The origin of the faeces: snail tales of earthworm dinners

The view from Stockton Plateau

‘Scrubs’ is one of my favourite comedy series. I really enjoyed the (often extremely) odd jokes and situations. Scrubs was around long enough that they qualified for a musical episode, called My Musical. Many long running series eventually have an episode where the cast sings. The concept suited Scrubs as in the episode a patient has a brain tumour which means she hears everyone singing. There are eight great songs, including ‘Guy Love’ but the one that I remember best is ‘Everything comes down to poo’. This song basically says that doctors learn a huge amount about a patient through their bowel movements. The song includes the great line “Your number one test is your number two”. Certainly, as ecologists, although not everything comes down to poo, there is a lot of information that we can retrieve from this great organic resource. Finding faeces in the field can tell you where an organism has been and what habitat it has utilised, maybe how many there are, and, obviously, what it has been eating (which can also tell you about other species that are in the area). We can tell what has been eaten by looking at distinctive hard bits of prey, like otoliths or fur, or chemical signatures or DNA. DNA is particularly useful for diet species that are soft-bodied or mainly liquid.

New Zealand is home to carnivorous snails, mostly in the Powelliphanta genus. Although it is hard to imagine a snail moving fast enough to catch prey, Powelliphanta make a good living catching other slow moving species, like earthworms, and slurping them down like spagetti. Powelliphanta are often large (sometimes as big as your hand) and many species are found in north western South Island. Being so large also makes Powelliphanta a target for other, often introduced, carnivores and many of these snail species are threatened or endangered. One such species is Powelliphanta augusta which is found on the Stockton Plateau on the West Coast. Unfortunately, it’s habitat overlaps significant coal measures which are accessed through open cast mining. In order to preserve this species while accessing the coal, over 6000 adult snails have either been relocated to adjacent undisturbed habitat or cultured in captivity for eventual release back to the remediated site. One of the key pieces of knowledge to help with the success of this project is understanding what the diet of these snails is made up of. Powelliphanta are largely nocturnal and cryptic and the best way forward was to collect their poo and sort out the origin of the faeces.

Stephane Boyer from Lincoln University has worked closely with the relocation process. A new paper in PLOS One details his work on establishing the diet of P. augusta. Snails were collected at Stockton and individually placed in plastic containers for 48 hours. Faecal strings produced during this time were collected for DNA analysis. There were few to no hard parts in the faeces. In order to look at the potential food available about 1500 earthworms, over a period of two years, were collected from Stockton. DNA was obtained from 139 worms and 18 species were identified. The earthworm DNA provided a library of earthworm species found in the area that could be compared to the snail diet. This comparison would allow Stephane to test whether P. augusta is a generalist (feeding on any earthworms they can find) or a specialist (needing certain species of earthworms to survive). Faeces from 46 snail individuals were analysed. Earthworm DNA was detected in 75% of the snails. Around three to four different worm species were usually found in each faeces string with one species, Deinodrilus gorgoni, making up 40% of the earthworm samples. D. gorgonis was found in almost all snails (that had earthworm DNA) and overall there were five worm species that were in more than half of the snails. Six of the worm species were found rarely (less than 10% of the snails) and two were not found at all. The pattern of abundance of the earthworm species in the environment was similar to the proportions in the snail diet (e.g. if they were rarely found at Stockton they were rarely found in the snail faeces) which suggests that P. augusta forages randomly through the leaf litter (i.e. eating common things more often than rare things).

Powelliphanta high tea… “soft & wriggly”

The diet study reveals that P. augusta is a generalist predator of earthworms and are happy to eat anything they find as long as they are soft and wriggly. This is good news for conservation of P. augusta as they are not tied to a having to eat specific species of earthworms which would also have to be carefully preserved. The prospect of successfully re-introducing the snail back to its remediated site looks a lot more likely to work. This finding could only have been discovered looking at DNA in the faeces. As the song says “It may sound gross, you may say shush,but we need to see what comes out of your tush”! Advice for the ages…thanks Scrubs!

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