Synchronous and Non-Synchronous Semelparity in Sibling Species of Pulmonates

Diverse life histories have been documented in terrestrial pulmonates, which inhabit different regions in climate. Life history traits are often phenotypically plastic and vary depending on the environment. Thus, surveys using designs that control for the confounding effects of environment are needed to evaluate the evolutionary differences between populations of closely related species in the wild. We examined the life histories of sibling species of terrestrial pulmonate within two regions of similar climates. Bradybaena pellucida (BP) is endemic to Japanese islands, and has recently been expanding its distribution northeastward, whereas B. similaris (BS) has been introduced by humans into temperate and tropical regions worldwide. We found that these species exhibit discrete differences in population dynamics and life cycle, despite their close relatedness. The annual life cycle of BP is synchronized among individuals in a population. Thus, BP is univoltine with discontinuous generation. In contrast, BS individuals do not synchronize their growth or reproduction, and thus exhibit overlapping generations. Our results indicate that synchronized and non-synchronized population dynamics diverge relatively rapidly in semelparous pulmonates. This type of difference has not been documented in pulmonate life history, and may have been overlooked because only a few studies have explicitly compared life cycles of closely related species within the same climate. Our results provide a basis for further studies of life history evolution in pulmonates.ArticleZOOLOGICAL SCIENCE. 32(4):372-377 (2015)journal articl

Life cycle and population dynamics of Discus perspectivus (Megerle von Muhlfeld, 1818) (Gastropoda: pulmonata: Endodontidae)

Life cycle and population dynamics of Discus perspectivus (Mühlf.) were studied in the field and in the laboratory. No courtship, copulation or uniparental reproduction could be observed. The eggs, laid in June, July and August on rotting timber are calcified, ellipsoidal, ca. 1 mm in major diameter; the number of eggs per batch is 1–9 (mostly 3–4), with a total of 17–33 eggs per individual per season. The incubation period is 24–35 days, hatching is nearly synchronous; the proportion of hatching eggs laid by individuals brought from the field is 51% (for eggs brought from the field 38.7%). The hatchlings have shells of 1.8–2.3 whorls; they consume their egg envelopes immediately after hatching. The egg cannibalism is prolonged into adult stage; only conspecific eggs are consumed; juveniles eat eggs of their own and alien batches. The growth rate in the laboratory is 1 whorl per 49–188 days (slower in the field); the growth is faster in juvenile and slower in mature snails. The snails reach maturity at slightly over 5 whorls (mature gametes present in the gonad), the life span is 173–849 days, the reproductive life constituting ca. 7% total life span. In the field juveniles hatch from June till October, with the maximum in August; till their first hibernation they reach 2.6–3.5 whorls and become mature in their second season to reproduce in the same or next year. The population density exceeds 50 m-2 in summer, and ranges from 25 to 30 m-2 during the remaining seasons. D. perspectivus shows aggregated distribution in August and October, in the remaining months the distribution is even. Some laboratory-born individuals have very much elevated spires and their shells become scalariform at the level of 5.3–5.75 whorls

Age structure and growth rate of Aegopinella epipedostoma (Fagot, 1879) (Gastropoda: pulmonata: Zonitidae)

Growth rate and age structure of Ae. epipedostoma (Fagot) were studied based on monthly samples from a population in Muszkowice (SW. Poland). The life cycle is probably a three-year cycle: juveniles hatch from July till September, and winter over at a size of 2.6–3.5 whorls. The growth rate is ca. 0.5 whorl/month. In their second season, the snails resume growth in spring to reach 4 whorls in May-July; they reproduce in the same season and at least some of them winter over again

Shell variation in the genus Discus Fitzinger, 1833 (Gastropoda: pulmonata: endodontidae)

In Europe the genus Discus Fitzinger, 1833 is represented by two subgenera: Gonyodiscus Fitzinger, with D. rotundatus (O. F. Müll.) and D. perspectivus (Mühlf.), and Discus s. str. with D. ruderatus (Fér.). Studies on their shell variation were to ascertain if variation among laboratory-bred individuals differed from such variation in natural populations. The number of whorls, shell height, body whorl height, aperture height, aperture width, shell major and minor diameter, umbilicus major and minor diameter, shell height/major diameter ratio, relative height of body whorl, relative umbilicus diameter and umbilicus major/minor diameter ratio were analysed. Variation ranges of most shell characters in laboratory specimens of D. rotundatus and D. ruderatus were much wider than those found in natural populations. The natural populations of each species differed statistically significantly among themselves in many characters but the differences were much smaller. Laboratory-bred D. perspectivus showed a tendency to produce scalariform shells or descending and partly detached body whorl. Laboratory-bred D. rotundatus and D. ruderatus tended to form a descending body whorl; in all three species the descending/detached part of the body whorl was formed after sexual maturity had been attained

Life cycle and population dynamics of Discus rotundatus (O.F.Muller, 1774) (Gastropoda: Pulmonata: Endodontidae)

The following life history traits of D. rotundatus (O. F. Müll.) were established based on field and laboratory observations: fully calcified eggs ca 1 mm in diameter, of a size correlated with the number of parent’s whorls; 1–11 eggs per clutch; 1–178 eggs per lifetime, in 1–36 clutches; incubation period 14–36 days; egg cannibalism with a strong preference for conspecific eggs; quick growth (33–35 days per whorl) prior to maturity and slow growth in mature snails, resulting from energy expenditure for egg-laying; sexual maturity at 5.5 whorls (2nd–3rd year of life); life span 2.5–3.5 years. Isolated individuals produce fewer eggs of lower viability than individuals kept in groups, they lay their eggs later and at a larger size. Most eggs in the laboratory arelaid in June, in thefie ld themaximum of there productivepe riod falls in July/August

Life cycle of Discus ruderatus (Ferussac, 1821) (Gastropoda: pulmonata: Endodontidae)

Life cycle of Discus ruderatus (Fér.) was studied in the laboratory, with occasional field observations. No courtship or copulation could be observed; uniparental reproduction is very rare. In the laboratory the eggs are laid in all months, with maximum in June/July and December/January, in the field the youngest age class appears in May. The eggs are laid on rotting timber; they are calcified, nearly sphaerical, ca. 1.5 mm in major diameter; the number of eggs per batch is 1–6 (mostly 3–4), with a total of 6–15 eggs in 2–5 batches per lifetime. The incubation period is 17–34 days, hatching is nearly synchronous; the hatching success is ca. 54%. The hatchlings have shells of 1.5–2.5 whorls; they consume their egg envelopes immediately after hatching. The egg cannibalism is prolonged into adult stage. The overall mean growth rate is 1 whorl per ca. 179 days (54 days per whorl till maturity, 487 days per whorl in mature snails). The snails reach maturity at slightly over 4 whorls (mature gametes present in the gonad), the mean life span is 580 days, the reproductive life constituting ca. 35% total life span. Life cycle parameters of three members of Discus are discussed comparatively

Age structure and growth rate of Zonitoides nitidus (O. F. Müller, 1774) (Gastropoda: Pulmonata: Gastrodontidae)

Growth rate and age structure of Zonitoides nitidus (O. F. Müller) were studied based on monthly samples from a population in Muszkowice (SW. Poland). The life cycle is probably a three-year cycle: juveniles hatch from June till October, and winter over at a size of 2.0–4.5 whorls. The growth rate is ca. 0.5–1.0 whorl/month