Sunday, June 26, 2011
Sunday Spinelessness - Snails can be speedy too
Sorry for anyone looking forward to the next part of a series on the first animals - that posts needs more editing than I have time to do this evening.
Instead, I'm going to jump on an internet bandwagon and show you a surprising video that's been doing the rounds:
As I've said before, we have some seriously big invertebrates in New Zealand, but none of them are more impressive than our giant carnivorous snails. We tend to think of snails and slugs as pests that destroy our lettuce plants, but snails are the most diverse group of molluscs and they have adapted to eat a whole range of food. Most snails scrape algae and fungi off surfaces, others are plant eaters, a few are parasites with no mouth at all and a surprisingly large number of them are carnivores. The scrapers and the herbivores eat by extending a rasp-like organ called the radula out from their mouths to chip away add the food at hand and rake into their mouths:
Great as this method is for eating immobile plants and algae, it doesn't really work for carnivorous snails whose prey has the ability to run away. Indeed, most carnivorous snails have seriously re-arranged their feeding anatomy to accommodate their lifestyle. The video above gives us a rare chance to see it in action. Once the snail has worked out where the worm is (using its two sets of tentacles - the smaller ones below are for smelling while the longer ones have eyes on their tips) its pharnyx fulls with blood and is rapidly thrust outside of its body, surrounding the worm. Once the worm is enveloped, the snail's sharp radular teeth will hold on, and start to break its body up as its dragged deeper into the digestive system. Although the actually moment of capture happens with a swiftness that belies snails' reputation as slow moving animals, the rest of the eating process takes a bit longer. The radular teeth are not particularly efficient and it will take several passes for the (still living) worm to be sloughed off into edible pieces.
All of New Zealand's carnivourous snails are from the Southern Hemisphere family Rhytididae. All told we have around 60 species in 6 genera. The video doesn't tell us what species were looking at, but it's probably from the one of the two related genera Powelliphanta and Paryphanta (if you forced me to pick, I'd says this was Po. augusta since the video comes from DoC and, as we'll see, they have a population of that species in captivity). Both these genera contain large worm-eating species with extraordinarily beautiful shells:
Most of the Powelliphanta species aren't yet formally described, and seems like there is some interesting evolutionary biology going on in this group. A number of species appear to be linked to each other in what is called a 'ring species' - a long chain of populations in which those adjacent to each other can interbreed but populations distant from each become quite distinct (and probably couldn't interbreed given the chance). I'd really love to get the chance to apply some genetic tools to understanding what's going on there, and, in fact, sorting out the number of species in this genus has important conservation implications. Unfortunately, for all their fearsome eating habits, most of our rhytidids are at risk of extinction. Like the rest of our fauna, they have no natural defence against introduced mammalian predators like possums. Habitat destruction also threatens their future, since some species appear to be adapted to very fine-scale differences in habitat, which makes the risk we took in translocating an entire species that had the temerity to live on mountain with a coal seam seem utterly crazy to me.