If you see a snail in the Queensland rainforests or sandstone gorges near Alice Springs, the chances are it will be a camaenid. Australia has hundreds of species, many of them big and strikingly coloured. But despite their size and splendor, they're seriously undervalued as a subject for study. And now it appears that what we thought we knew about their evolutionary history is mostly … wrong.
Texts will tell you that Camaenidae has a disjunct distribution. It occurs in Australia, eastern and south-eastern Asia, central America (including the Caribbean) and northern South America. The family is largely tropical, although a few species live in southern Australia and the milder parts of China and Japan. In many parts of the range, they live in humid forests and woodland, but rugged Australian species dwell among the red rocks and spinifex of the central deserts. (That's not a shell. This is a shell.)
Fossils assigned to Camaenidae have been found in the Palaeocene of North America, the Pliocene of South America and Pleistocene of the Caribbean. In Australia, the earliest camaenids are known from late Oligocene deposits in NW Queensland.
On the basis of the modern and prehistoric distribution, Alan Solem (1959) postulated that camaenids originated there and dispersed southwards into South America and eastwards into Asia and Australia. Subsequent extinction in North America and Europe split the family into two geographically isolated groups. He suggested that they arrived in Australia in two waves, which are represented by an older endemic lineage and a more recent one with much in common with the fauna of New Guinea.
But his hypothesis presupposes that Old World and New World Camaenidae belong to the same group. How strong is the evidence supporting that?
Short answer: Not very.
Long answer: Not very. And here's why ...
There's a problem in actually knowing what a camaenid is. The family is defined by the absence of certain characteristics — by what its members don't have rather than by what they share with one another. Pilsbry (1939: 411) referred to them as 'Helices without dart apparatus' in which the 'penis [is] continued in an epiphallus and a flagellum (the latter sometimes vestigial or wanting) [and the] spermathecal duct [is] not branched.' Is there significance to the absent dart apparatus? Was it never there? Was it lost? What about the unbranched spermathecal duct? Lots of nots but nothing unique to tie them all together. Camaenidae defined in this way could be nothing more than the bunch of wallflowers left after other species have been assigned to families.
Craterodiscus pricei is a good example of this. On describing the species from the shell only, Don McMichael (1959) tentatively assigned this tiny, rather nondescript North Queensland snail to Helicarionidae. When Solem (1973) examined anatomical material, he shifted it to Camaenidae — but only after a process of elimination. He classified the species by what it wasn't rather than what it was. Craterodiscus remained in Camaenidae until Simon Tillier (1989) moved it into Corillidae. Once again, the assessment was based an absent morphological character. There it stayed until Christopher Wade and colleagues (2001, 2006) re-examined its affinities as part of a study of land snail phylogeny. Using molecular data, their work revealed that Craterodiscus was neither camaenid nor corillid. In fact, McMichael had pretty much got it right. The tiny, rather nondescript snail belonged with the dyakiid – ariophantid – helicarionid semi-slugs*.
So does these molecular data shed light on the taxonomic tangle of Camaenidae?
I'm glad you asked.
Yes. Yes, they do.
The study that put Craterodiscus back in its place also looked at a number of camaenids from Japan, the Philippines, Australia and the Caribbean. The camaenids did not resolve into a single lineage. They didn't even resolve along biogeographical lines. They were mixed in with helicoids (with and without dart apparatus) from a range of families. But not only did the molecular data demonstrate that Camaenidae was a freakin' mess — quelle surprise! — it also sorted them into neat(ish) piles.
Maximum-likelihood phylogenetic tree showing the evolutionary relationships among the Helicoidea (Wade et al., 2007).
It appears that Old World 'camaenids' are closely related to Bradybaenidae, a group of Asian snails possessing a dart apparatus. According to a further analysis (Wade et al., 2007), the species are mixed together like a well-shuffled pack of cards. New World 'camaenids' show a different pattern. Some species cluster with the European Hygromiidae (with dart apparatus), whereas others hang out with Caribbean Sagdidae (also with dart apparatus). Did I mention that the absence of the dart apparatus was a crap characteristic on which to base a classification?
So, is there any morphological character that might be helpful when trying to sort of camaenid affiliations? Well, yes … sort of. Many bradybaenids possess a structure called, unglamorously, a head wart. (But that's what it is — a patch of enlarged tubercles that sits between and slightly behind the upper pair of tentacles.) A number of Old World 'camaenids' have a similar, if not identical, structure. In some, it is permanently everted (the state in bradybaenids); in others, it can be tucked away into a transverse pouch. (Just the thing for Tropical Friday.) Very little work has been done on the head wart but it might be a useful character. Someone start looking.
Having illuminated some of the camaenid conundrum, the molecular analyses now leave us with a nomenclatural problem. Do we take the plunge and call the Old World group something different from the New World group? Or do we deploy the inverted commas with aplomb? Sometimes systematics is like wallpapering — you smooth down one bubble and it just pops up somewhere else.
* And the dyakiid – ariophantid – helicarionid semi-slug thing is a story for another day.
Wade, C.M., Hudelot, C., Davison, A., Naggs, F., Mordan, P.B. (2007). Molecular phylogeny of the helicoid land snails (Pulmonata: Stylommatophora: Helicoidea), with special emphasis on the Camaenidae. Journal of Molluscan Studies, 73(4), 411-415. DOI: 10.1093/mollus/eym030
McMichael, DF. (1959). A new species and genus of land snail from North Queensland. Journal of the Malacological Society of Australia 1: 31 – 32.
Pilsbry, HA. (1939). The land Mollusca of North America. Proceedings of the Academy of Natural Sciences of Philadelphia, Monograph no. 3 2: 1 – 573.
Solem, A. (1959). Systematics and zoogeography of the land and fresh-water Mollusca of the New Hebrides. Fieldiana, Zoology 43: 1 – 359.
Solem, A. (1973). Craterodiscus McMichael, 1959, a camaenid land snail from Queensland. Journal of the Malacological Society of Australia 2: 377 – 385.
Tillier, S. (1989). Comparative morphology, phylogeny and classification of land snails and slugs (Gastropoda: Pulmonata: Stylommatophora). Malacologia 30: 1 – 303.
Wade, CM, Mordan, PB and Clarke, B. (2001). A phylogeny of the land snails (Gastropoda: Pulmonata). Proceedings of the Royal Society of London, B 268: 413 – 422.
Wade, CM, Mordan, PB and Naggs, F. (2006) Evolutionary relationships among the pulmonate land snails and slugs (Pulmonata, Stylommatophora). Biological Journal of the Linnean Society 87: 593 – 610.