Several species of sea urchins are found on the west coast.  Each species may live singly or in aggregations of hundreds or even thousands of individuals.  Red sea-urchins Strongylocentrotus franciscanus represent the largest species and are thought to be indigenous to the west coast. A commercial fishery exists for this species.  Green sea-urchins Strongylocentrotus droebachiensis have an Arctic/circum-boreal distribution and are found in the north Pacific nd north Atlantic regions. Their west-coast distribution extends south to Cape Blanco, Oregon and they are absent from California.

Purple urchins Strongylocentrotus purpuratus, unlike the other two species, are often found intertidally, especially in rough-water areas. In sandstone rock they often live in burrows hollowed out by teeth and spines over many successive generations.  Two subtidal species S. fragilis and S. pallidus are described, but their taxonomic status is uncertain.  Apparently, these 2 species hybridise readily.

NOTE  two excellent reviews have appeared recently with up-to-date literature lists in Edible sea urchins: biology and ecology (2007 Lawrence, ed.) Elsevier, Amsterdam: for green urchins see Scheibling & Hatcher p. 353 in this publication, and for red and purple urchins see Rogers-Bennett p. 393

NOTE  for lab studies on factors influencing aggregating behaviour of S. droebachiensis in Nova Scotia see Hagen & Mann 1994 J Exp Mar Biol Ecol 176: 107

Two species of sea urchins Strongylocentrotus droebachiensis and S. pallidus occur together over much of their ranges on the west coast.  Breeding times are similar and embryos of both species co-occur in the plankton.  In laboratory experiments at Friday Harbor Laboratories, Washington, eggs of S. droebachiensis are fertilised by sperm of S. pallidus, but the reciprocal cross yields low percentage fertilisation. The lab-produced hybrids are viable, with the offspring resembling S. pallidus in the larval stage and S. droebachiensis in the adult stage.  Moreover, the hybrids can backcross with S. pallidus.  The question is whether hybridisation occurs naturally.  This is not known, but if there is a barrier to genetic exchange in the San Juan Islands, the author conjectures that it may consist of small separations in space or time during spawning, or in substantially reduced viability and fecundity of hybrids in nature as compared with laboratory-produced hybrids.  Strathmann 1981 J Exp Mar Biol Ecol 55: 39. Photograph of S. pallidus courtesy Dave Cowles, Walla Walla University, Washington www.wallawalla.edu.

NOTE  the author notes that the adults “are similar and variable”, and it is difficult to distinguish hybrids on anatomical evidence

Green sea-urchins Strongylocentrotus droebachiensis are found in both the Pacific and Atlantic oceans and the question arises as to the extent of genetic differentiation between these populations.  With high fecundity and free-living larval lives of several-weeks duration, the expectation is that there will be high levels of gene flow.  Researchers at Dalhousie University, Nova Scotia collect specimens from San Juan Channel, Washington and compare frequency data of 4-locus microsatellite alleles of these with specimens collected from populations in the Atlantic region.  Results show no differentiation among populations in the northwest Atlantic, but strong differentiation between Pacific and Atlantic populations.  The authors present confirmatory evidence of an extinction in the northwest Atlantic followed by extensive recolonisation from the Pacific during Pleistocene times when the Bering Seaway opened (3.5mya).  Addison & Hart 2004 Mar Biol 144: 243.

NOTE  to illustrate this the authors estimate that larvae spawned off the coast of Nova Scotia are capable of travelling more than 1000km before settling

NOTE  a total of 11 populations are studied, most in the region of Nova Scotia to Newfoundland, but extending as far east as Norway and Iceland.  However, as the interest here is on Pacific-Atlantic differences, the comparative information from the Atlantic sites is not included here

Contrary to results of previous modeling-type studies showing that purple urchin Strongylocentrotus purpuratus are more densely populated at the southern end of their distribution, a researcher at San Diego State University, California finds highest densities between 35-37^oN latitude (Vandenburg-Santa Cruz).  The author uses “foot-on-ground” technology and actually visits 41 sites over a 3-yr period from 30^o (Punta Baja, Baja California) to 50^o (northern Vancouver Island, British Columbia), recording tide-pool densities and other information relating to demographic patterns.  The study shows that highest numbers of recruits are found in the latitudinal range 34-38^o, and maximum and minimum diameters at 43^o and 34^o, respectively.  There is no significant latitudinal pattern to growth or survival.  The author speculates that the southern range limit is best explained by thermal tolerance of adults and the northern one by developmental times of larvae at low temperatures.  Ebert 2010 Mar Ecol Progr Ser 406: 105.

NOTE  these include about 65% of the species’ reported range

Growth of jaws of the Aristotle's lantern
is determined after tetracycline injection,
then converted to growth in test diameter

Fisheries & harvest refugia

Research study 1

Red sea-urchins¹ Strongylocentrotus franciscanus are fished commercially for the sushi² trade from Alaska to California.  Both sexes are harvested and marketed.  Other than Alaska, which has a small and somewhat sproadic fishery, most west-coast fisheries peaked in 1988-92.  The graphs below show total catch for fisheries along the west coast, and the one for Washington state³ includes CPUE (catch per unit effort, that is, the amount of time spent by fishers in relation to yield). CPUE in all areas has been dropping since the inception of the fisheries in the late 1970’s.  Andrew et al. 2002 Oceanog Mar Biol Ann Rev 40: 343.

NOTE¹  some green urchins S. droebachiensis are also fished, but represent a minor component of overall harvest

NOTE²  sushi made with sea-urchin gonads is known as uni-zushi

NOTE³ in Washington there is a small recreational fishery, and similar sports fisheries likely exist in other States. Tuya et al. 2000 J Mar Sci 57: 1218.

Research study 2

Observations on red urchins Strongylocentrotus franciscanus¹ in Bodega Bay, California suggest that shallow-water populations, if maintained in harvest refugia, could possibly enhance recruitment by their large spawnings and sheltering of juveniles. Individuals in shallow beds (at 5m depth) have significantly larger gonads (mean 63g) than individuals in deeper beds (14-23m depth; overall mean=12g).  Presumably, there is more food available for the urchins in the shallow habitat, and this may help explain why the shallow urchins move around less. The density of urchins in the shallow beds is also more than twice that in the deeper beds (see histograms). Although absolute spine lengths in the shallow population are less than that in the deeper populations, sheltering of juvenile urchins² under the spine canopies of adults is higher in shallow than in deeper habitats (28 vs. 2%, respectively).  Part of the reason for this may be that shallow-water adults live in rock “bowls” in which they move less than their more motile deep-water counterparts.  Principal-component analysis reveals several morphological distinctions between shallow- and deep-inhabiting S. franciscanus.  These are shorter spines, larger gonads, thicker tests, smaller lanterns, and smaller peristomial openings. The authors think that the feature of short spines³ would be enough to distinguish the different populations, and a policy of protecting urchins in shallow-water refugia would offer another management strategy for sea-urchin harvesting in northern California.  Rogers-Bennett et al 1995 Ecol Applic 5: 1171.

NOTE¹  the authors remark that in 1991 red urchins had become the state of California’s most valuable fishery

NOTE²  juveniles of both S. franciscanus and S. purpuratus

NOTE³  the authors’ data indicate that prohibiting harvest of urchins with spines <52mm in length would protect over 80% of the shallow-water urchins while allowing >90% of the deeper-water ones to be harvested

This is an interesting idea, but what other factors might be involved? Consider these possibilities, then CLICK HERE to see explanations.

Harvesting is more difficult at depth. 

It is the sushi-trade that dictates harvest value. 

Simple harvest-economics (“catch” of gonads per unit effort) will work against exploitation of the deeper population. 

Removal of gonads is easier from short-spined (shallow) urchins than from long-spined (deeper) ones. 

What about establishing more MPAs (Marine Protected Areas) designed to act as refugia for sea urchins?  Do they even work?  This question is addressed in a study in San Juan Islands, Washington where several marine protected areas have been established.  Three of these are special University of Washington research preserves (UWPs), designated in blue on the map, and two are MPAs, designated in yellow on the map. The UWPs are “no take” zones except for scientific research and then only by special permission.  They have been in existence for about 5yr prior to the study.  The MPAs are voluntary “no take” zones, except for fin-fishes that are specifically prohibited.  The MPAs have been in existence for only about a year prior to the study.  Three other sites are marked on the map in purple.  These are Unprotected areas used as control sites in the study.  Censuses for red sea-urchins Strongylocentrotus franciscanus in 1998 (1yr after establishment of the two MPAs) show similar levels of abundance between the newly established MPAs and the Unprotected areas (the graph shown data only for large urchins). The authors could find no differences in abundance of small urchins among the 3 categories of sites, and they think this is attributable to a lack of effective protection within the protected sites.  The significantly greater abundance of large-sized urchins in the UWPs probably owes to the fact that these areas lie within an urchin-fishery closure zone established in the late 1970s.  Small urchins tend to be virtually absent at all sites in comparison with larger-sized ones.  The authors credit this to possible gamete dilution from low densities of adults (in the MPAs and Unprotected areas).  Presumably densities of adults in the UWPs (225 individuals ^. 300m ^-2) are high enough to mitigate this effect, but their larvae may drift beyond their capability for colonising the other sites.  The answer to our research question, then, is a mitigated ‘yes’.  Marine protected areas in the San Juan Islands have been effective in enhancing survival of medium and large size classes of S. franciscanus, but more work will need to be done to assess their effectiveness as “broodstock feeders” even in such an enclosed area as the San Juan archipelago.  Tuya et al. 2000 J Mar Sci 57: 1218.

NOTE  many other invertebrate and fish species are also recorded in systematic transect-line surveys used in the censuses, and analysed using interesting cluster-analysis techniques, but the data of most interest here concern the sea urchins

NOTE  the lack of enhancement of densities of large urchins in the MPAs over densities in the Unprotected areas likely reflect their relatively short history