On the dispersion, population structure, and life history of a basommatophoran snail, Helisoma trivolvis, in central Alberta

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Abstract
We studied the dispersion, population structure, and life history of a northern population of Helisoma trivolvis inhabiting an artificial pond in central Alberta in 1968 and 1969. In both years, snails were concentrated from May to October within the first 6 m from shore (150 cm in depth). In spring the number of overwintered snails rose from 21∙m−2 in 1968 to 42∙m−2 in 1969, but juvenile recruitment fell from 160∙m−2 in 1968 to 0∙m−2 in 1969. Based on the number of varices, some individuals survived up to five summers. Varix counts on samples of empty shells suggested that on average about 65% of the adult population was 1 year old with proportionately fewer in each older age category. Maturation among 1-year-old snails appeared to require growth to more than 9.0 mm maximum diameter. Although the diameter attained by young snails in their first summer ranged from 8 to 16 mm, none was recorded spawning before their second summer. Survivors spawned in each succeeding summer unless rendered infertile by trematode infections. Peak spawning occurred in late May. Mean number of eggs per egg mass varied between years from 30 to 37. Juveniles appeared in early July and reached peak numbers by mid-July. Survival of young snails changed dramatically between years. The role of iteroparity as an adaptive strategy of this species is discussed in the context of fluctuating productivity.