Strongylocentrotus purpuratus | MARINe

Strongylocentrotus purpuratus (Purple Urchin)

Strongylocentrotus purpuratus (Stimpson 1857)

Kingdom Animalia, phylum Echinodermata, class Echinoidea, order Echinoida, family Stronylocentrotidae

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Purple sea urchins are echinoids encased in a round, radially symmetrical endoskeleton of interlocking calcium carbonate plates (also known as a “test”). The test is covered with spines, tube feet, and pincer-like organs called pedicellariae (Light and Smith 2007) S. purpuratus are generally smaller urchins (50-100mm) with bright purple spines, occasionally pale green in juveniles (<30mm) (Light and Smith 2007 and Morris et al. 1980). Female and male purple urchins are monomorphic (not physically different from one another) (Morris et al. 1980).

Habitat and Geographic Range

S. purpuratus are commonly found in cracks, pools, and mussel beds in the mid to low intertidal, and extending subtidally to 160m from Alaska to Cedros Island, Mexico (Light and Smith 2007, Morris et al. 1980, Olhaussen and Russo 1981). In sedimentary rock they are often found in round hollows or pits that are formed by erosion from the urchins’ spines and teeth (Light and Smith 2007).



Similar species

The most notable lookalike to S. purpuratus is the red sea urchin (Mesocentrotus franciscanus). Red urchins are primarily a subtidal species, extending to depths of 90m, but can occasionally be found in the very low intertidal zone on open coasts' rocky shores from Japan and Alaska to Isla Cedros, Baja California. Tests of M. franciscanus are typically larger than S. purpuratus (~100mm or more) and bear long (50mm) red to bright dark purple/brown spines. Red urchin gonads are considered a delicacy by man and animals alike (Morris et al. 1980). Another species that can be confused with S. purpuratus in the northern portion of its’ range (Alaska to Washington) is the green sea urchin, S. droebachiensis. Green urchins can be common in calmer, inland waters (e.g., Salish Sea), whereas purple urchins occur mainly on the outer coast (Kozloff 1993). As with red urchins, green urchins are primarily subtidal (to 1,140 m) but also occur in the low intertidal zone (Lamb & Hanby 2005). Tests are similar in size to purple urchins, but spines are finer in green urchins and greenish in color, with reddish brown tones near the top (Kozloff 1993).

Natural History

Purple sea urchins are an ecologically important coastal species that aids in regulating kelp forests densities through algal grazing (Osovitz and Hoffman 2015). Purple urchins are usually detritovores, however when kelp litter is reduced urchins can shift from drift feeding to active foraging on macroalgae (Carr and Reed 2016). Dense subtidal populations move across rocky reefs grazing areas of kelp down to the holdfast, forming urchin “barrens” (Light and Smith 2007).

Historically, sea otters were the main predator of purple sea urchins. However, fur hunters nearly eradicated the sea otter population during the 1900s, resulting in cascading changes to the kelp forest system. Sea otter prey, like abalone and urchins, became more abundant with the loss of this keystone species. Divers harvested abalone until their populations diminished and then began harvesting the larger red urchin, leaving the small purple urchins behind (McEachern et al. 2016). Although sea otter populations are increasing in central and northern California, we do not see otters foraging on urchins as frequently as they once did (McEachern et al. 2016 and Smith et al. 2020). In areas where kelp has been greatly diminished, urchins are starving and their gonads are reduced, making them a less appealing snack to a sea otter (Smith et al. 2020).

In addition to reduced predators and food supply, urchins are also susceptible to infections from common oceanic bacteria of the Rhodospirillum genus. The purple bacteria cause necrotic spots to form on the exterior of the test. Tube feet and spines in this area fall off as the bacteria eats its way through the test. Urchins can survive months with the hole through their test until they eventually are killed by the infection. Infection rates can be exacerbated by years of poor weather conditions (i.e., increased water temperatures and pH) and decreased food supply. In fact, sea urchins are used as an indicator organism for water quality because they are one of the first animals to show signs of stress (Sea Urchin Embryology). For more information and photos for identifying disease in urchins please see our guide.

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