obtusata sporting an olive green color morph
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.
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).
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).
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).
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).
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).
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).