Hey hey, it's the end of May

After a long, cold spring, I've been watching all kinds of habitats and species wake up and get 2013 moving, including our garden pond. To celebrate this, I'm going with a non-technical post today, just enjoying what can be seen with a bit of careful design and construction, and a bit of patience in a small-to-medium urban back garden...

A portrait of the hoverfly Heliophilus pendulus basking on a pond plant

A small hoverfly (I haven't identified it yet) feeding on a buttercup (Rancunculus repens) by the pond edge.

A smooth newt gulping air.

Nymph of large red damselfly.

And that's it for me today - enjoy the sunshine, enjoy the wildlife and happy ecology! Back with something more technical in a few days...

Snails - reaching the crunch-point

I like snails; I may garden, but I like them anyway, and I really hate standing on them as they roam across footpaths at night. However, there are many dangers awaiting snails - hungry corvids for example - and damage can occur that doesn't leave them squashed. However, apart from minor chips and so on, it often feels like they must die if their shell is broken, after all dehydration seems likely aside from any internal damage. This specimen of the common garden snail (Cornu aspersum or Helix aspersa depending where you look - I won't go into the taxonomic disagreements here) found in our garden yesterday shows that this is not necessarily the case...

A garden snail showing a large shell-injury

The shell injury showing where membranes have rejoined to it from the body. There are also cracks on the spire, indicating the considerable extent of the damage it survived.

Although it may not be obvious from the photos, this is not a new injury and the snail was highly active. The exposed surface is dry and has re-attached to the broken edges of the shell; although snails can, and do, mend minor shell-damage as they grow (by secreting new calcareous material), that is unlikely to happen with such a large hole. So, how did the snail survive?

Well, assuming no majo damage to the body itself, the main issue is water loss. The shell retains moisture but of course water can be lost through the main opening (aperture). Therefore, during dry and/or cold conditions, the snail withdraws and seals the aperture using a thin 'skin' of dried mucus, called an 'epiphragm'. Less obviously, it can also avoid damage from freezing, not only by seeking sheletered locations, but also through an antifreeze mechanism in its haemolymph (equivalent of blood). In warmer dry conditions, the edge of the internal 'skin' or mantle (i.e. where it meets the edge of the aperture) can change its permeability to water and thus further help prevent drying-out (Machin, 1966).

A broken shell however presents a whole new aperture through which water can be lost. A search for publications on snail-shell mending, show that Andrews (1935) looked at snails of the genus Neritina and noted a variety of mechanisms and examples, but Durning (1957) described the process from a medical perspective. He noted that the snail secretes a glycoprotein matrix in order to provide a substrate onto/into which calcium carbonate can be secreted in turn i.e. shell material can't be produced directly onto the edge of breaks, and there are few cells there to secrete directly. So, a matrix is needed. This is something that will be happening here to some extent, but the membrane around the mantle also seems to have toughened and, unless the damage is much more recent than it appears to be, this snail survived the recent long winter with this hole - a common species that I'm now looking at from a totally new perspective.

References

Andrews, E.A. (1935). Shell repair by the snail, Neritina. Journal of Experimental Zoology 70(1): 75–107.
Durning, W.C. (1957). Repair of a defect in the shell of the snail Helix aspersa. J Bone Joint Surg Am 39-A(2): 377-93.

Machin, J. (1966). The evaporation of water from Helix aspersa IV. Loss from the mantle of the inactive snail. Journal of Experimental Biology 45: 269-278.

Peering at pond predators

As our garden pond develops, so does the community of invertebrates that live within it - and this of course means predation. Predators come in all shapes and sizes, from tiny water mites to herons. One vertebrate predator is a neighbour's cat (which does not like being sprayed with water!); another is the smooth newt Lissotriton vulgaris. Our pond currently has a pair of these, so we will be looking out for eggs - they are also voracious predators of frog tadpoles (of which there are many) and one was seen yesterday taking a tadpole as prey.

Female smooth newt Lissotriton vulgaris

Other predators are smaller (in the absence of fish which are likely to devour/reduce the biodiversity of a garden pond) in the form of various invertebrates. Some of these are familiar and commonly seen, such as the pond-skater Gerris lacustris (there are other species, but this one is often an early coloniser of newly created ponds). Being a true bug (Hemiptera), it has piercing mouthparts (which curve back under the head) and can be seen skating on the surface held up by surface tension. Pond skaters can move quickly when disturbed or darting towards prey such as smaller invertebrates that have fallen onto the water's surface. Their feet and legs dimple the surface and the large middle pair of legs push backwards, creating a pressure wave that allows the insect to glide forwards (e.g. Guthrie 1989).

Pond skater Gerris lacustris. The dimples in the water's surface are clearly visible - also note that this specimen is fully winged; spring dispersal flights are an important part of this species' life history.

Lastly, a quick look at a group well known as predators in their juvenile form - the Odonata - dragonflies and damselflies. There are several small early-instar nymphs of the Large Red Damselfly Pyrrhosoma nymphula in our pond, but also a late/final-instar nymph as shown below. Although often lurking in sediment (this one was underneath an aquatic plant pot/basket), they can swim actively, have excellent vision and of course hinged mouthparts (the 'mask') which can be quickly thrust forwards to grab prey. Nymphs of the Odonata will attack many types of prey - not only other invertebrates, but also amphibians, especially small/juvenile individuals. It is likely that our large population of frog tadpoles may start to be reduced soon...

Late-instar nymph of the Large Red Damselfly Pyrrhosoma nymphula. The partly darkened caudal lamellae at the tip of the abdomen can be seen, plus the bulges and central indent at the back of the head - these features are useful for identification.

Late-instar nymph of the Large Red Damselfly Pyrrhosoma nymphula. The large eyes, useful for locating prey, are shown - the jawed 'mask' is retracted beneath the front of the head.

Reference

Guthrie, M. (1989). Animals of the Surface Film. Richmond, Slough.

Pondnet diary day 2

Continuing the Pondnet survey that began about a week ago, having done a preliminary survey of the main environmental features and whichever species could be spotted from the side, this time a net and white tray were required.

My wife/field-assistant doing some pond-netting.

the first thing we notcied was the great increase in frog tadpoles - not only in number (from a couple of hundred to at least a thousand as a rough estimate) but in size. A week previously the ones we saw were newly hatched - these had developed their typical fat-headed shape. They hadn't all hatched in the last week, so many must have been well hidden. The net also meant that we could confirm the species of the numerous small fish seen previously - they looked like sticklebacks but it's always worth checking, and all the ones we caught (and yes re-released) were indeed three-spined sticklebacks (Gasterosteus aculeatus).

A three-spined stickleback Gasterosteus aculeatus - the dorsal spines are just visible

Of course, it wouldn't be a proper sampling day (for me) if there weren't invertebrates involved. The netting meant we could see beyond the larger surface-dwelling species and maybe find some that, even if common, are less immediately familiar.

The water-slater or hog-louse Asellus aquaticus

The water-slater or hog-louse Asellus aquaticus is an isopod crustacean i.e. related to woodlice, and is common and widespread in Britain. It can be found in a very wide range of water bodies and qualities, but especially under aquatic foliage, stones and wood (Gregory, 2009), so is rarely seen by the casual observer without a net. It can be separated from the similar Proasellus meridianus by the head pattern although the photo doesn't show it as clearly as could be seen on the live specimen.

Another common-but-overlooked species is Plea minutissima, the 'least water-boatman'. It is broadly similar to other water-boatmen seen 'rowing' beneath the water's surface, but is tiny (around 1.8 - 2.8mm long) and its domed shape means that it is initially more likely to look like a small beetle than a water bug at a glance (Denton, 2007) - it certainly did to me when seen among the debris and other small species from the net. However, under the microscope (or even a squinted eye) it is quite different and the pointed mouthparts can be seen.

The least water-boatman Plea minutissima is the sole British species in the family Pleidae.

Lastly, I'd like to keep moving down the size scale to look at the water mites - arachnids of the suborder Hydracarina. These are mostly bulbous and the body in not separated into separate sections (cephalothorax and abdomen) as would be seen in the suborder Oribatei. Identification is tricky, but Hopkins (1961) can be downloaded from free here and is very useful for those beginning to study this tricky, and again often-overlooked, group. I won't go into detail about the identification here, but the water mite I investigated was Piona coccinea - a red species around 2-3mm long, red and globose in form.

That's where I'll leave pond-related matters for today - more soon!

The water mite Piona coccinea.

References

Denton, J. (2007). Water Bugs and Water Beetles of Surrey. SWT, Woking.
Gregory, S. (2009). Woodlice and Waterlice (Isopoda: Oniscidea & Asellota) in Britain and Ireland. NERC/BRC, Wallingford.
Hopkins, C.L. (1961). A key to the water mites (Hydracarina) of the Flatford area. Field Studies 1(3): 45-64.