Monday, 19 January 2009

The Hannibal Lecter snails

Most land snails are mild-mannered leaf- and mushroom munchers. Oh, sure, they might transmit the occasional rat lungworm (Angiostrongylus cantonensis) that takes a wrong turn and ends up in your brain but — generally — land snails are not very threatening.

Unless they belong to the family Rhytididae.

Now, I know I overstated the case for those carnivorous slugs but let me quote a passage about rhytidids from Dai Herbert and Dick Kilburn's book on snails and slugs of eastern South Africa.
Cannibal snails have long borne some degree of notoriety as a result of an early account by Edgar Layard (1824 – 1900), one-time curator at the South African Museum. In this he referred to a specimen of N. cafra collected by 'the late Mr H.J. McKen, who told me he procured it on a battlefield in Zululand, feeding on the putrefying corpses of the natives slain in one of their tribal fights'. Even less credible is a lurid account by the Rev. A.H. Cooke, in which he quotes Layard to the effect that the best time to collect Natalina in quantity is after tribal conflicts 'when they might be observed streaming from all points of the compass towards the field of slaughter'!

Preposterous, you say? Utter madness? The passage continues:
Not surprisingly, these reports have never been confirmed, but they are not beyond possibility. In captivity, N. cafra will readily devour minced beef and the smaller, southern Cape species N. trimeni (Melvill & Ponsonby, 1892) has been seen attempting to get at dead mice caught in mammal traps ...

Oh, they're scavengers. That's all right, then.
A word of caution is perhaps necessary here. Local conchologist Markus Lussi tells of an instance when he rescued a medium sized specimen of N. cafra from the path of the lawn mower, whereupon the snail twisted its neck around 'at lightning speed' and bit him on the thumb, leaving a half centimetre cut, deep enough to bleed.

Rhytididae is a Gondwanan group with representatives in South Africa, the Seychelles, Australia, New Zealand and islands of the western Pacific. All species look very much the same. They have thin, glossy shells with a low spire and (usually) an open umbilicus. The neck is long and tapering, which allows the animal to get its head deep into the shells of snail prey. Skin and shell are shades of grey, yellow or brown. What they lack in glamour, they make up for in voraciousness. Herbert and Kilburn describe the feeding behaviour of the southern African N. cafra:

The first bite, during which the radula is thrust out of the mouth, is accompanied by a slow-motion lunge and usually results in a deep gash in the victim's skin, often exposing the viscera. Then, with subsequent bites, the victim is gradually drawn into the mouth and swallowed. Even sizeable slugs such as species of Elisolimax and Laevicaulis can be devoured in as few as six bites.

They don't stop there. Having cleaned up a hapless snail, they then carry its shell around with them like a trophy. (I'm beginning to think that Thomas Harris had a hand in writing this field guide.) But they're not showing off; they're not parading the spoils of war. While carrying the shell, the snail's foot secretes weak hydrochloric acid, which dissolves the calcium carbonate. The calcium is absorbed and used by the rhytidid to reinforce its own shell or produce eggs. Sometimes, they store prey shells for use later on. (Right, that's it. Now I'm sure that Harris was involved.)

As far as I'm aware, this behaviour has not been observed in any of the Australian species. The most closely studied Aussie rhytidid is the Otway black snail (Victaphanta compacta), which occurs in rainforest only a few hours drive from Melbourne. It preys on other snails but does so in a straightforward and not-at-all-weird way. The giant New Zealand Paryphanta are similarly unadventurous in their diets, feeding on earthworms and other snails. They are also known to recycle calcium from shells but do so by scraping it with the radula not dissolving it with acid secretions.

All I can say to the snails on the eastern side of the Indian Ocean, is lift your game.

Strangesta gawleri from Portland district, SW Victoria. Shell width = 17 mm

Strangesta franklandensis (above and below), North Queensland. Shell width = 30 mm.

Paryphanta busbyi (above and below), New Zealand. Shell width = 62 mm.

Herbert, D. & Kilburn, D. (2004) Field guide to the land snails and slugs of eastern South Africa. Natal Museum, Pietermaritzburg.



Actually, egg cannibalism is quite common among pulmonate land snails: the first to hatch get to eat their unhatched siblings.

Baur, B. (1992): Cannibalism in gastropods. In: Elgar, M.A. & Crespi, B.J.: Cannibalism: Ecology and Evolution among Diverse Taxa.

Mel said...

Wow, didn't know all that :S

Snail said...

It's fun and games in the world of snails!

Robert Nordsieck said...

Hi there,
I have a friend who also likes snails very much because they do no harm to anyone, but are pursued by (almost) all of nature.
But in my recent studies of snails I keep on coming by rather disagreable species, such as the northwest American robust lance tooth (Haplotrema vancouverense), which makes the jumping slugs jump (no kidding), and of course there is the rosy wolf snail (Euglandina rosea), in spite of its flowery name a rather disagreable character. The European predator snails at least only hunt earthworms. Usually. It makes one glad those snails are only so small...
But I love this part of the book on South African snails, that is something you can tell people in the pub who want to know, what's so special about snails ;)
Kind regards

Snail said...

I once made the mistake of testing if a freshwater snail, Lymnaea stagnalis, would feed on my finger tip. The answer is yes and the radula is so effective that it doesn't take long before there's quite a bit of damage. Lesson learnt!

Apart from the vicious snails, they're also important in transmitting 'orrible parasitic diseases, so they're not all blameless and harmless!