Interesting little creature from a Budawang pool

I wrote yesterday about the trip into the Budawangs from Tianjara Creek in search of migrating Honeyeaters. While there, at what is known as the "cross-roads", just below Kangaroo Hill, I was exploring the exposed rock shelves, while we had a cup of tea.There were many shallow pools of water, in natural depressions in the rock face. One of them (and probably all of them, if I had checked) had small black creatures swimming around in the bottom of the pool. I took a photo of one of these mysterious creatures on the sandy bottom of this little pool.At first I took them to be tiny tadpoles. But on closer examination they did not have tails.
If not Tadpoles, what were they?

Then I was able to pick one up easily. That confirmed it was not a tadpole, for it was not hard to catch, nor was it slippery. It was less than 1o mm long. It had a hard shell - looking exactly like a Mussel. But any Mussels I have seen were fixed to a rock base or jetty pole. This one moved around. What was it?Furthermore, it was obvious from watching them move that they did so by beating small protruding "legs" (for want of a better word). What sort of Shellfish could this be?
I sought assistance from some of my Aussie Nature Blogger colleagues, and "Snail" came up with the goods. Thanks to Chai for suggesting Snail as being the right person to ask.Snail replied very quickly - as follows:

• "Very interesting critter. It's a crustacean with a bivalved shell. It might be an ostracod or seed shrimp, but I'm pretty sure it's a clam shrimp. These are related to shield shrimps and water fleas. They often live in temporary water bodies. Really, really temporary ones like your rocky rain pools. Their embryos are enclosed in tough cysts that are resistant to drying out, so persist for a long time after the water's gone. Once the water returns, they continue development. Because of this, they appear very quickly after rainfall. Cysts are also spread by the wind, so these animals can colonise pools. ponds and puddles in the middle of nowhere (but in a very hit and miss way)."

Snail was exactly right. My creature is NOT a shellfish, but a form of Crustacean, a branchiopod, a member of the group known as "Clam Shrimps".

There is an Australian researcher, Brian Timms, from Newcastle University, who has done work as a Research Associate at the Australian Museum (Sydney), specialising in Clam Shrimps.

Clam Shrimp - Illustration - Richter and Timms
His illustration is of a newly described species - not my specimen,
but it is close enough to confirm that my creature is a Clam Shrimp.
Note the antennae (left) and tail (right) protruding, as in my two photos above.
The strong muscle which controls the opening and closing of the carapace
is shown in the top left of this anatomical sketch.
It is inside the carapace, so is not visible in my photographs.

Note: Scale line shown represents 1 mm

While the books do not mention the term, it seems to me that these creatures have adapted in what is known as "parallel evolution", to closely resemble a Mussel shell, despite being totally unrelated to those creatures.

Image of an as yet undescribed Clam Shrimp.
One valve of the carapace is removed to show the animal's body.
These conchostracans live in temporary water in a desert.
Photo by Brenton Knott
Source: Uni of WA, School of Animal Biology - Image Gallery

So, it turns out my little Clam Shrimps are freshwater creatures, close to prawn or a miniature Crayfish, which has evolved a bivalve shell, as a casing. It lives within this shell, and filters the water, in search of its food. The movement comes from beating of its long protruding antennae (not its legs, as one might assume).

Image of Clam Shrimp, (genus: Eulimnadia) showing body - through shell.
In this image the antennae are protruding on the right.
Source: Uni of WA, School of Animal Biology - Image Gallery

Their life cycle is pretty amazing. I have seen this area in a time of total drought, and the moss beds on the rocks were blackened and dry. There were no pools left for these creatures to live in. Where did they come from?
Wrong question.
They were probably there all the time - surviving the drought as cysts enclosing the embryos - a form of suspended animation. They were there, as "cysts" waiting for the rain to fill their temporary pools. A perfect adaptation to Australia's harsh climate on top of a sandstone plateau.

Even though this area is in a high rainfall region, the specific habitat, on an exposed rock shelf, has very little water-holding capacity. So what they do, is lay minute eggs which have a hard shell (cysts), and these cysts can survive an extended drought. Indeed the cysts can even be transported from one place to another on the wind.

These Clam Shrimps are related to "Brine Shrimps" which are known to occur in places like Uluru. Now that is a truly harsh environment for any aquatic creature. It makes the sudden appearance of my little Clam Shrimps on the Budawang Plateau as not remarkable, compared to Uluru.

They are also related to creatures which the Americans refer to as "Sea Monkeys", which are apparently sold in pet shops at the egg stage, which are used as food for aquarium fish, and as pets - for their novelty value. They have the ability to hatch and grow very quickly - exactly the key to the survival of these little creatures in a rock pool on an exposed sandstone plateau in the Budawangs.

As "Snail" said: "Hope this helps. They're really fascinating animals."
It certainly did help - I would never have though of it being a crustacean hiding inside a carapace, looking like a classic bivalve. Too tricky for me. These things make Orchids look easy to study.
Fascinating creatures indeed.

Technical Notes:
Brian Timms and Stefan Richter published a paper naming a new species of Clam Shrimp, apparently the first Australian record in the genus Eocyzicus, and it is a salt-tolerant species which is also unusual (in a group of fresh-water animals).

Within that article, they listed all known Australian species of Clam Shrimps.

• "With this new species the Australian fauna comprises 26 valid species of
  clam shrimps. We provide a list of all described species, including their known localities and a key to the genera of Australian clam shrimps."
  

Most of their records are of arid zone areas, or in the tropics. Only one species, Limnadia stanleyana is shown as having a known distribution which includes "Coogee near Port Jackson, Moore Park, Maroubra, Sydney (New South Wales). Also North Head, Sydney and Kanangra Walls, via Oberon, NSW".

That is the only species which has a distribution which includes "Sydney Sandstone country" at altitude (Kanangra Walls). That is in the Upper Blue Mountains, which is quite close (similar in habitat type) to the Budawangs. Although my record is at approximately 600 metres, which is not nearly as high as Kanangra Walls, that species would be the only one in a roughly similar habitat and geology to my report of this Clam Shrimp.

I shall attempt to contact Brian Timms, and I will provide him with documentation of this sighting. At least that way, someone who has a detailed appreciation of their biology and distribution of Australian Clam Shrimps will be able to note this report.

UPDATE
I have just spoke with Martin Schwentner, a visiting researcher at the Australian Museum. He was very interested to hear of my little Clam Shrimp. I have sent him my photos. Lets see what he can make of them. I might have to retrace my steps, to find these little creatures again.

FURTHER UPDATE:

Martin got back to me:

• I just had a look at those pictures and your blog. That looks very
  interesting.
  I have to say that identifying these crustaceans is very difficult.
  That is one of the reasons I am doing my PhD on this. I try to use
  molecular and morphological approaches to to be able to identify
  species (old and new ones). From the picture I would guess that it
  might be a Limnadia species, so Limadia stanleyana could be right (as
  you have guessed). Brian knows these animals better, but I doubt that
  he will be able to name the species just by the photo.
  
  It would be great if you could send me the coordinates.

EVEN FURTHER UPDATE

Late on Wednesday afternoon (just on dark) I stopped at the Budderoo Plateau Track, and realised there were pools beside the track, with sandy bottoms (over sandstone rock shelves) - just like where I had found my first Clam Shrimps, out at the Budawangs. I looked and sure enough there were Clam shrimps here too. Great. This place is a mere 10 minutes down the road from Robertson - much easier to find them there than a full day trip out to the Budawangs. I will also report this locality to the Clam Shrimp specialists.