Habitat ecology of the native subtidal snail, Lacuna vincta, in the Gulf of Maine and the impact of two introduced species
Lacuna vincta (Montagu) is a small native herbivorous gastropod that feeds and lives on algae within the rocky subtidal zone of New England. In this study, a combination of field samplings and laboratory experiments were conducted in order to examine the seasonal abundance, feeding preference, and growth of snails on nine different algal species. The results showed that there were differences in the seasonal abundance patterns of L. vincta populations on different algal species. However, the snail populations in all size classes were found year round on Laminaria saccharina (L.) Lamour. Feeding preference experiments showed that L. vincta preferred Antithamnionella floccosa (O. F. Mull.) Whittick, Ulva lactuca L., and Laminaria saccharina. However, the growth experiments demonstrated that growth rates of snails fed on L. saccharina tended to be higher than those fed on other algae. Because of the introduction of non-indigenous species, several introduced taxa have become established within subtidal ecosystems of the Gulf of Maine. The impact of the introduced bryozoan, Membranipora membranacea (L.), and the green alga, Codium fragile (Sur.) Hariot subsp. tomentosoides (van Goor) Silva, on Lacuna vincta was investigated. Results from field samplings, reciprocal transplantations, and laboratory experiments suggest that these two introduced species have an apparent negative impact on L. vincta. The preferred habitat and food of snails in the kelp canopy was reduced by bryozoan coverage and/or replacement of kelp by C. fragile ssp. tomentosoides . The overgrowth of M. membranacea on kelp blades decreases the available grazing spaces and affects the growth of L. vincta. The coexistence of these two species in the same habitat has created intraspecific and interspecific competition for this resource. In addition, changing algal composition from a kelp-dominated canopy ( Laminaria saccharina) to one dominated by C. fragile ssp. tomentosoides also had a negative impact on this snail. Field sampling showed that the density of L. vincta at sites dominated by C. fragile ssp. tomentosoides was less than half that of sites dominated by L. saccharina. The parallel success C. fragile ssp. tomentosoides and M. membranacea, may lead to an even higher negative impact on L. vincta populations.
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