Whilst kneeling upon the muddy bank of a freshwater spring (like ya do) I spotted something quite unusual. The undersides of the rocks I was turning over were speckled with little black blobs. As I lifted them up to the light, they began to stir, unfurling, slender and jet black, moving back towards the water.
The spring is a small one, tinkling down through rocks and swathes of water mint - it feeds into a brook near a cluster of crab-apple trees. Flowing fresh from the belly of the hill, the water is clear and crisp, completely pure. I have always turned rocks here to see freshwater shrimps spiralling beneath them - there's something very relaxing about it!
So I returned later, armed with a pocket microscope, plant pot tray and my camera...
They were about a centimetre long, and a few millimetres wide - absolutely tiny! With black bodies, beige undersides, and no outstanding features, they were proving tricky to research...
I sent my pictures to Zack (@theaquaticadventure - instagram), an awesome biologist who specialises in aquatic organisms, who helped me identify that I'd spotted an aquatic slug of the family Acochlidiacea!
Acochlidiacea is an order of opisthobranchs (aquatic gastropods) which contains a few freshwater species. These shell-less creatures have elongated, worm-like bodies, and very little is known about them. A good identification key is their head-gear… Two stalks sprout from their faces, acting as satellites to the surroundings. These are in fact ‘rhinophores’ - chemosensory, ear-like structures which detect chemicals dissolved in the water, allowing the slug to locate food and mates!
Speaking of food, I was puzzled as to what the slugs were eating - there wasn’t any visible algae or weed, the stereotypical plant diet I was expecting. It turns out they are ‘deposit feeders’. This means they feed on particles mixed with the sediment, and sometimes even the sediments themselves - stripping them of microbes and detritus. At least they won’t be bothering my lettuces….
As I said, little is known about Acochlidiacea - I have delved into some pretty complicated documents, only to find a snippet of information. This was laborious, yet rewarding as I found out that they have quite an interesting life cycle. These creatures start out as planktonic ‘veliger larvae’ - which are assumed to be interstitial (existing in the gaps between sand grains), thus avoiding being carried downstream. Funnily enough, when I scrabbled around in another part of the brook, there were none to be seen!
There is a lot left to discover, such as how these slugs crossed over from marine to freshwater habitats, and how they reproduce.
To me, the smallest things are always the most fascinating! In a world where we have a name for everything we see, there is nothing more exciting than something we have no clue about...
So next time you're near a stream, lift some pebbles, see what you can find ;)
P.S. if you are turning over stones, be sure to replace them as you found them!