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Interactions between the two herbivorous marine gastropods, Littorina littorea and L. obtusata
Master of Science
Littorina, Competition (Biology), Coexistence of species, Competition, Coexistence, Littorina littorea, Littorina obtusata
Herbivorous gastropod species such as Littorina spp. may have overlapping foodand habitat requirements, resulting in competitive interactions in temperate rockyintertidal areas. L. littorea is a habitat and food generalist, found on rockyshorelines, salt marshes and tidal flats and is capable of feeding on a variety of macroalgae, microalgal mats and invertebrate eggs; while L. obtusata is a specialist, relying on Fucus vesiculosus and Ascophyllum nodosum for food and habitat. Through both field and laboratory experiments, I investigated intra- and interspecific interactions between these species, focusing on food and density effects on grazing and growth rates. Although abundances of both herbivores at field sites in Maine and Rhode Island were relatively stable through time, removal of L. littorea from fucoid canopies resulted in significant increases in densities of L. obtusata at both sites. In the laboratory, paired-choice dietary preference experiments indicated that Ulva lactuca was the preferred food of L. littorea, while L. obtusata showed no grazing on this ephemeral algal species. Further, when held in the presence of U. lactuca, L. obtusata suffered nearly 100% mortality due to a toxic exudate produced by this algal species. In contrast, F. vesiculosus was highly preferred by L. obtusata over all other algal species offered. When offered F. vesiculosus or A. nodosum that had been pre-grazed by its conspecific or un-grazed, neither herbivore had a significant preference, showing similar grazing rates between treatments. However, although L. littorea failed to induce phlorotannin levels in either fucoid species, L. obtusata demonstrated reduced grazing rates on fronds pre-grazed by L. littorea, revealing a strong negative effect suggestive of interference competition. Grazing by L. obtusata was also reduced when held in the presence of L. littorea or its waste, and growth rate determinations under varying snail densities and food availability indicated that the presence of L. littorea reduced growth in L. obtusata, further highlighting an adverse interspecific interaction. L. littorea demonstrated the opposite pattern: lower growth rates were observed in the presence of its conspecific, indicating intraspecific competition. L. littorea showed no effect on growth when in the presence of L. obtusata, indicating no interspecific interaction by L. obtusata. Taken together, these results demonstrated that L. littorea exerted competitive interference on L. obtusata, affecting grazing and growth rates as well as population densities. Due to the mobility of these gastropods and adequate food supply in most intertidal areas, these closely related species are able to coexist through resource partitioning, thus avoiding strong competitive interactions.
Putnam, Alysha Barbara, "Competition and coexistence : interactions between the two herbivorous marine gastropods, Littorina littorea and L. obtusata" (2016). Masters Thesis, Smith College, Northampton, MA.
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