Red Sea Urchin

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Red Sea Urchin
Red Sea Urchin. (PD) Photo: Michael Nishizaki
Red Sea Urchin.
(PD) Photo: Michael Nishizaki
Scientific classification
Kingdom: Animalia
Phylum: Echinodermata
Class: Echinoidea
Subclass: Euechinoidea
Superorder: Echinacea
Order: Echinoida
Family: Strongylocentrotidae
Genus: Strongylocentrotus
Species: S. franciscanus
Binomial name
Strongylocentrotus franciscanus
(A. Agassiz, 1863)

The Red Sea Urchin, Strongylocentrotus franciscanus (A. Agassiz, 1863), is a species of marine invertebrate belonging to the phylum Echinodermata or "spiny-skinned" animals. Typically found in the Pacific ocean from Alaska to Baja California, red urchins inhabit shallow waters from the low-tide line to depths of 100 m. In general, this species prefers wave-sheltered rocky shorelines.

Physical Description

During larval development, the body of a sea urchin transitions from radial to bilateral symmetry. This bilaterally symmetrical larva, called an echinopluteus, subsequently develops a type of pentaradiate symmetry that characterizes echinoderms. As adults, the body, or test, can reach up to 24 cm and can vary in color from red to dark burgundy. The test is covered by sharp spines that can grow up to 8 cm in length. The exterior of the body is also covered with small pincer-like structures called pedicellaria and rows of tiny tube feet. Like all echinoderms, urchins have a water vascular system that aids in respiration, digestion, circulation and locomotion. An endoskeleton made of calcareous ossicles provides the body with additional structure

The mouth of an urchin, called the Aristotle's lantern, consists of 5 teeth and is located on the underside (or oral) of the body. Waste is excreted from an opening on the upper side (or aboral) of the body. During reproduction, eggs and sperm are also released from the aboral side of the body through 5 small openings called gonopores.

Feeding Habits

As larvae, urchins feed using bands of cilia to capture food particles from the water column[1], while also absorbing amino acids directly from seawater[2]. After settling on the ocean floor, juveniles begin feeding about 9 days post-metamorphosis. As adults, red urchins feed on a wide range of foods, both plant and animal though kelps are the preferred food item. In the Pacific Northwest, urchins preferentially graze bull kelp (Nereocystis leutkeana)[3], whereas in southern California, Macrocystis pyrifera is targeted[4].

Behavior and reproduction

(PD) Photo: Michael Nishizaki
Juvenile sea urchin.

Lifespan in red urchins often exceeds 30 years, and recent evidence indicates that some individuals are over 100 years old.[5]. Urchins are known as broadcast spawners, where gametes are released into the water and fertilization occurs in the water column. Spawning peaks between June and September and planktonic larvae (echinopluteus) remain in the water column for about a month before settling on the bottom of the sea floor. Once settled, larvae undergo metamorphosis into juvenile urchins. These juveniles use a set of chemical cues to locate and crawl underneath of near-by adults[6]. Aggregations of adult urchins form a "spine canopy" that provides protection from predation and extreme water motion[7]. It is not surprising then, that juveniles are found almost exclusively sheltering under adults[8] until they reach a test diameter of 40mm[9]. Juveniles are not necessarily related to the adults under which they shelter[10].

Similar species

There are two congeneric species generally found living in the same habitats as the red urchin. The purple urchin, Strongylocentrotus purpuratus is smaller, with shorter spines and a deep purple color. Purple urchins are usually found in slightly shallower and more wave-exposed areas. The second species is the green urchin Strongylocentrotus droebachiensis, which is smaller than the red urchin and has shorter and thinner spines.


Fig. 1. Historical sea urchin harvest.

Red sea urchins are primarily harvested for their reproductive organs, or "roe,". Most sea urchin roe is exported to Japan where it is used as an ingredient in sushi. Harvested in California since the 1970's, the sea urchin fishery expanded until it was the state's second most valuable fishery. Red Sea urchins are now harvested on the west coast from California to Alaska (Fig. 1).


  1. Strathmann, Richard R (1971). "The feeding behavior of planktotrophic echinoderm larvae: mechanisms, regulation, and rates of suspension feeding". Journal of Experimental Marine Biology and Ecology 6: 109–160.
  2. Manahan, Donal T.; James P. Davis and Grover C. Stephens (1983). "Bacteria-Free Sea Urchin Larvae: Selective Uptake of Neutral Amino Acids from Seawater". Science Magazine 220 (4593): 204-206. DOI:10.1126/science.220.4593.204. Retrieved on 2007-11-02. Research Blogging.
  3. Vadas, Robert Louis (1977). "Preferential feeding: An optimization strategy in sea urchins". Ecological Monographs 47 (4): 337-371. DOI:10.2307/1942173. Research Blogging.
  4. Leighton, D.L. (1966). "Studies of food preference in algivorous invertebrates of Southern California kelp beds". Pacific Science 20: 104-113.
  5. Ebert, Thomas A; John R Southon (2003). "Red sea urchins can live over 100 years: confirmation with A-bomb 14 carbon - (Strongylocentrotus franciscanus)". Fishery bulletin 101 (4): 915-922. ISSN 0090-0656. Retrieved on 2007-11-02.
  6. Nishizaki, Michael T; Josef Daniel Ackerman (2005). "A secondary chemical cue facilitates juvenile-adult post-settlement associations in red sea urchins (Strongylocentrotus franciscanus)". Limnology & Oceanography 50 (1): 354-362. ISSN 0024-3590.
  7. Nishizaki MT and JD Ackerman (2007). Juvenile–adult associations in sea urchins (Strongylocentrotus franciscanus and S. droebachiensis): protection from predation and hydrodynamics in S. franciscanus). Marine Biology 151:135-145.
  8. Low, CG (1975). The effect of grouping of Strongylocentrotus franciscanus, the giant red sea urchin, on its population biology. Ph.D. thesis, University of British Columbia, Vancouver, BC.
  9. Tegner MJ and PK Dayton (1977). Sea urchin recruitment patterns and implications of commercial fishing. Science 196: 324-32.
  10. Moberg, PE & RS Burton (2000). Genetic heterogeneity among recruit and adult red sea urchins, Strongylocentrotus franciscanus. Marine Biology 136:773-784.