Growth and reproduction in Hawaiian succineid land snails

The native land snail fauna of the Hawaiian Islands is one of the most threatened snail faunas in the world; a large proportion of the species, perhaps as many as 90%, may now be extinct. The impacts of introduced predators [e.g. rats and the predatory snail Euglandina rosea (Férussac)] and other human-related impacts (e.g. habitat destruction, collecting by shell collectors), superimposed upon long pre-reproductive life and low fecundity (as in achatinelline tree snails), have been implicated in the demise of many of the species. Certain native snails, however, have remained relatively abundant. Among these are some of the rainforest succineids. Aside from the Achatinellinae, little is known of the life histories of most Hawaiian land snails. However, just as knowledge of life-history characteristics has been important in understanding the high extinction rate of the achatinelline tree snails, such information may also be key to understanding why some succineid species remain abundant in the face of similar pressures. In this study, the oviparous rainforest succineids Succinea thaanumi Ancey and Catinella rotundata (Gould), which lay gelatinous, translucent egg masses, usually on aboveground vegetation, were raised in captivity to investigate a range of life-history characteristics. Three recently hatched juvenile Succinea thaanumi and one egg mass from Olaa, Island of Hawaii, were brought into a captiverearing facility on 3 November 1999. The juveniles were assumed to have hatched very recently because their bodies appeared white through the transparent shells and their digestive glands were not dark, indicating that they probably had not yet fed. Fifteen Catinella rotundata from Makaleha, Waianae Mountains, Island of Oahu, were brought into the facility on 30 March 2000. Snails and egg masses were maintained as colonies in plastic containers (10 cm wide, 10 cm deep, 16 cm tall, with mesh tops; one for each species) subject to night (12 h) and day (12 h) light cycles, and intermittent spraying with water to simulate rain (3 min, every 8 h) in an environmental chamber (16°C at night, 20°C during the daytime), as used for achatinelline tree snail breeding. The natural diet of these succineids is poorly known, but probably includes fungal material obtained from leaf surfaces. Each container was provisioned with fresh leaves and branches of the native Hawaiian tree Metrosideros polymorpha (Myrtaceae), partly covered with fungus, plus supplemental cultured fungus fortified with a trace of calcium carbonate. The M. polymorpha and fungus were replaced approximately every 14 days when the containers were cleaned. At that time, maximum shell dimension (length) of adults and offspring, number of births and deaths, and number of eggs laid were recorded. Reproductive characteristics of Succinea thaanumi are summarized in Table 1. Average shell length of the three juveniles (termed Cohort A) on 10 November 1999 (7 days after they were collected) was 1.8 mm. The mean maximum length attained by these three individuals was 9.3 mm, on 15 March 2000, after a period of 134 days in captivity. The first egg mass laid by them was found on 1 March 2000, when their average length was 8.8 mm, about 4 months after they had been brought into captivity (as juveniles). Additionally, two of them were observed mating (by ‘shell-mounting’) on 15 March 2000 (shell lengths 9.8 and 10.1 mm). One was found dead on 30 March 2000, the other two on 28 April 2000, giving a life span of at least 178 days for these latter two snails. The first S. thaanumi hatched from the wild-collected egg mass (Cohort B) was observed on 22 December 1999. Because the previous observation date was 8 December 1999, the snail could have hatched any time during this period and the time to hatching was, therefore, at least 36–50 days. The average shell length of juveniles that were first seen on 28 April 2000 was 0.85 mm (n = 15) and for those first seen on 9 May 2000 was 0.90 mm (n = 26). Data from the two hatchings were combined after 9 May 2000 as Cohort C. They reached a mean maximum length of 7.8 mm on 13 October 2000, approximately 5 months from birth. The first reproductive event witnessed for Cohort C snails was a mating on 4 August 2000. Shell lengths of these snails were 7.0 and 7.7 mm, when the average length of Cohort C snails was 6.0 ± 1.1 mm (mean and SD; n = 11). Time to reproductive maturity is therefore approx. 3 months. The first egg mass laid by Cohort C snails was found on 1 September 2000, when the snails were about four months old. The minimum maximal lifespan for Succinea thaanumi, calculated for Cohort C from the earliest possible birth date (13 April 2000) and the date of death of the last Cohort C snails (22 December 2000), was 253 days. Reproductive characteristics of Catinella rotundata are also summarized in Table 1. All wild collected individuals except the largest died soon after coming into captivity, most before reachCorrespondence: R. J. Rundell; e-mail: rundell@uchicago.edu