Life cycle and population dynamics of Helicodonta obvoluta (O. F. Müller, 1774) (Gastropoda: Pulmonata: Helicidae)

Abstract

Life cycle of Helicodonta obvoluta (0. F. Mull.) was studied in the field and in laboratory. Mating lasts 2-3 hrs and includes: meeting of the partners, recognition, courtship dance, copulation, resting phase and parting. No spermatophores were observed upon dissection of a total of 90 adult individuals which seems to indicate that H. obvoluta does not produce them. The egg-laying snail embeds anterior part of its body 4-6 mm deep in rotting timber. The egg-laying lasts from about a dozen hours to two days. Freshly laid eggs are white, calcified, slightly translucent and get opaque in a few days. They are slightly oval, of 2.10-2.85 mm major and 2.00-2.60 mm minor diameter. In laboratory, eggs are laid in spring (March June) and autumn (August-November), in the field, the egg-laying periods are somewhat shorter (April-beginning of June, end of August-beginning of October). Most laboratory snails laid eggs only once in their lifetime, the maximum number of egg-laying periods was four. The percentage of hatching eggs in laboratory is ca. 59%. The number of eggs per clutch ranges from 9 to 27. The incubation period ranges from 14 to 31 days, and is shorter for spring (14-18 days) compared to autumn (19-23 days) clutches. Hatching is asynchronous, lasting from 1 to 4 days. With approaching hatching, the white colour of the egg disappears, so that the young snail is surrounded only by a translucent membrane, which gets broken as a result of its movements. Newly-hatched snails have shells of 1 whorl, devoid of periostracal hairs. No egg cannibalism was observed. Out of 174 young hatched in laboratory, 159 reached maturity. During numerous dissections of adult individuals no eggs were found in the reproductive tracts; if there is an egg-retention, it must be very short-lasting. Placing eggs in rotting timber and covering them with mucus protects them from drying-out, ensures a more favourable temperature and limits accessibility to predators. The number, relative and absolute size of eggs, number of clutches per year and per lifetime, and the life span seem to be correlated with size rather than with phylogenetic position of the species. No uniparental reproduction was observed. In laboratory the time elapsing between hatching and maturity (lip completely formed) ranged from 140 to 624 days; it varied between individuals hatched in particular years and seasons, e.g. young of the spring 1997 grew much faster (mean 354 days) than those of the spring 1999 (mean 442 days). The time required to reach full size was not correlated with the ultimate number of whorls. The growth shows three distinct phases: a quick initial phase of 3-4 months, a slow phase, and a short quick phase preceding lip formation. The monthly increment depends on the growth phase: 1.15 whorl in phases 1 and 3, 0.30 whorl in phase 2. The growth rate in the field is similar to that observed in laboratory, though with a wider scatter within growth phases and some differences between years and seasons. Depending on weather conditions, the youngest age class (1) appears from April till June or from May till July, and from the end of August till the beginning of October. Thus each season two new generations are produced: spring and autumn. The spring generation, depending on the month of hatching, may complete its growth in the same season and winter as adults, or reach stage 4 or 5 and complete their growth next spring. The autumn generation always winters as immature snails which complete their growth in late spring next year. Adult snails (c. 1 year old) dominate in all the monthly samples, while older individuals are few. In laboratory, the life span ranged from 516 to 1,187 days; thus the life span of some individuals exceeds 3 years. The estimate of life span in the field, based on marking-release-recapture, is less exact, but on the whole the oldest snails in the field lived slightly over 3 years, while life span of some was only 2 years. In laboratory and in the field, the snails reproduce in spring and autumn. In laboratory, they are active (feeding, crawling) throughout the year, but in winter resting periods of a few days at a time are observed, with the aperture covered by a thin epiphragm. In the field, the snails enter winter torpor at the end of October/beginning of November, having penetrated rotting logs from below, and stay in rotting wood with their apertures covered with thick, calcified epiphragms. Depending on weather, they get active at the beginning or end of April. Under conditions of constant temperature and humidity (indoors) adult snails show two activity peaks: late evening and early morning, with a resting period during the day. Immature individuals are more active, with a constant high activity since afternoon till mid-morning, and most remain active during the day. In conditions of variable temperature and humidity (out-doors), the activity of all age classes depends on humidity and temperature, the immature snails, like in constant conditions, being generally more active. The mobility of H. obvoluta is rather high, individual snails cover the distance of 4-5 or even 7 metres during a month.