Mitchell’s Rainforest Snail

 Jonathan Parkyn gave a detailed and intriguing presentation on the current state of the local, and critically endangered, Mitchell’s rainforest snail and the research been done to learn more. Jonathan is a PhD candidate at Southern Cross University

Snails are molluscs, from the Latin word mollis, meaning soft. Molluscs are soft-bodied animals including Mitchell’s rainforest snail (Thersites mitchellae), in the same group as squid, cuttlefish, octopus, oysters, scallops, pipis, and many more. Unfortunately, approximately a quarter of all Australian molluscs are under threat. The gastropods, namely snails and slugs, contain over 80% of all terrestrial and aquatic molluscs. In Australia we have about 2,000 species of land snails, with most species occurring in only one state or territory. Nationally, there are 15 listed threatened land snail species. In NSW we have three critically endangered land snail species, including our own Mitchell’s rainforest snail (the first invertebrate to be listed as threatened in NSW), the Cumberland Plains snail, west of Sydney, and the Lord Howe Island flax snail.
Mitchell’s rainforest snail has a restricted distribution occurring on coastal plain between Ballina and Tweed Heads. Historical records suggest that the snail used to be common in areas such as the Big Scrub rainforest which was previously extensive on the north coast. The snail has been protected since 1999 under the NSW Threatened Species Conservation Act, and is also listed as critically endangered nationally. In 2001 Stotts Island Nature Reserve was declared critical habitat for Mitchell’s rainforest snail. A Nature Reserve since 1971, it is approximately 165 ha in area, and is located in the lower reaches of the Tweed River, near Murwillumbah. The Reserve supports 120 ha of relatively undisturbed subtropical rainforest. Prior to our studies in 2009 this area contained the largest verified population of the species. The entire population at that time was estimated to be less than 500 individuals throughout its range. The snail was also known to occur at a few other locations including Banora Point, Lennox Head and Byron Bay, though sometimes from a single observation.
In 2009 students from Southern Cross University had an opportunity to study Mitchell’s rainforest snail at the Byron at Byron resort, near Suffolk Park. Management and staff have been fully supportive of these studies, and weed control strategies and bush regeneration techniques routinely include consideration for the snail’s habitat. Guests are informed about the snail, and the environment in general, providing occasion to educate the public in a relaxed and beautiful setting. People seem genuinely fascinated in the snail project, and being able to walk along boardwalks amongst threatened species adds a special dimension to their rainforest experience.
My first encounter with Mitchell’s rainforest snail was with shell fragments from predation by the noisy pitta. A rainforest bird, the pitta is a natural threat to the snail but the greatest threat is, by far, human beings and land clearing. We have created habitats that are relatively small and fragmented. These areas are further threatened by fire, exotic weeds, and predation by introduced species such as rats and cane toads. Part of our work is to determine the level of these threats, and what can be done to alleviate them.
Mitchell’s rainforest snail is a large native land snail with a shell up to 5 cm in height. The shells vary from a red-brown to black in colour with two prominent yellow bands. The body colour is black with a thin white dorsal line. The snail has two sets of tentacles that can be retracted independently so that you might see them without either, one or the other or both. The shape of a land snail shell is a clue to where you might find them. Mitchell’s is triangular in profile, and they tend to shelter upside down during the day (see picture below). They are primarily a litter dwelling species. The juveniles are thought to be arboreal and we have found them sheltering under bark or in crevices at the base of trees. We distinguish a snail as an adult or a juvenile by looking for a small hole on the underside of the shell, the umbificus. If you imagine a spiral staircase with a hole through the centre then so it is with a shell; when the snail reaches adulthood the hole grows over and the lip of the aperture thickens and is reflected. ‘Juvenile’ and ‘adult’ are convenient working terms, as we are not really sure at this stage if a snail with an umbificus, a supposed juvenile, can reproduce or not.
Palm forests are an element of lowland subtropical rainforest, and we have spent many hours searching through palm fronds on the forest floor. Intuitively, such a habitat would supply a relatively safe environment from predation; it would be dark and moist, and offer a hard substrate to hang upside down and shelter during the day. More often we have found the snail sheltering during the day under low shrubs that retain moist soil, and provide protection from desiccation and predation. Sometimes we find them burrowed in leaf litter.
Not much was know about the breeding behaviour or feeding habits of the snail when we began our study last year. They were known to be herbivorous rather than carnivorous, from their radula structure, or mouthparts. We have often observed the snail feeding on fungi, including bracket, gill, and cup types. Cataloguing local fungi species and analysing miniscule snail scats for fungal spores is another interesting aspect to the study into the snails’ feeding preferences. People often ask me what the purpose of a snail is, and part of the answer lies here. Land snails are ecologically important as many species spread seeds and fungal spores attached to the mucus or passed in faeces; they also recycle nutrients, particularly calcium. Despite their importance in soil ecology and association with vegetation, there are few studies on the physiology and biology of land snails.
We’re attempting to estimate the number of snails in the Byron at Byron population, using a ‘capture-mark-recapture’ method where you mark an animal and then you go back again and try to recapture it. Of course you don’t need ethics approval to work on invertebrates but you do need a scientific licence from the NPWS to handle threatened species. We tested our methods on the common introduced European snail, painting them with nail polish, numbering them with a fade-proof marker pen and coating all of that with a clear lacquer: if the identification marks come off in the field then you will over-estimate abundance. We monitored the test-snails for about six weeks, during which time they had no ill effects, so we went ahead and did our research on the threatened species. I’ve since recaptured numbered Mitchell’s rainforest snail’s a year and a half after tagging.
At the site we marked out a plot 30 x 30 metres (900 m2), measuring all sorts of variables including vegetation, understorey/canopy cover, pH, calcium, leaf litter, micro-elevation, and more. We sampled on six occasions that were at least two weeks apart. On each occasion we sampled over three consecutive nights. With this design we can effectively estimate the population sizes at each primary session from closed population models while apparent survival between sessions is estimated from an open population model. We can also estimate the temporary emigration between the primary sessions. Data such as temperature, rainfall and relative humidity was recorded on all sampling occasions. We also measured the snails’ weight, width, height, umbificus, and counted whorls. So, our modelling accounts for recruits and losses, climatic conditions, and capture and recapture probabilities based on measurements taken on individual animals.
Overall, we numbered 105 snails during this period. This is encouraging, as we appear to have a persistent breeding population. But there is a word of caution as some people have commented, “you’ve caught 105 snails in 900 m2, there’s hundreds of hectares like that, so there must be thousands of snails and they can’t be critically endangered!” Unfortunately this is not the case at all as land snails form a clustered distribution around a resource. In fact, we purposely conducted this study in a specific location that was known to us. Localised flooding at the time and other habitat factors ensured we had a particularly high population density. We have used information gathered from this recapture research to design a patch-occupancy study that will take us into the broader landscape.
Already we have found Mitchell’s rainforest snails in areas where there are no previously recorded sightings. In part, this has been due to the local community who have responded positively to a number of recent media releases. Often a snail turns out not to be Mitchell’s rainforest snail but rather Fraser’s snail, a snail very similar in size and looks, but it has multiple yellow bands on each whorl. Often the snails are found together as they share particular habitat requirements, so keep looking. If you think you may have seen Mitchell’s rainforest snail you could take a photograph and e-mail it to me. Otherwise give me a call and I’ll be happy to come and have a look.
Jonathan is a PhD candidate at Southern Cross University, Lismore, and has lived in the Byron Shire for 15 years. He may be contacted at: jonathan.parkyn@scu.edu.au or telephone 0404 798734

 

Leave a Reply

Your email address will not be published.