Cladocerans are small crustaceans commonly found in most freshwater habitats, including lakes, ponds, streams and rivers. While there have been a few marine species, cladocerans have definitely not been successful in seawater. Bodies of freshwater that lack an abundance of fish that act as predators provide the most suitable habitats. Many species of cladocerans can be found residing in the open water of lakes, as do plankton. Some other species live either on or near vegetation near the bottom of lakes.
The term cladoceran is useful in describing crustaceans of similar appearance. More commonly referred to as water fleas, cladocera are members of the class Crustacea, and the phylum Arthropoda. Cladocerans can be split into four orders, Anomopoda, Ctenopoda, Onychopoda and Haplopoda (Fryer 1993). In the United States, less than 20 species of cladocerans are known and they have been collected and studied in only a few sites. There is still much work to be completed in studying cladocerans (Pennak 1978).
Cladocerans vary in size from 0.2 to 3.0 mm long and identification is usually only possible with the help of a microscope (Fryer 1993). In most species, the body is not segmented, but it is covered by a secreted shell. The shell has an appearance that looks like a bivalve, however, the shell is one continuous piece, folded in half. Looking at a lateral view of a cladoceran, the shell may be oval, circular, elongated, or angular. The head of a cladoceran is a very small, compact structure. It is bent in a downward position and is easily recognized by the large compound eye. The eye rotates constantly and seems to be rather jerky in its movements. Another defining characteristic of the head is the rostrum, or beak. The rostrum points downward just as the head does, and lies in between and just below the two pairs of antennae located on the forehead. There are 5 or 6 pairs of lobed legs, and each is covered with setae and hairs. In general, males are smaller than females, have larger antennae, and their first pair of legs contains a hook at the end used for grabbing and holding. As for the coloring of cladocerans, species found in the open waters or in the limnetic zones of lakes are usually light in color and translucent. All other species, including species found in more eutrophic bodies of water and species found on the bottoms of lakes are darker in color, ranging from a yellowish-brown to black (Pennak 1978).
Generally, cladocerans are in motion most of the time, swimming by vigorously stroking their antennae, which are used as their main form of propulsion. Most species of cladocerans also move in a series of hops, in the same manner as fleas, hence their nickname "water fleas" (Fryer 1993). In addition to swimming and hopping, many cladocerans spend a large amount of time crawling in mud, leaf surfaces, or other bottom debris. They accomplish this crawling motion by kicking their legs, which also resembles hopping (Pennak 1978).
A few species of cladocerans are predacious, but most species are herbivorous. The herbivores mainly feed on phytoplankton, decaying organic material, and bacteria. They feed as filter feeders, using complex movements of their legs to induce a constant stream of water rushing towards their mouths. The many setae on their legs serve as filters and capturing devices. The collected food particles are stored in a ventral groove at the base of their legs. The food is then taken towards their mouths and crushed in the mandibles before ingested (Pennak 1978). Even though they are filter feeders, cladocerans are not known to use any sort of selective mechanism in their feeding. They have been proven to prefer some types of algae and protozoans to others, but as long as a particle is of desirable size, it will be ingested. Two genera, Polyphemus and Leptodora, are predators and they capture prey with the help of their legs, which are modified for seizing prey. Rotifers are the main source of food for these predators (Pennak 1978).
Most of the internal organs of cladocerans are easily distinguishable. They contain a complex muscular system which envelopes the other bodily systems. The digestive system of cladocerans is unspecialized, with the exception of the rectum (in some species). The rectum performs what is known as "anal drinking". This process increases the efficiency of food utilization. Their hearts are simple and some pond species contain hemoglobin, which leads to an increased life span. Respiration is performed through an exchange of oxygen and carbon dioxide through the body surface, especially through the surface of the legs. The nervous system of cladocerans is extremely simple, as is the excretory system (Pennak 1978).
During most of the year, reproduction is parthenogenic or asexual, and only females are produced. The number of eggs per clutch varies significantly, but is usually between 10 and 20, and each female adult will produce only one clutch in her lifetime. In the early spring the cladoceran population consists mainly of females recently hatched from resting eggs. Once the water temperature reaches 6° to 12° C, reproduction resumes. The population will then grow and when the water temperature gets warmer, 14° to 17° C, and when the environment starts to deteriorate due to overcrowding, eggs are produced that develop into males and females capable of sexual reproduction. The eggs that are then fertilized sexually by males are capable of surviving winters, and once spring arrives, these eggs will hatch. Many studies have been conducted aiming to figure out this phenomenon of sexual versus asexual reproduction, but there has not been a definitive answer as of yet (Pennak 1978). However, according to Walls and Ventela (1998), temperature has shown to have an influence on reproduction and specifically on the age at first reproduction. They found that cladoceran species reared at 20° C are significantly larger at first reproduction then are species reared at 16° C.
The life cycle of cladocerans is also very complex. There are known to be four distinct life stages: the egg, juvenile, adolescent and adult. Their life expectancy has been documented at anywhere between 25 and 100 days (MacArthur and Baillie 1929).
Cladocerans are among the most easily cultured of all the freshwater invertebrates. Manure, cottonseed meal, dried yeast, dried milk and chopped hay are all suitable mediums. Cladocerans are extremely important as a source of food for other freshwater organisms such as hydra and immature and mature insects. Capturing of cladocerans can be accomplished with the use of a plankton townet or a Birge cone net (Pennak 1978).
In conclusion, cladocerans are an extremely interesting invertebrate. They are extremely mobile and have a specialized feeding mechanism. They are very sensitive to their habitats as shown by the experiment conducted by Walls and Ventela, and they will continue to thrive in great numbers due to their high reproduction rates.
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