Under Control?

Several methods can help manage sea urchin overpopulation:

Restoring and protecting predator species like lobsters, crayfish, snapper, and otters can help keep sea urchin populations in check.
Divers can also manually remove sea urchins from affected areas to allow kelp forests to recover.
Sustainably harvesting sea urchin for human consumption can likewise help reduce their numbers.

How Can Removing Sea Urchins Lead to Positive Ecosystem Recovery?

Studies have shown that areas where sea urchins are removed can experience rapid recovery. In trial removals monitored by Dr. Spyksma, for instance, kelp forests began to regrow within 12 months. This led to:

A return of fish and invertebrates that rely on kelp for shelter and food.
Re-establishment of other marine organisms, which then led to increased biodiversity.
A healthier, more resilient ecosystem that can better withstand environmental changes.