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Littorina Species |
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Atlantic Shoreline Ecology Homepage Gulf of Maine Organisms Nerita Versicolor Experiment
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In New England rocky intertidal systems, Littorina spp. are the most dominant grazers by far (Bertness, Mark D. 1999). They are rather common in the Gulf of Maine, with wide distribution within the intertidal zone. They feed vigorously with an evolutionarily modified device called a radula. This device is made out of a supportive base studded with a ribbon of recurved teeth (Brusca, Richard C. & Brusca, Gary J. 1990) specially suited for their herbivorous diet. In the littorines the radula is taenioglossate (Brusca, Richard C. & Brusca, Gary J 1990). They consume diatoms, algal sporelings, and soft algal tissues (Bertness, Mark D. 1999). Like all gastropods, Littorina spp. have a corkscrew-like twist to their bodies. This is the product of two 90° twists at different stages of their life cycles (Brusca, Richard C. & Brusca, Gary J. 1990). The first occurs in the larval stage, and is due to a velar muscle. This muscle attaches to the left side of their head and foot. A contraction in this muscle causes to shell and internal body structure to twist 90° in a counter-clockwise direction (Brusca, Richard C. & Brusca, Gary J. 1990). This twist occurs very rapidly taking anywhere from a few minutes to a few hours. The second twist in the adult morphology comes from differential tissue growth in the development of the organism (Brusca, Richard C. & Brusca, Gary J. 1990). This takes months to achieve. Littorine movement in the intertidal adult takes the form of a ciliated sole (Brusca, Richard C. & Brusca, Gary J. 1990). The cilia on this "foot" with the assistance of muscle undulations across the sole allow for movement of the organisms. The pedal gland secretes a slimy mucous on which the snails can slide and move (Brusca, Richard C. & Brusca, Gary J. 1990). The pedal retractor muscle shortens and lengthens the sole so that the organism can have the option of hiding in its shell for periods of time (Brusca, Richard C. & Brusca, Gary J. 1990). The littorine shell consists of a few layers of calcareous material. This includes a prismatic layer, a lamellate layer, and an epidermal later right under the shell (Brusca, Richard C. & Brusca, Gary J. 1990). Littorina littorea: This species of Littorina, often called the common periwinkle was introduced into the Gulf of Maine from Europe in the late 1800s (Steneck, Robert S. and Carlton, James T. 2001). Since then, it has drastically altered the New England intertidal community structure by allowing slow growing Chondrus crispus to overtake faster growing green algal species (Menger, Bruce, A. & Branch, George M. 2001). In both open coast and estuary habitats L. littorea can be found often at densities of 200-500 individuals per square meter (Menger, Bruce, A. & Branch, George M. 2001). L. littorea has a planktonic development stage and may have reached Canada before the 1800s (Vermeij, G.J. 1982). It is more likely however, that it was introduced by settlers in Nova Scotia in the middle of the 1800s and from there quickly moved south. At the turn of the 19th century it spread south and reached Cape Cod by the 20th century. Chesapeake Bay was infested by the 1950s (Carlton, J.T. 1982). L. littorea will settle on anything with a hard substrate as it is a habitat generalist (Bertness, Mark D. 1999). It reaches 2-3 cm in length and can vary greatly in shell morphology. It has become the most common herbivore in its range and has been very vigorously studied due to the ecological implications it poses (Lubchenco, J. 1978). In estuarine cobbled beaches it can reach densities of up to 1000 individuals per square meter (Bertness, Mark D. 1999). When experimentally removed from a protected New England rocky beach, an increased success of soft-sediment organisms was seen, with a reduced success of hard substrate organisms (Bertness, Mark D. 1984).
Littorina obtusata: Known as the smooth periwinkle, this species is smaller than L. littorea and native unlike its larger cousin. It is almost always found living on Ascophyllum nodosum canopies (Bertness, Mark D. 1999). It develops directly, differing in life history stages from L. littorea. It is known for its phenotypically plastic features. Shell shape and thickness, much like L. littorea is highly variable, and responds to exposure to prey (Bertness, Mark D. 1999). Overall shell thickness in New England communities has increased recently in L. obtusata due to the invasion of Carcinus maenas (Trussell, G.C. 1996). When exposed to predatory crab excretions, shell thickness in the experimental population was found to rapidly increase compared to a control population by Trussell and Smith (2000). When found in open environments where wave exposure is high, it develops a larger foot to grip to the substrate with (Trussell, G.C., Johnson, A.S., & Rudolph, S.G. 1993).
Littorina saxatilis: The smooth periwinkle L. saxatilis like L. obtusata is both smaller than L. littorea and lacks a pelagic life stage (Bertness, Mark D. 1999). It grows up to 8mm in length and can be found in the high intertidal above where L. littorea is usually located(Gosner, K.L. 1978). It resembles the juvenile form of L. littorea and can easily be mistaken for its invasive cousin. In Europe L. saxatilis grows faster and larger on wave protected shores (Hughes, R.N. & Roberts, D.J. 1981; Janson, K. 1983) It has received little experimental attention in New England.
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Littorina spp. feeding on intertidal hard rocky substrate . Littorina obtusata Phylum: Mollusca Class: Gastropoda Order: Mesogastropoda Genus: Littorina Species: littorea obtusata saxatilis |
