Branchiopods are a subclass of the Class Crustacea, that branch of the Arthropods that have mandibles and live in the water. Branchiopods are one of the most primitive groups of crustaceans and first show up in the fossil record back in the middle of the Paleozoic era, but because they have been around so long, they also show highly specialized characteristics along with the primitive traits. There are a few marine species but the vast majority live in freshwater.
Branchiopods are divided into three orders. The Anostraca are the fairy shrimps and have no carapace. Artemia or brine shrimp are the best known of the Anostraca and are used to feed fish and young aquaculture species. Artemia tend to swim on their backs, with little legs beating the water above them. The compound eyes are on stalks and all the legs are on the thorax. They are also called sea monkeys and one of my childhood memories is ordering pet sea monkeys from a comic book ad and being so disappointed when I hatched them and they were so small. I didn’t have a good microscope back then or I would have realised how fascinating they really were.
The second order is Notostraca, the tadpole shrimps. These shrimp have a broad, flat carapace covering the head and thorax, so they look like miniature horseshoe crabs. The segmented abdomen is long and uncovered and ends in a ‘furca’ or forked tail. I once saw these animals living in a puddle of water on a rock in the middle of the Australian desert. I was lucky enough to be there after a rainstorm. The waters had run like rainbow serpents and pooled in rocky depressions and eggs that had lain dormant for tens of years or more hatched into shrimp. They lived fast and died when the pool dried up a few days later but in that time they had made more eggs which could survive being dried and out and would roll around in the desert winds until the rains came again, in weeks, months or years.
The third order, Diplostraca, is divided into two distinct suborders: the clam shrimps (Conchocera) and water fleas (Cladocera). Both have laterally compressed carapaces which cover the abdomen. In the clam shrimp the carapace is a bivalved shell like that of an oyster or other bivalve mollusc. It even has an umbo and adductor muscles to open and close it. Inside though is a typical crustacean body with compound eye, antennae and antennules, claspers, legs and a telson. They swim around like mad little clams, gobbling up detritus and poking around everything in their habitat. They are interesting to watch under a microscope.
Daphnia, commonly bred for fish food, is a typical cladoceran water flea and looks quite different from its relative, the clam shrimp. Water fleas are upright in the water with their tail pointing down and their faces pointing forward. They have large compound eyes and big, branched antennae modified for swimming. Water fleas can be seen often in pond water samples observed through a dissecting microscope. They are fascinating animals to watch because they are active and also transparent, so one can see the action inside their bodies as well. They have a large dorsal heart that can be seen beating, so students can measure heart beat rates. What they had for breakfast is visible in their stomachs and if they are gravid, the eggs are clearly visible in the brood pouch.
All Branchiopods are filter feeders and all use similar tactics to catch their food. They move their thoracic appendages to create water currents which aerate the gills and deliver food, mostly algae, to a food groove between the appendages. These little animals are important members of their food chains. In most ponds, there is a thriving planktonic community that contains lots of algae, protozoa, and bacteria plus these little crustacean filter feeders to eat them. Little fish then make meals of the Branchiopods and larger fish eat the little fish and then we can catch the big fish for fun or food. It’s a great system.
Branchiopods also feed aquatic insect larvae such as dragonfly and stonefly nymphs. These insects in turn feed not only fish, but when they mature into winged adults, feed terrestrial insectivores such as birds as well. So the branchiopods have nurtured not only aquatic food chains but terrestrial food chains too.
Branchiopods have simple reproductive systems with separate sexes and simple paired gonads. Their sex lives though are quite complex. Males have appendages modified as clasping arms to hold the female during copulation. In some species the males have paired penises. In other species, males have never been found and the females reproduce parthenogenetically. Most species brood their young for at least a period. Some species produce different eggs in summer and just before winter. Overwintering eggs are tougher with heavy membranes to resist cold and desiccation.
I could write more about these fascinating animals but I hope that others will contribute their knowledge to this title instead. It certainly deserves to be more than a one article title.
References: Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press
