Now you see me. Now you don't.

The history of the rocky intertidal of the northeastern coast of North America is a history of alien invasion.  A stroll, or more realistically scramble, along the coast today reveals a community that is dramatically different than that observed by the early European colonists in New England.  Today, you would see an abundant and widespread periwinkle snail dispersed on and amongst the rocks.  You would see green or orange trapezoidal crabs hiding beneath algae and rocks in tide pools and regions closer to the low tide mark.  You would see brown striped, square-shaped crabs quickly scuttling away as you lifted boulders.  In the 1600’s, you wouldn’t have seen any of these now common animals. 

Rocky intertidal during low tide. Maine, USA.  Photo credit: Sara Edquist

Rocky intertidal during low tide. Maine, USA.  Photo credit: Sara Edquist

Approximately 200 years ago, in the early 1800s, Littorina littorea, known today in the US as the “common periwinkle” and in Europe as the “edible winkle”, was introduced from Northern Europe.  Not surprisingly, due to the habitat similarities between the Northeast Atlantic (Europe) and the Northwest Atlantic (N. America), this snail proved to be very successful in adapting to the environment.  Today it is an established and integral part of the community. 

Littorina littorea , the common periwinkle or edible winkle.  Photo credit: Sara Edquist

Littorina littorea , the common periwinkle or edible winkle.  Photo credit: Sara Edquist

A little over 100 years ago, in the late 1800s, another organism was introduced from northern Europe.  Carcinus maenas, the “European green crab”, similarly assimilated to the Northwest Atlantic and became an established predator in the community.  Over the last century, the common periwinkle and European green crab have both adapted and established flourishing populations.  Today’s community is a reflection of the substantial impacts both of these organisms had on an intertidal community with relatively low diversity and limited competitors.

The European green crab, Carcinus maenas .  Photo credit: © Copyright Malcolm Storey 2011-2112

The European green crab, Carcinus maenas .  Photo credit: © Copyright Malcolm Storey 2011-2112

However, times they are a-changin’.  More recently, another organism has been introduced to this region.  In the 1980s, Hemigrapsus sanguineus, the “Asian shore crab”, made it’s was to the east coast of North America.  The Asian shore crab was first documented south of Cape Cod, MA.  By the late 1990s, it had been found north of Cape Cod.  Since its introduction, the Asian shore crab has displayed an incredible ability to adapt and spread.  Rocky intertidal sites that were previously dominated by European green crabs are now dominated by the Asian shore crab.  The green crabs which have had such a large impact on the rocky intertidal community over the last 100 years are now becoming relatively rare as the Asian shore crab population steadily increases.

The Asian shore crab, Hemigrapsus sanguineus .  Photo credit 

The Asian shore crab, Hemigrapsus sanguineus .  Photo credit 

The Asian shore crab is not only becoming very abundant, but it is also a more aggressive predator and competitor.  This crab also has a much broader tolerance of stressful environmental conditions.  Regardless of what ocean or sea around the world, the intertidal zone is a difficult habitat in which to live.  As the tide rises and falls, temperature, salinity, and exposure to air all fluctuate; survival depends on the ability to tolerate these regular changes.  Consequently, an organism’s ability to tolerate these harsh conditions determines where it resides in the intertidal.  The Asian shore crab’s broad tolerance of stressful environmental conditions means that it can inhabit parts of the rocky intertidal that historically had very low predator presence.  Other organisms that are adapted to live in these regions of the intertidal with little to no predators now have to cope with an abundant and aggressive predator.
Aggressive posturing of the Asian shore crab, Hemigrapsus sanguineus .  Photo credit: http://home.kpn.nl/faassema/Hemigrapsussanguineus.html

Aggressive posturing of the Asian shore crab, Hemigrapsus sanguineus .  Photo credit: http://home.kpn.nl/faassema/Hemigrapsussanguineus.html

The Asian shore crab’s distribution in New England is continuing to expand.  Within the last month, it has been reported for the first time as far north as Casco Bay.  The full impact of the Asian shore crab on the New England rocky intertidal is not yet understood.  However it is clear that the community is changing.

BIOLOGY NOTES 

 Littorina littorea, the Common Periwinkle

  • The dominant grazer in the New England rocky intertidal.

  • Commercially harvested, both in Europe and Northeastern US (especially Maine), as escargot. 
  • In its native range, L. littorea is host to over 11 species of flatworm (trematode) parasites. In its introduced range, it is host to only 5 species.
An aggregation of the common periwinkle, Littorina littorea .  Photo credit: Dysmachus 

An aggregation of the common periwinkle, Littorina littorea .  Photo credit: Dysmachus 

Hemigrapsus sanguineus , the Asian Shore Crab

  • Similar to C. maenas , H. sanguineus  can also breath air for a limited time by "blowing bubbles".
  • Unlike most crab species, the Asian shore crab and other related grapsid crabs do not need to molt to mate.
  • Sexual dimorphism is very apparent in this species, with males have significantly larger claws.
Asian shore crab, Hemigrapsus sanguineus , "blowing bubbles" to breathe.  Photo credit: GALJ at Vital Signs, Gulf of Maine Research Institute 

Asian shore crab, Hemigrapsus sanguineus , "blowing bubbles" to breathe.  Photo credit: GALJ at Vital Signs, Gulf of Maine Research Institute 

Carcinus maenas , the European Green Crab

  • Has the ability to breathe a limited amount out of air by creating “blowing bubbles”.
  • As time since molting passes, the exoskeleton changes from the green color (its namesake) to orange.  Hence older crabs that have not molted recently will often be orange.
Two European green crabs, Carcinus maenas, mating.  Note the orange color the male, in comparison to the green female beneath that has recently molted is is now ready for mating.  Photo credit: Ar rouz 

Two European green crabs, Carcinus maenas, mating.  Note the orange color the male, in comparison to the green female beneath that has recently molted is is now ready for mating.  Photo credit: Ar rouz 

  • Intertidal predation by the introduced C. maenas in New England over the last 100 years has induced a change is the shell shape of native snail species. 
  • In the native range, it is host to a parasitic barnacle which castrates the crab and suppresses molting of the exoskeleton (necessary for growth, mating, and clearing the exoskeleton of encrusting organisms).
 A European green crab, Carcinus maenas, infected with a parasitic barnacle (Sacculina carcini ).  Photo credit: Ar rouz

 A European green crab, Carcinus maenas, infected with a parasitic barnacle (Sacculina carcini ).  Photo credit: Ar rouz

References:

Blakeslee, AMH, Byers, JE, and Lesser MP. 2008. Solving cryptogenic histories using host and parasite molecular genetics: the resolution of Littorina littorea's North American origin. Molecular Ecology 17: 3684-3696. http://dx.doi.org/10.1111/j.1365-294X.2008.03865.x

Epifanio, CH. 2013. Invasion biology of the Asian shore crab Hemigrapsus sanguineus: A review. Journal of Experimental Marine Biology and Ecology 441: 33-49. http://dx.doi.org/10.1016/j.jembe.2013.01.010.

 

Resources:

Eat the Invaders: Periwinkle recipes 

Maine Periwinkle fishery landings 

Massachusetts Invasive Species Monitoring 

Check out more posts on introduced species