Mid-winter Pond Dipping

Thursday's Guardian Country Diary is about freshwater invertebrates that remain active all through the winter. There may be no bees or butterflies about at the moment, but there are plenty of aquatic animals to look at - the only problem is that the water is bone-numbingly cold, so collecting them can be a bit of an ordeal! 

All the animals below (except the bottom one) were found in a single jam jar of water, dredged along the edge of our garden pond which had been frozen over on the previous night.

This is a freshwater orb-cockle Sphaerium sp. There must the hundreds of these little bivalves (the largest was about 8mm. across) in our pond. The two pink protruding tubes on the left are the inhalent and exhalent siphons that circulate water over the gills. You can just see the muscular foot protruding through the shell valves in the 5 o'clock position. 

Orb-cockles are surprisingly mobile, especially when they are small. This one, whcih was about 4mm. long, is using its muscular foot (which you can see protruding from the back of the shells) to scoot across the bottom of the container.

There are scores of these red ramshorn snails in our pond but it's providing difficult to identify it precisely. I posted the pictures on the excellent iSpot web site and there are varying opinions as to its identify. It might be Planorbis corneus rubrum or it could be Planorbella duryi which is the American ramshorn snail. Either way it's an introduced species that probably came in with pond weed bought from a garden centre.

If there are scores of ramshorn snails, then there must be many thousands of these flatworms (Polycelis nigra?) in our garden pond - there were over 50 in a single jam jar sample. They glide along like magic carpets, propelled by a covering of beating cilia which you can see on a marine species by clicking here.

Our pond has a thick layer of decomposing leaves at the bottom - perfect conditions for water slaters (aka water hog-lice), so there are large numbers present.

Gammarid shrimps like these prefer well oxygenated flowing water so our somewhat stagnant pond is far from ideal, but there are still plenty present in the shallows and they are very active swimmers, making them difficult to photograph.  Mostly they swim on their sides, or sometimes upside down, curling up when they are alarmed. These have an interesting history - I introduced them from a population that I rescued from the bottom of Wearmouth Colliery before it closed; you can read the details here.

The young gammarids are almost transparent - I think this must be a male, judging from those prominent claws on the front pair of legs.

And finally, this animal (below) is not from our pond. I collected it a few days ago from the gravelly bottom of a ditch, with snow melt water flowing over it, on the edge of the moors at St. John's near Wolsingham in Weardale, and it's ........

...... an exquisite little caddisfly larva, with a tube constructed mostly from quartz grit. They can be coaxed to build their tubes from more exotic materials - for a stunning example, click here

Pond Life

 A quick sampling of the tadpoles in our garden pond showed that many of them were still some way off developing limbs, which I guess is a sign of the late spring - the frogs spawned very late this year. (Double-click for a larger image).

This tadpole is a sharing the trough that I photographed it in with a flatworm.

A water log-louse Asellus aquaticus. There are vast numbers of these scavengers down at the bottom of the pond, where there's  thick layer of decaying leaves.

Underside of a water hog-louse. These crustaceans are isopods - all their legs are more or less the same length. For more isopods, click here and here.

Possibly the least lovable animal in the pond - a leech. I suspect that these parasitize the pond snails. The dark patterning is its digestive system, visible through the body wall.

Water Hog-louse

Our small garden pond (the same one that's home to the newts mentioned in a previous post) is also home to a thriving water hog-louse (aka water slater) Asellus aquaticus population - and this is their breeding season. This is a mating pair and eventually the female will carry her fertilised eggs around with her under the front of her flattened body. These are crustaceans which exhibit a considerable level of maternal care of their offspring and when the eggs have hatched the female will continue to carry her young under her body, between the front two pairs of legs until they're old enough to fend for themselves. Water hog-lice aren't too fussy about water quality and so will thrive in small aquaria, where it's easy to watch their reproductive cycle. 

The brown pigmentation of their exoskeleton takes time to develop and in the youngsters it's transparent, which makes it easy to watch the workings of their internal organs under a microscope. This one is a juvenile that is only just beginning to develop the vestiges of pigmentation .....

..... and this one is a little further along the road to adulthood.


Asellus is an isopod crustacean, meaning that all of its legs are of similar length, unlike the amphipods that have a combination of long and short legs.