Research Article |
Corresponding author: Robert G. Weck ( bob.weck@swic.edu ) Academic editor: Stefano Mammola
© 2022 Robert G. Weck.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Weck RG (2022) Life history observations of the Illinois state endangered Enigmatic Cavesnail, Fontigens antroecetes (Hubricht, 1940) made under simulated cave conditions. Subterranean Biology 43: 185-198. https://doi.org/10.3897/subtbiol.43.87277
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The Enigmatic Cavesnail, Fontigens antroecetes (Hubricht, 1940), is a cave adapted hydrobioid snail listed as state endangered in Illinois. It is known from only one cave in Illinois, Stemler Cave, and from several caves in the eastern Ozark ecoregion of Missouri. Little is known about the snail’s reproductive habits, embryological development, or growth rates. I attempted to gain basic life history information by breeding Enigmatic Cavesnails under simulated cave conditions in the laboratory. Six adult snails were collected from Stemler Cave and held in aerated containers of cave water with one or two cobbles from the cave stream. Containers of snails were housed in incubators set at the average cave water temperature of 13 °C. The snails produced 49 embryos in captivity over the course of 34 weeks. Eggs were deposited singly, attached to the underside of rocks within small pits or crevices. Nearly 82% of embryos developed to hatching. Mean estimated development time of embryos was 70.7 days. Survival of hatchling snails was poor. Limited data available from surviving hatchling snails suggests slow growth rates. The process was replicated with nine Enigmatic Cavesnails collected from Cliff Cave in St. Louis County MO. Captive Cliff Cave snails produced 34 embryos over 46 weeks and varied from the Stemler population in their oviposition behavior, with a majority of eggs deposited on the top surface of rocks. Cliff Cave snail embryos also had longer mean estimated development times (82.17 days).
Captive breeding, Fontigentidae, gastropod, stygobiont
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, Fontigens antroecetes (Hubricht, 1940) is a minute stygobitic hydrobioid limited to caves in the eastern edge of the Ozarks karst region, USA (Fig.
High levels of fecal coliforms and other contaminants have been documented in Stemler Cave (
Six individuals of F. antroecetes, ranging in size from 2 mm to 3 mm shell length, were collected from Stemler Cave, St. Clair County, Illinois USA between 28 August 2016 and 30 May 2017. The snails were variously paired in three different combinations in an attempt to induce reproduction in the laboratory under simulated cave conditions. Fontigens antroecetes is gonochoristic and females store sperm once mated. Because externally visible sexual dimorphisms are absent in this species, the sex of individual snails was not determined and it was not known whether any individuals had mated prior to collection.
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 F. antroecetes collected from Cliff Cave in St. Louis County, Missouri (USA) on 22 December 2018. The snails were divided equally between three containers. Water and cobbles from Stemler Cave were added and the containers were placed in simulated cave conditions on 8 January 2019. The null hypothesis that numbers of eggs laid on top, bottom and side of cobbles did not differ between Stemler Cave snails and Cliff Cave snails was evaluated using Fisher’s exact test, followed by post hoc pairwise Fisher’s exact test with Benjamini-Hochberg FDR method for correcting p values in R version 4.1.2 using packages “stats” and “statix” (
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 Fontigens antroecetes observed under simulated cave conditions in the laboratory a egg capsule containing two-day old embryo at the four-celled stage b well developed embryo at 75 days development c newly hatched snail d adult lab raised individual. Scale bars represents approximately 1 mm.
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
Timelines and details about egg deposition for each of the three Stemler Cave snail pairings are presented below.
Pair 1 – Two snails collected from Stemler Cave on 28 August 2016 were housed in cave water at ambient room temperature (circa 20 °C) for 142 days, during which 10 eggs were deposited. The snails were transferred to simulated cave conditions on 17 January 2017. Pair 1 snails produced a single egg under simulated cave conditions on 21 February 2017, which failed to develop to hatching. The pair was maintained for total of 70 days until one of the adults was accidentally crushed during data collection on 4 April 2017. The remaining snail was maintained alone under the simulated cave conditions until it was paired with another specimen on 30 May 2017.
Pair 2 – Three snails were collected from Stemler Cave on 4 March 2017 and immediately combined under simulated cave conditions. One snail was found dead on 28 March 2017. The remaining two snails were maintained for 181 days and produced 40 embryos. Egg production appeared to be cyclical during the 25 weeks that the snails were maintained (Fig.
Pair 3 – A single snail was collected from Stemler Cave on 30 May 2017 and combined with the surviving individual from Pair 1. The pair produced eight embryos during 61 days in simulated cave conditions. All embryos successfully developed to hatching. Both snails were found dead on 31 July 2017.
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 F. antroecetes reproduction in Stemler Cave can be gleaned from observations of embryos already attached to rocks collected from the cave stream for potential use in the captive breeding efforts. Well-developed embryos, apparently near hatching, were observed on 14 October 2016, 11 December 2016, 21 June 2017, and 13 November 2021. Early stage embryos were seen on 13 November 2016, 18 December 2017, 29 December 2020, and 13 November 2021. Partly developed embryos were observed on 13 February 2018 and 13 November 2021.
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.
Timelines and details about egg deposition for each of the three Cliff Cave snail combinations are presented below.
Container 1 – The three snails in container 1 produced no eggs after 225 days in captivity when all were preserved on 4 August 2019.
Container 2 – A total of six eggs were produced with the first egg observed 14 February 2019, 37 days after the snails were placed in simulated cave conditions. One snail was found dead on 2 May 2019. One additional egg was deposited beyond that date. A second snail was accidentally crushed during handling on 27 June 2019. No additional eggs were deposited beyond that date, and the remaining snail was preserved on 4 August 2019.
Container 3 – The first egg was discovered on 18 April 2019, 100 days after the snails were placed in simulated cave conditions. Sixteen additional eggs were deposited between 18 April and 16 July 2019. At that time, each of the three snails were sequentially isolated in an attempt to gain information about egg production by individuals. The first snail isolated produced no eggs before it was preserved on 6 August 2019. The two remaining snails were left paired between 16 July and 30 July 2019, during which four additional eggs were produced. These two individuals remain alive at this time, over 1000 days after entering simulated cave conditions. One produced a single additional egg on 18 August 2019. The remaining snail produced six additional eggs, with the last egg deposited on 6 March 2020.
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. Potamopyrgus antipodarum (Gray 1843) reared in aquaria at 12 °C grew at a rate of 0.007 mm/day over 250 days, and had a predicted life span of 16–18 months based on observations of a wild population (
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, Antrobia culveri, Hubricht, 1971, at the Tennessee Aquarium. The project utilized Antrorbis breweri, Hershler & Thompson, 1990, the Manitou Cavesnail from Alabama, and Fontigens spp. as surrogates. They were able to propagate some snails, but not in appreciable numbers sufficient for repopulating a portion of a cave. Future studies could explore more effective methods for maintaining laboratory populations of stygobitic snails as a possible refugium to protect against catastrophic events such as chemical spills. These laboratory populations could then provide a source for reintroduction after the cave stream has sufficiently recovered. This approach has been used for subterranean aquatic species in the Edwards Aquifer of Texas (
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.