Page September 1980 No. 26 NEW YORK SHELL CLUB NOTES 
MARSH SNAILS 
The Gentle Grazers 
By Thomas M. Baird 
tween the grasses of the coastal salt 
ae herbivorous snails that find a 
ge snails exist in vast numbers within 
entle creatures. If the visitor to 
f grass and 
the salt marsh carefully removes a snail from a stalk o 
places it on his open = the snail will begin crawling around 
within a few seconds, tentacles waving about. The interaction be- 
tween these snails and the marsh grasses is one intriguing aspect of 
the marsh habitat. 
Coastal marshes have their own unique 
to offshore marine life has been repeatedly demonstrated. Growing 
nearest to the water, with its roots and lower stalks covered daily 
by seawater, cord grass (Spartina alterniflora) abounds. Ona 
slightly higher elevation in southern marshes is the needle rush 
(Juncus roemerianus). living on the stalks of these grasses is a 
small tan or brown snail, the salt-marsh periwinkle (Littorina ir- 
rorata). 
Life begins for the salt-marsh periwinkle in the form of a plank- 
tonic egg; adult sexes are separate and fertilization is internal, 
The fertilized egg develops into a typical trochophore larva, and 
eventually becomes a swimming veliger larva. As its shell develops, 
the young snail settles onto a marsh plant. Periwinkles are most 
plentiful in summer and fall, when the young snails colonize the 
cord-grass stalks. The size of the overall population is kept in 
check, however, by stone crabs, which feast on them. 
Periwinkles crawl up and down the leaves of the cord grass, staying 
up high during high tide. At low tide, they may be seen crawling 
around the bases of the plants. Equipped with the typical trap doors, 
or operculums, found in most marine snails, periwinkles can withdraw 
into their shells for protection against cold weather or to prevent 
dessication. a ae ogee periods in the marsh, or when the tempera- 
ture falls below 46°F, they withdraw into their shells and secrete a 
ace Sy Cees anchoring themselves to the cord grass above the high 
water mark, 
Using their rasping, filelike radulas, the salt-marsh periwinkles 
eat small epiphytic algae that grow on the moist leaves of the cord 
grass. On their way down the grass stalks, the snails consume bac- 
teria and detritus, as well as algae growing around the base of the 
plants, This activity is beneficial to both the plants and the 
snails. Without the snails, the cord-grass leaves would soon become 
coated with algae and bacteria, which would block the Sunlight neces- 
sary for photosynthesis. As the snails keep the surface of the 
leaves clean, they obtain -- in turn -- food for themselves. 
There is another, more far-reaching benefit, howe erl- 
wWinkles feed, they loosen much of the algae, sated = Mee ty 
not all of which are consumed. At the next high tide, this material 
is carried into the nearby estuary or ocean, thus providing a valu- 
able energy source to near-shore aquatic food webs. Tn addition 
A unique relationship exists 
marshes and three species of 
home on and around them. ‘The 
the marsh, and they are totally & 
beauty, and their importance 
