A General Introduction.
Cladoceran Crustaceans Gallery.
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Crustaceans are distinguished from all other arthropods (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:
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.
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.