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A General Introduction.
Cladoceran Crustaceans Gallery.

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Female Diaptomus (a small copepod crustacean) swimming.
Female Diaptomus, swimming.


Main/Cladoceran. Copepods. Ostracods. Shrimps.

Crustaceans are distinguished from all other arthropodsAnimals having a hollow, chitinous, jointed exoskeleton. (insects, spiders, scorpions, mites etc.) by the possession of two pairs of antennae on the head. The largest crustacean found in fresh water is the freshwater crayfish -- not shown here. The freshwater crustaceans in these galleries are much smaller -- for the most part around 2mm and less in size.

This gallery divides crustaceans into four groups of interest to freshwater microscopists:
  1. Cladocerans.
    Commonly called Water fleas; includes Daphnia, Simocephalus.

  2. Copepods.
    Includes Cyclops, Diaptomus and similar small crustaceans.

  3. Ostracods.
    Freshwater forms include Cypris.

  4. Amphipods.
    Includes Gammarus, the common freshwater shrimp.

  5. Freshwater Isopods (water lice) and Decapods (crayfish) are not represented here.
Small crustaceans, including water fleas and shrimps of various kinds are the most prominent arthropods in freshwater, and similar tiny crustaceans are also present in the oceans in vast numbers. Despite their small size, they are very important ecologically not only on account of their numbers, but as filter feeders, they consume even vaster amounts of the organic detritus and free-swimming algal unicells which are their food. They are the most abundant animals at the base of the food pyramids which sustain fish, dolphins and whales, sea birds and thus most of the marine and freshwater animal foods consumed by humans.

As a footnote on crustacean ecology, it has been recently discovered that the cholera vibrio, much feared as an agent of disease when it appears in the human environment, plays a vitally important role in the oceans as one of the principal agents in the breakdown of the chitinous exoskeletons of dead crustaceans.

  Cladoceran Crustaceans (Water Fleas).

In 1669, the Dutch microscopist Swammerdam in his work on "insects" described "pulex aquaticus arborescens" -- the water flea with branching arms. So if not the discovery, the detailed description of these creatures dates from the earliest days of the microscope.

"Water Fleas" is a term loosely applied to a variety of small freshwater crustaceans which swim in a rapid jerky manner by flipping their antennae or other appendages. More specifically, the term is usually applied to the cladoceran crustaceans. Of these, Daphnia and Simocephalus are two of the most common. Their average size is in the 0.5 to 2mm range.

During the majority of the summer season, they reproduce parthenogenetically, producing only female offspring from eggs which require no fertilization. The young mature to a well-developed stage within the brood chamber of the parent, sustained initially by their egg yolk, and then by a secretion from the inside of the brood chamber. Towards autumn, a combination of conditions not entirely predictable sees the production of males which fertilize special eggs, capable of surviving winter and hatching the following spring into females which will continue the parthenogenetic cycle.


Female Daphnia with egg. Daphnia (whole view) with one egg. Most water fleas are females. This female Daphnia waterflea is seen with a single egg in its brood chamber. The simple tubular digestive tract is empty except for a small accumulation of fecal matter seen as yellowish particles in the end closest to the two terminal claws (caudal furcae).
Darkfield, x65.
Daphnia: Closeup of head.  Daphnia: Close-up of the Head. The single large compound eye is covered with lenses, and an ocellus (small black speck) can be seen below the compound eye.
Darkfield, x150
Daphnia: Single Antenna. Daphnia: Closeup of the antennae. Crustaceans are distinguished from the other arthropods by the possession of two pairs of antennae. In Daphnia the second pair are large and branched, have long bristles and are used for swimming, whilst the first are unbranched,vestigial and cannot be seen in these pictures.
Darkfield, x150.


This series of pictures shows the live birth of young Simocephalus water fleas by an adult female with a brood chamber filled with fully developed young. Most reproduction in water fleas is parthenogenetic -- unfertilized eggs develop to maturity within the body of the adult female, which then gives birth to all-female offspring which will in turn produce females from unfertilized eggs, etc.

Sexual reproduction usually occurs towards the end of summer when the females produce special eggs which hatch into males. The eggs which result from fertilization have thick resistant shells, and endure the winter to hatch into parthenogenetic females the following spring.

Click for a detailed diagram of Simocephalus: male and female.

1. Simocephalus: Female with Eggs. Picture captions anticlockwise from upper left.
All pictures use blue Rheinberg illumination.
  1. Adult female Simocephalus with five immature eggs in the brood chamber. It will take a week or so for the young water fleas to develop fully. x50.
  2. A different female Simocephalus (pics 2-5) with mature young. They have already broken free of their egg shells. x50.
  3. The birth process is initiated by the parent moving the limb-like digestive canal forward, creating an opening between the halves of its carapace through which the young can escape. x50.
  4. Closeup of an escaping young water flea. x90.
  5. Mother and Young. Around twelve daughters were released.
    The newly-emerged water fleas take up water to rapidly increase in size in the first couple of minutes after birth whilst their exoskeleton is still supple. Ten minutes later it is difficult to believe that so many offspring can have come from so small a space as the parent's brood chamber. x25.
2. Simocephalus: Adult Female with Developed Young. 3. Simocephalus: The Birth Process Begins. 4. Simocephalus: Young individual emerging. 5. Simocephalus: Parent with offspring.

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