Taxonomy

Ancestral Origins
 

Distribution


Anatomy
 

Reproduction
 

Ecology

Littorina obtusata, also known commonly as the “smooth or flat periwinkle” or the “smooth littorine”, is a conspicuous resident of rocky intertidal communities here along the northwestern shores of the United States.

Taxonomy:
phylum: Mollusca
class: Gastropoda
order: Mesogastropoda
family: Littorinidae
genus: Littorina
species: obtusata

Molluscs are said to be highly successful due to the fact that there are more species of molluscs in the ocean than any other group (Castro and Huber 2000).

Beginning approximately 3.5 million years ago during the Pilocene epoch, the North Atlantic and Arctic experienced a large invasion of species from the North Pacific through the Bering Strait (Bertness 1999).  On the western side of the North Atlantic, it is thought that some 83% of rocky intertidal mollusks and 50% of mollusks from sandy and muddy shores of New England and Canadian shores arrived from the Pacific or are the derived descendants of Pacific invaders.  L. obtusata and its cousin, Littorina saxatilus, are both derived from a common ancestor in the subgenus Neritrema (Bertness 1999).  Today, L. obtusata are found on both sides of the Atlantic Ocean.  In North America, they range from New Jersey to Newfoundland and southern Labrador (Trussell 1997).  Genetic evidence comparing the DNA of modern Littorina obtusata from New England and L. obtusata from Europe has suggested that the snails may have recolonized the North American side of the Atlantic from Europe following local extinction due to Pleistocene glaciation about 22,000 years ago (Wares and Cunningham 2001).  The evidence is based on the hypothesis that populations which are founded more recently are expected to represent a subset of the genetic diversity of the source population (in this case the European population).  This, in turn, translates into more recently founded populations having much lower genetic diversity with a high frequency of alleles that are identical or descended from alleles in the European population (Wares and Cunningham 2001).  Indeed, this was the case where the New England population possessed a high-frequency haplotype shared with the European population but possessed lower haplotype diversity overall relative to snails in Europe (Wares and Cunningham 2001).

 Littorina obtusata can be found below the splash zone throughout intertidal shores but is the most abundant at midshore.  This herbivorous snail feeds during high tide or moist conditions directly on several fucoid algal species such as Fucus spiralis, F. vesiculosus, F. seratus, and Ascophyllum nodosum as well as epiphytic microflora growing upon such algae (Trussell 1997).    In fact, L. obtusata have become specialized in grazing upon a few species of fucoid algae and can induce A. nodosum to increase defensive chemical phlorotannins production (Pavia and Toth 2000).  It is also hypothesized that obtusata grazing on diatoms and epiphytes could benefit A. nosodum by limiting algal fouling (Bertness 1999).

Asophyllum nodosum
As characteristic of most molluscs, L. obtusata possesses a soft body surrounded by a thin layer of mantle tissue that produces their protective calcium carbonate shell (Castro and Huber 2000).  They have an unsegmented body with bilateral symmetry, a ventral muscular foot for locomotion, and a head with eyes and sensory organs (Castro and Huber 2000).  L. obtusata uses its tongue like radula (a ribbon of small teeth), composed largely of chitin, to scrape diatoms, algal sporelings, and algal tissue into their mouths which is then mixed with mucus and digested (Bertness 1999, Castro and Huber 2000).  Being gastropods (stomach footed), the snails are basically a mass of vital organs enclosed within their shell.  Their internal structures include a heart, gonad, digestive gland, stomach, intestine, anus, gill, and salivary gland.  The head and foot can be retracted into the shell leaving their operculum as the ‘door’ protecting the shell opening from predation dangers and desiccation (Castro and Huber 2000).

 Littorina obtusata are gonochoric and undergo internal fertilization.  The sperm can be stored in the bursa copulatrix for short time periods and then for 3 months or more in the seminal receptacle organ deep within the oviduct (Paterson et al. 2001).  Females may mate many times before fertilizing the eggs, which can result in multiple paternity of any given brood (Paterson et al. 2001).  Females will then lay benthic egg masses containing 50-150 eggs on fucoid algae (Trussell 1997, Paterson et al. 2001).  The embryos undergo direct development and emerging juveniles begin a benthic existence immediately upon crawling away from egg masses (Trussell 1997, Paterson et al. 2001).
The green crab, Carcinus maenas, and other crabs are formidable predators on L. obtusata (Trussell 1997).  In Europe, green crabs prey heavily upon juvenile L. obtusata restricting them to high intertidal heights (Bertness 1999).

Carcinus maenas
Other evidence of this relationship are shown by L. obtusata shells collected prior to 1900 before the introduction of the green crab that were relatively thin with high spires, while shells collected in the 1980’s after green crabs were well-established were significantly thicker with lower spires (Bertness 2001).  However, it is unclear how much of this defensive change is genetically based or due to phenotypic plasticity (Trussell and Smith 1999).  Another way in which L. obtusata may attempt to avoid predation is by cryptic coloration when associated with Ascophyllum nodosum perhaps along with mimicry of A. nodosum’s gas-filled vesicles (Wilbur and Steneck 1999).