| Summary: | The trophic ecology of two predatory marine gastropods, Lunatia heros and Polinices duplicatus, feeding on Mya was monitored under field conditions. Feeding and growth rates were affected by prey size, temperature and prey type. Both predatory species selected prey according to size, Lunatia consumed smaller clams than did similar Polinices. Lunatia actively fed throughout the year, peaking at 0.42 Mya/snail/day at Gloucester during the summer, continuing to feed through the winter (0.15 Mya/snail/day); whereas the feeding activities of Polinices varied seasonally, reaching a maximum of 0.6 Mya/day during the summer, but ceasing for the winter. Feeding and growth rates were also studied with moon snails using an alternative prey, Ilyanassa obsoletus. While both species consumed similar amounts of these prey species, moon snail growth was limited with a diet of Ilyanassa. Due to their ecological similarity, these species compete in areas of overlap by utilizing common resources. Intra- and interspecific pairs of moon snails were maintained on limited diet of Mya. Both species grew less under competitive conditions as compared to the solitary feeding trials. The effects of interspecific competition on snail growth were less severe than the growth impact and intraspecific competition; individuals of both species grew more with individuals of the other species than with individuals of their own species, possibly leading to a stable coexistence between the two predatory species. Several temporal and spatial mechanisms may allow these species to partition the habitat and reduce competitive interactions. When both species were present, Lunatia normally remained in the lower intertidal (i.e., lower temperature), whereas Polinices inhabited the upper beach to low mean water. Lunatia also exhibited limited burrowing abilities (i.e., hunting) as compared with the actions of Polinices, restricting Lunatia to the smaller Mya near the surface of the sediments. Lunatia was less aggressive than Polinices, avoiding direct encounters unless Lunatia had a significant size advantage over its competitor. Finally, due to Lunatia ability to remain active during the winter, it was able to reproduce before Polinices became active. Larval Lunatia may limit or escape competition through time.
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