Sunday, 21 January 2007

Slugs in my garden II

I was off doing stuff yesterday, so apologies for the hiatus.

It rained. Not exactly a deluge but enough to fill the big plastic saucer that the birds use as a bath*. About 15 mm overnight. With any luck there's more on the way. As far as I'm concerned, it can keep raining until Christmas. (As long as it stops long enough on weekends for the washing to dry. That's not much to ask.)

The heat and humidity have brought out interesting crawling and flying things, mostly of the six- or eight-legged kinds. But whereas insects and spiders are everywhere, slugs and snails seem reluctant to poke their tentacles above the parapet.

I found a couple of juveniles of the introduced species Limacus flavus and Limax maximus (leopard or tiger slug) but no adults. It's a shame that none of the adult leopard slugs were roaming abroad because they're spectacular animals. Full-grown individuals are about 20 cm long and marked with Texta-black spots and stripes. They're fairly benign in the garden, feeding on soft material including decaying vegetable matter, dog and cat food and dead animals. Unfortunately, seedlings tend to come under the heading 'soft material', so these slugs aren't perfect.

They're probably best known for their exhibitionist tendencies. Leopard slugs mate while dangling from a thread of mucus. David Attenborough featured footage of this on his Life in the undergrowth. Natural history blogger David Nelson has an excellent sequence of photos showing leopard slugs at it in his New South Wales garden.

Like most other terrestrial slugs and snails, they are simultaneous hermaphrodites. That is, they possess both male and female reproductive organs. (Compare this with sequential hermaphrodites, which change from one sex to the other.) When they mate, individuals act as male and female at the same time. Sperm from A fertilises the eggs of B and vice versa (reciprocal fertilisation). None of this is unusual. (At least, not for slugs and snails.) But what sets them apart from the other species you're likely to encounter in a garden is the way in which they exchange sperm.

On meeting, the slugs circle one another and then climb up a vertical surface, usually a tree trunk. Entwined, they lower themselves from the twisted cord of mucus. Once in mid-air, each everts its penis, which has an expanded, scalloped end. Continuing the coiled theme, the penes twine around one another and swap sperm.

The slugs disengage and ascend the mucus thread, consuming it as they climb. As they do so, each retracts its penis, bringing with it the other's sperm. Sperm exchange is external but fertilisation is internal.

So just be careful where you walk on a humid summer night. Don't say you haven't been warned.


* I had to bring in the towel rack and soap dish.


Trevor said...

Aaaah - the humble slug. Well - not so humble it seems. Thanks to you I now know a whole lot more about these interesting creatures.

Have you seen the movie "Flushed Away.?" The slugs are the stars of the show, IMHO.

Snail said...

I haven't seen it yet. It does sound like my sort of film though. Animation and slugs---a winning combination!