Corresponding author: Robert G. Weck (
Academic editor: Stefano Mammola
The Enigmatic Cavesnail,
Weck RG (2022) Life history observations of the Illinois state endangered Enigmatic Cavesnail,
Subterranean ecosystems are likely the most widespread nonmarine environments on Earth, yet specialized subterranean organisms are among the least documented and studied groups (
The Enigmatic Cavesnail,
Distribution of the Enigmatic Cavesnail (
High levels of fecal coliforms and other contaminants have been documented in Stemler Cave (
Six individuals of
Cave-like conditions were created within 15 × 10 × 5 cm lidded plastic containers. Each container included cave water and a cobble collected from the stream in Stemler Cave. Cobbles were inspected carefully to exclude any new snails or snail embryos. The cobbles included biofilms on which the snails grazed and substrates for egg deposition. The average cobble size was 12 × 9 × 3 cm. Water depth in the containers was maintained at 3.5 cm. The containers were aerated by aquarium air pumps with a tube fitted through a hole drilled in the lid. The containers were placed in a darkened 46 × 46 × 51 cm electronic cooler set at 13 °C, the mean annual water temperature in Stemler Cave (
The experimental containers were removed from the cooler once or twice a week for data collection. The condition of the adult snails was assessed and all surfaces of the cobbles were inspected under a stereozoom microscope for the presence of eggs and to check the condition of developing embryos. The position of any newly deposited eggs was noted (top, side, or bottom surface of the cobble). Water was changed on average every 7.84 days (± 3.3 SD) and cobbles were replaced when biofilms were depleted and to isolate newly detected eggs. Cobbles were replaced on average every 10.26 days (± 6.13 SD). 38.46% of cobble changes were done to isolate newly deposited eggs. Embryonic development times were estimated because embryos were not checked daily to avoid excess handling and potential damage. Mean estimated development times were calculated by averaging the longest and shortest possible intervals between oviposition and hatching, which varied depending on the frequency of observation for each embryo. Newly hatched snails were isolated in separate containers as space in the cooler permitted. Growth rates of a limited number of offspring were estimated by periodically measuring shell length to nearest 0.25 mm with a ruler.
Similar procedures were followed using nine
Wild collected snails from Stemler Cave produced a total of 49 eggs under simulated cave conditions. Eggs were deposited singly, within a clear capsule approximately 1 mm in diameter (Fig.
Life stages of
Hatching success rate was 81.6%. A total of five embryos were accidentally damaged or destroyed during handling for data collection and four embryos failed to fully develop for unknown reasons. Mean estimated embryonic development time was 70.7 days (range 56.5 to 81 days). The uncertainty in development time ranged from 2 to 8 days, with an average of 5.3 days. The hatching process was directly observed in one instance. The snail became more active within the egg capsule in the two days preceding hatching and was observed chewing on the capsule. At the time of hatching the snail crawled out of the capsule through a well-formed hole that it had apparently created. This hatching process has also been documented by
Growth profile of
Timelines and details about egg deposition for each of the three Stemler Cave snail pairings are presented below.
Pattern of egg production by
On 27 June 2019 two laboratory-produced offspring snails, raised in isolation for over two years, were combined in an attempt to yield a second generation of lab-reared offspring. The first egg produced by this pairing was deposited on 10 July 2019, at which time the parents were isolated in an attempt to determine the sex of each snail. A total of 13 eggs were deposited by the female between 10 July and 30 September 2019. Nine embryos successfully hatched (70%). Like the wild collected snails, the female deposited single eggs within in a gelatinous capsule, almost exclusively on the underside of cobbles, often in small pits or crevices. Only one egg was deposited on the top surface of a cobble. The mean estimated development time for the second generation was 73 days. The female, progeny of Pair 1, hatched on 11 December 2016, was 2.5 years old at the time of pairing, and had a shell length of 3 mm. The male, progeny of Pair 2, hatched circa 30 September 2017, was 2.25 years old, and had a shell length of 2.75 mm. The adult female survived until 21 October 2019 and likely died of injuries due to mishandling. The male survived until 22 April 2021. Two sibling second generation lab-reared snails were then reared together. These snails hatched in November 2019 and reached 2.5 mm shell length by the end of November 2020. This pair produced the first viable embryo in September 2021.
Some information about
Captive Cliff Cave snails deposited a total of 34 eggs during the course of this study. The first egg was observed on 14 February 2019 and the last on 6 March 2020. All eggs were deposited singly, within an approximately 1 mm wide gelatinous capsule. Unlike the pattern seen in Stemler Cave snails, the Cliff Cave animals mostly utilized pits and crevices on the top and side surfaces of cobbles for oviposition (Fig.
Comparison of egg deposition on top, side, and bottom surface of cobbles in captive
Timelines and details about egg deposition for each of the three Cliff Cave snail combinations are presented below.
A few Cliff Cave offspring were successfully raised to maturity. Three individuals hatched in September 2019 had reached 2.75 mm shell length by July 2020 and began producing a second generation in the lab by late August/early September 2020. Twenty-six eggs were observed between September 2020 and September 2021. Oviposition varied from the wild-collected snails in that a higher percentage of eggs were deposited on the bottom side of cobbles (54%). One surviving individual continued to produce viable embryos in September 2021.
The deposition of single eggs within capsules, attached to substrates appears to be typical for several genera of hydrobioid snails (
Almost nothing is known about stygobitic snail reproduction in nature.
Mean estimated embryonic development time of 70.7 days for Stemler snails and 82.17 days for Cliff Cave snails is surprisingly long considering the small size of the embryo, and likely reflect slow growth and lower metabolic rates typical of cave-adapted organisms (
No comparable studies of sygtobitic snail growth and longevity are available, although a few studies of epigean hydrobioids have been published.
The results obtained in this study provide a possible approach to gaining life history information about snails from other subterranean systems. Dr Paul Johnson (personal communication, 2 December 2021) attempted to develop protocols for captive rearing the federally endangered Tumbling Creek Cavesnail,
I thank Jeff Arnold, Steve Taylor, and Patrick Weck for assistance in producing the figures. Steve Taylor also provided help with statistical analysis. Nicole Linskey and Olivia Tarantella assisted with data collection and snail care. This work was permitted by the Illinois Department of Natural Resources under endangered and threatened species permit no. 10804.