Strongylocentrotus droebachiensis, green sea urchin
Background
In the western North Atlantic portion of its circumpolar range the green sea urchin is
distributed from the Canadian Arctic to New Jersey (Atkinson and Wacasey 1989,
Gosner 1978). It is a common occupant of both sheltered and exposed kelp beds and
rocky areas of the low intertidal zone and subtidal zone and lives to 1,200 meters in the
Atlantic Ocean. Green sea urchins feed on macroalgae and a wide variety of other
organisms, and scrape diatoms and coralline algae from rocks. Green sea urchins spawn
in March to April in the western North Atlantic, with a smaller spawning in the autumn
(Brady and Scheibling 2006). In Nova Scotia spring spawning was found to coincide
with the annual temperature minimum while fall spawning coincided with the annual
maximum of 8–10°C (Brady and Scheibling 2006). Eggs and sperm are broadcast into
the water column where fertilization occurs. Larvae are planktonic from four to 21
weeks (Scheibling and Hatcher 2001) and typically settle in July in Nova Scotia (Balch
and Scheibling 2000, in Brady and Scheibling 2005).
Brady and Scheibling (2005, 2006) described the key role that green sea urchins play in
the alternating subtidal community states of lush kelp beds and urchin barrens dominated
by coralline algae. Mass urchin mortality caused by the amoeba Paramoeba invadens
occurs during peak seawater temperatures, with the severity related to the magnitude and
duration of temperatures exceeding 12°C. Mortality is greatest in the shallowest depths;
urchins in waters deeper than 20-25 m experience a thermal refuge from the disease.
However, the deeper waters are a food-limiting environment negatively impacting urchin
growth and reproduction. With autumn cooling the disease subsides and urchin
repopulation of the shallows begins through settlement of planktonic larvae and
migration of adults from nearby deeper waters. Green sea urchins can migrate up to
several hundred meters a day. Within two to three years following a mass mortality event
kelps can grow to dominate the subtidal environment. However, with urchin
repopulation the feeding pressure on the kelps grows, reverting the shallows once again
to urchin barrens.
Green sea urchins are harvested commercially for their roe in Newfoundland, southwest
New Brunswick, and southwest Nova Scotia
(http://marvdc.bio.dfo.ca/pls/vdc/mwmfdweb.auth,
http://www.gov.nf.ca/fishaq/species/underutilized/pdf/sea_urchin_green.pdf,
http://www.bofep.org/underuti.htm), and also in Maine
(http://www.st.nmfs.gov/st1/commercial/landings/ds_8850_bystate.html).
Temperature limits, critical thresholds, vulnerability, and barriers to adaptation
Sea surface temperatures in the current distribution of S. droebachiensis range from a
February minimum of -2.1oC to an August maximum of 24.9oC.
Chapter 3
109
This is a cold water species. Critical temperatures are from Scheibling and Hatcher 2001.
Green sea urchins are found in our waters from -1oC for all life stages to 18oC for larvae
and 20oC for benthic individuals. Though growth rate is not strongly temperature
dependent recently settled juveniles grow best at 9–13°C (Pearce et al. 2005). Maximum
lethal temperatures are 19oC for larvae and 22oC for settled individuals. Urchins were of
average sensitivity compared to other species in our analysis of sensitivity to climate
change. The larval stage may be the most critical thermally because of its lower natural
and lethal temperatures.
Critical temperatures for the disease-inducing Paramoeba invadens were given by
Scheibling and Hatcher (2001). Growth is optimum at 12–20°C with arrested
development at 10–12°C (urchins can recover from infection at 8°C or less). The amoeba
cannot survive below 2°C, indicating periodic introductions from warmer regions.
Impacts
A 4oC rise in global temperature will impact the future distribution of green sea urchins
in the western Atlantic. Results from all models and scenarios are generally similar and
show potential loss of thermal habitat in waters south of approximately Cape Cod.
Model GFDL Scenario A2 shows potential loss in the southern Gulf of St. Lawrence, as
well. Model CCSR shows loss off southern to most of Labrador. All other waters
generally will remain suitable for this species. No habitat gain is predicted in our study
which does not extend to the northern range limit of the green sea urchin. No loss of
habitat is predicted where urchins are harvested commercially in the Atlantic provinces
and Maine.
Because the green sea urchin is a cold water species the waters in its southern range
which do not reach lethal summer temperatures as the climate changes nevertheless will
make living conditions suboptimal. The resulting negative effect on growth and
reproduction of Laminaria spp., a principal urchin prey, will reduce food quality and
quantity for the urchins in southern waters. More importantly, mass urchin mortality
caused by the amoeba Paramoeba invadens occurs during peak seawater temperatures,
with the severity related to the magnitude and duration of temperatures exceeding 12°C.
It is very likely that such mortality events will become much more prevalent and perhaps
widespread with global warming. This impact should have a positive effect on the algal
prey of urchins.
References
Atkinson, E.G., and J.W. Wacasey. 1989. Benthic invertebrates collected from Hudson
Strait, Foxe Channel and Foxe Basin, Canada, 1949 to 1970. Canadian Data Reports
of Fisheries and Aquatic Sciences No. 746, 98 p.
Brady, S.M., and R. E. Scheibling. 2005. Repopulation of the shallow subtidal zone by
green sea urchins (Strongylocentrotus droebachiensis) following mass mortality in
Chapter 3
110
Nova Scotia, Canada. Journal of the Marine Biological Association of the United
Kingdom 85: 1511-1517.
Brady, S.M., and R. E. Scheibling. 2006. Changes in growth and reproduction of green
sea urchins, Strongylocentrotus droebachiensis (Müller), during repopulation of the
shallow subtidal zone after mass mortality. Journal of Experimental Marine Biology
and Ecology 335: 277–291.
Gosner, K.L. 1978. A field guide to the Atlantic seashore. Invertebrates and seaweeds of
the Atlantic coast from the Bay of Fundy to Cape Hatteras. The Peterson Field Guide
Series. Houghton Mifflin Co., Boston.
Pearce, C.M., S.W. Williams, F Yuan, J.D. Castell, and S.M.C. Robinson. 2005. Effect of
temperature on somatic growth and survivorship of early post-settled green sea
urchins, Strongylocentrotus droebachiensis (Müller). Aquaculture Research 36(6):
600-609.
Scheibling, R.E. and B.G. Hatcher 2001. The ecology of Strongylocentrotus
droebachiensis. In: Edible sea urchins: biology and ecology (J.M. Lawrence, ed).
Developments in Aquaculture and Fisheries Science No. 32, Elsevier Science Press,
Amsterdam, London, New York, Oxford, Paris, Shannon, Tokyo.
Chapter 3
111