By Sarah Haselton
Purple sea urchins, strongylocentrotus purpuratus are invertebrates which range from Vancouver Island to Isla Cedra, Baja California. Purple sea urchins are in the phylum echinodermata which means they are related to sea stars, sand dollars and sea cucumbers. They are found in rocky intertidal ecosystems as well as kelp beds. The animal is round in shape due to the inner shell, referred to as a “test.” The test is covered with pincers, tubed feet and purple spines. The spines functional purpose is for feeding and protection from predators such as sea otters. The tubed feet are used for mobility along the substrata; in addition, the tubed feet are utilized for “breathing” or gas exchange.
In the last couple of years, the purple sea urchin population has increased in California dramatically. The population explosion has undoubtedly had an effect on the ecosystem dynamics of kelp forests. The purple sea urchins have consumed entire kelp forests. They are outcompeting aquatic species within the kelp forest ecosystem such as red urchins. This is a complex issue with countless contributing factors that have ultimately led to the current state of many kelp forests in California. There was a die off of several sea star species due to star fish wasting disease, most notably the sunflower sea star which served as a natural predator of purple sea urchins. Another contributing factor to the explosion of their population is the depleted sea otter population. Sea urchins are also considered resilient animals because they are able to survive up to 40 years without any food source and can reproduce successfully during shorter periods of starvation, unlike some invertebrates such as abalone.
Abalone divers in northern California are suffering the consequences of increases in urchin populations. This 2018 recreational abalone season was closed due to the near collapse of abalone. Massive efforts to restore the abalone have included vacuuming purple sea urchins off their substrata. Divers are utilizing fiberglass pipes (vacuum) which are attached to a large net that collects the spiny creatures. The purple sea urchins do not have very much commercial value whereas abalone are a commercially valuable species. Marine conservation groups are well aware of the severity of the issue and raised $80,000 in 2018 in order to fund projects removing the sea urchins from the coast of Northern California. The California Department of Fish and Wildlife scientists have been working on removing urchins since 2016. The urgency of the issue is related to the die off of bull kelp. As previously stated the kelp forests support many commercially valuable species such as abalone and red sea urchins but also small fishes and invertebrates. This ultimately can have a larger effect on ocean dynamics as these smaller fish serve as a food source for larger fish such as salmon. Environmental scientists who have been involved with this issue do not predict kelp forests will bounce back until purple sea urchin populations decline.
Sea urchins are important marine invertebrates and serve a structural and functional purpose both as grazers and prey. However, the ecological imbalance has created urchin barrens (unchecked growth of urchins) and a balance is required for overall kelp forest health and restoration. The issue of overpopulated sea urchins does not apply in all areas where they are found but rather, is a primary concern for northern California.
I hope you continue to learn more about the ecosystem dynamics and interactions that affect our local fisheries!