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Taxonomy
Kingdom: Anamalia
Phylum: Echinodermata
Class: Echinoidea
Order: Echinoida
Family: Strongylocentrotidae
Genus: Strongylocentrotus
Species: S. purpuratus
Habitat, Distribution, and Life History
The Pacific Purple Sea Urchin can be found in subtidal and intertidal zones on the West coast of North America. This region ranges from Cook Inlet in Alaska to Northern Mexico. These little guys like to make their home in rocky crevices, kelp forests, and tidepools. They can be found as deep as 525 feet and prefer water temperatures below 73 degrees Fahrenheit. They can live for an average of 20 years and tend to breed from January to March annually (Fretwell 2014). Females are oviparous, which means that they are egg-laying organisms. Once the eggs are laid and fertilized externally, they will hatch and become pleutue larvae. After three months of a pelagic lifestyle, these larvae will turn into juvenile sea urchins and eventually become sexually mature adults (Heyland 2014).
Morphology
The Pacific Purple Sea Urchin is a small echinoderm ranging from 6 cm to 12 cm in length, not including the spines. They have pentamerous radial symmetry meaning that they have five lines of symmetry and have a skeleton made from fused calcareous ossicles (Gosselin-Ildari 2001). They also have tube feet which aid in securing them in a crevice and are a part of the respiratory system (Kazilek 2009). This group of organisms has a unique structure surrounding the mouth called Aristotle's Lantern. This interesting feature consists of five calcareous plates, called pyramids, with a tooth on each plate's end. The mouth is located at the center of the five pyramids. This feeding structure evolved to aid in scraping off algae and other marine plants from hard substrates like rocks. It has also been seen that urchins use Aristotle Lantern to make crevices large so they can inhabit them (Gosselin-Ildari 2001).
Now for the most famous feature of all sea urchins, the spines! The spines' length can range from 2.5 cm to 6 cm, depending on their environment. Typically in subtidal regions, the spines are longer (Fretwell 2014). The spines are all connected to the test (shell minus the spines) by ball and socket joints allowing for a range of motion. The function of these spines is locomotion, fitting in and clinging to crevices, protection, and prey capture. When an organism runs into the spines, they all move toward the organism to protect it from injuring the test. After an organism has run into the urchin, the spines face outward, revealing small pincer-like structures called Pedicellariae. These structures are what release the poison that is capable of paralyzing small animals. They use the pedicellariae both for immobilizing prey and for defense from predators (Gosselin-Ildari 2001). So if you ever go walking around in their habitat, watch where you step!
Ecological Role and Importance
The Pacific Purple Sea Urchin is found mainly in shallow water areas and tends to control the ecosystem they reside in as primary consumers. There are two main ecosystems that a Pacific Purple Sea Urchin can control and change: kelp forests and urchin barrens. Kelp forests are a hot spot for biodiverse ecosystems on rocky, cold coasts. They are mainly dominated by kelp, providing food and shelter to many species of fish, marine mammals, and invertebrates. As you can see from the name, urchin barrens are areas with a high density of sea urchins that overgraze on kelp, algae, and other marine plants (Pearse 2006).
The sea urchins cause a drastic change in the ecosystem going from a lively, biodiverse kelp forest to an empty, sea urchin-dominated urchin barren. This phenomenon has been a continuous struggle on the West coast of North America for many reasons, including a decline in sea otters, sea star wasting syndrome, and climate change. The decline in sea otters and sea star wasting syndrome relates to the same overall problem: a decline in their predators, allowing the sea urchin populations to grow unchecked (Burgess 2021). Climate change and the warming of the oceans have suppressed many deep water upwellings, which are nutrient-rich and vital to kelp forest survival, causing them to be vulnerable (Stephens 2021).
By eating away all the kelp in the kelp forests, biodiversity plummets, rapidly affecting species like salmon, orcas, and seals by taking away shelter and food sources. This directly affects the survival of kelp forest-dwelling species and has harmful effects on us humans, including a decrease in food availability (Burgess 2021).
Kelp forest off the coast of California
Urchin Barren off the coast of Oregon
Fun Facts About Your Favorite Echinoderm!
Adolescents have light green spines until they fully mature (Fretwell 2014).
They can adapt to future conditions by changing their offsprings genes (if there is a low pH the mother will change the coding of the developing offspring to better fit those conditions (Burgess 2021).
Uni Sushi is the gonads of the Pacific Purple Sea Urchin (Oceana 2022).
Us humans have a very similar amount of genes (20,000 - 25,000) when compared to the Pacific Purple Sea Urchin (23,500) (Burgess 2021).
Pacific Purple Sea Urchins are one of the first groups of organisms stress during an algal bloom. Some signs of stress include reduced feeding and dropping of spines (Monterey Bay Aquarium 2022).
In intertidal zones, Pacific Purple Sea Urchins collect shells, peices of rocks, and algae on themsleves. This behavior is thought to play a part in camoflauge from predators when the water retreats (Monterey Bay Aquarium 2022).
Adolescent Pacific Purple Sea Urchin
Uni Sushi
The top center displays an urchin collecting shells and small rock pieces
Citations
Burgess, D. (2021). Resilience and the Purple Sea urchin. Washington State Department of Ecology. Retrieved December 5, 2022, from
https://ecology.wa.gov/Blog/Posts/May-2021/Resilience-and-the-purple-sea-urchin
Fretwell, K., & Starzomski , B. (2013). Purple Sea Urchin • Strongylocentrotus purpuratus. Biodiversity of the Central Coast. Retrieved December 5, 2022,
from https://www.centralcoastbiodiversity.org/purple-sea-urchin-bull-strongylocentrotus-purpuratus.html
Gosselin-Ildari, A. (2001). Strongylocentrotus purpuratus (purple sea urchin). Digimorph. Retrieved December 5, 2022, from
http://www.digimorph.org/specimens/Strongylocentrotus_purpuratus/
Heyland, A., & Hodin, J. (2014). A detailed staging scheme for late larval development in strongylocentrotus purpuratus focused on readily-visible juvenile
structures within the rudiment. BMC developmental biology. Retrieved December 5, 2022, from https://pubmed.ncbi.nlm.nih.gov/24886415/
Kazilek, C. J. (2009, December 23). Tube feet. kazilek. Retrieved December 5, 2022, from https://askabiologist.asu.edu/tube-feet
Monterey Bay Aquarium. (2022). Purple Sea Urchin. Montereybayaquarium.org. Retrieved December 5, 2022, from
https://www.montereybayaquarium.org/animals/animals-a-to-z/purple-sea-urchin
Pacific Purple Sea Urchin. Oceana. (2022, July 14). Retrieved December 5, 2022, from https://oceana.org/marine-life/pacific-purple-sea-urchin/
Pearse, J. S. (2006). Ecological role of Purple Sea Urchins. Science, 314(5801), 940–941. https://doi.org/10.1126/science.1131888
Stephens , T. (2021). The collapse of Northern California kelp forests will be hard to reverse. UC Santa Cruz News. Retrieved December 5, 2022, from
https://news.ucsc.edu/2021/03/kelp-forests-norcal.html
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