Small water animals

This entry is about the small crustaceans we call water fleas, because they are about the size of fleas, and they live in water.

Water fleas at a glance

These animals may be flea-sized, but they are actually crustaceans, distant relatives of crabs, prawns and slaters. The ones you are most likely to see are Daphnia, Cypris and Cyclops, but you never know your luck! They move differently, but you need at least a hand lens to see any details, and they are excellent for low-power microscopy.

Technically, they are all branchiopods (not to be confused with brachiopods!). Daphnia are in the sub-order Cladocera, the similar looking Cypris is in the Ostracoda, and Cyclops is in the Copepoda, so you may need to look up cladocerans, ostracods and copepods to find them on the web. The copepods are much less flea-like.

Branchiopds are easy to collect, because they will be found in most bodies of water, and they are just as easy to cultivate. They also have some interesting biology: the Cyclops that you see here is carrying two egg sacs, and you can often see eggs inside Daphnia.

I want to begin, though, with an odd discovery about branchiopods. It was made by Jacques Loeb, a German-born physiologist who moved to America. Loeb made some important discoveries on how animals respond to stimuli, and also did some useful work in embryology. He never explained how he made this discovery, but it must surely have been during a laboratory party!

The writer found that certain freshwater crustaceans, namely Californian species of Daphnia, copepods, and Gammarus when indifferent to light can be made intensely positively heliotropic by adding some acid to the fresh water, especially the weak acid CO2. When carbonated water (or beer) to the extent of about 5 c.c. or 10 c.c. is slowly and carefully added to 50 c.c. of fresh water containing these Daphnia, the animals will become intensely positive and will collect in a dense cluster on the window side of the dish. Stronger acids act in the same way but the animals are likely to die quickly. . . Alcohols act in the same way. In the case of Gammarus the positive heliotropism lasts only a few seconds, while in Daphnia it lasts from 10 to 50 minutes and can be renewed by the further careful addition of some CO2.

— Jacques Loeb, Forced Movements, Tropisms, & Animal Conduct, Dover edition of 1973, pp. 113–114.

In the passage above, 'heliotropism' means "moving towards the sun". People now prefer to say 'phototropism', meaning "moving towards the light", instead. Strictly, heliotropism means "moving towards or away from the light", which is why Loeb speaks of a "positive heliotropism” to show that the animals moved towards the light. Negative heliotropism would involve a movement away from the sun.

Today, we can see the logic of the animals' reaction: high CO2 means less oxygen, so moving towards the light usually means moving upwards and getting closer to the oxygen-rich surface layers of the water.

As a rule, when you are cultivating water animals in bottles, leave the water level far enough down to keep the surface area large. This maintains oxygen levels.  On the other hand, if you want to collect animals to look at, fill the bottle almost to the top, and within 24 hours, most of the small crustaceans will be in the top centimetre or so.

The tiny crustaceans (which is what they are) thrive wherever there is food, so green water from a pond will usually have some, but puddles, horse troughs (if they still have those where you live) and so on are also worth trying. Now for the rest of this, I am going to call them all Daphnia. At its simplest level, half-fill a bottle with green water, add a pinch of all-purpose fertiliser, cover it to stop mosquitoes getting in or water flowing out too messily if it tips over, and leave the bottle in the sun for a week or so.

The best bottles to use for this are 2-litre (or larger) PET plastic fruit-juice bottles, with the labels scrubbed off.  PET plastic is clear, so you will be able to see the animals if they are there. They show up best when you stand the bottle on a table in sunlight, crouch down and look towards the sun, especially near the top of the water and loom for small dots that are moving around near the surface.

As a general rule, that is all you need to do.  On the other hand, some professional biologists prefer to feed their Daphnia on small amounts of brewer's yeast, so the choice is yours. The golden rule is to have several cultures of anything precious, and to feed them at different times. That way, if the yeast takes over, you will have other cultures to fall back on, though usually, if a 'dead' culture is left for a while, there will be eggs, spores or survivors which will bounce back.

The best way to breed large numbers of Daphnia quickly is to take some water from a murky green aquarium, without any filamentous algae. Add a small amount of hard-boiled egg yolk, mixed with water into a sort of soup, and stand back! Any Daphnia that you picked up with the algae will start to breed very rapidly.

In stagnant water, Daphnia develop more haemoglobin, up to ten times as much as in water with plenty of oxygen, so the Daphnia from stagnant water can be quite pink in colour. See if you can observe this.

To look at these animals under the microscope, you need well slides, and you need some method of slowing the animals down, so they don't whizz out of sight.  I will talk about that next time.