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Zooplankton - Waterfleas

Cladoceran Anatomy

Photo of Daphnia in lateral view with Anterior-Posterior, Dorsal-Ventral directions labelled

<= Lateral (side) view
of Daphnia sp. (typical cladocera).

The lateral view is more commonly seen and more useful for identification than the dorsal or ventral views.

Photo of Scapholeberis in dorsal view

Dorsal (back) view =>
of Scapholeberis sp. (typical cladocera)

Live free-swimming organisms frequently orient themselves dorsal side up (so this is what you see looking down at them through a microscope) - a few orient ventral side up. Identification is difficult with this view. Removing liquid from the slide may help to tip them to one side.

Photo of Daphnia with carapace outlined in red.

Cladocera have a bivalve carapace. Usually the carapace completely covers the animal with only appendages (antennae and postabdomen) extending outside its protection. In a few families, the carapace is reduced to cover only the brood pouch area.

Other important carapace features

Brood Chamber
Photo of Daphnia with brood chamber outlined in red.

Female cladocera have a brood pouch or brood chamber lying along the dorsal side of the body within the carapace. The brood chamber contains eggs, epphippia (resting eggs) or young. Eggs and/or young are typically released when the mother molts.

Male cladocera can be particularly difficult to identify and to distinguish from juvenile or non-gravid (pregnant) females. Males are usually smaller, usually have larger first antennae, and have a hook on their first legs (true legs are located on the ventral surface within the carapace and are very difficult to observe via typical light microscopy).

Photo of Daphnia with head outlined in red.


The cladoceran head may or may not be separated from the body along the dorsal surface of the carapace but is typically separated by a deep indentation along the ventral surface. The ventral-posterior angle of the head may or may not be extended to form a rostrum (beak). Nearly all cladocera (all Great Lakes species except Monospilus dispar) have a single large compound eye located near the center of the head. Cladocera may or may not have a much smaller ocellus (eyespot) located near the compound eye.

Other important head features

Photo of Daphnia with ventral margin of the rostrum outlined in red.Photo of Daphnia with the Compound eye outlined in red.Photo of Daphnia with the Ocellus highlighted in red.

Photo of Daphnia with a 1st antenna outlined in red.


Cladocera have two pairs of antennae. The second antennae are used for swimming and gathering food and are typically much larger than the first antennae (or antennules). 1st antennae contain sensory organs - they may be extremely small and hidden by the rostrum (as in this Daphnia) or nearly as large as the 2nd antennae. Except for the family Macrothricidae, the second antennae are attached anterior to the first antennae.

Additional 1st antennae forms

Additional 2nd Antennae forms

Photo of Daphnia with the 2nd Antennae highlighted in red.
Photo of Daphnia with the intestine outlined in red.


Cladocerans have a relatively simple digestive system consisting of a mouth (located anteriorly beneath the rostrum) and an anus (located posteriorly in the postabdomen) connected by a simple intestine. In only a few taxa is the intestine convoluted (coiled). Fine-scale characteristics of the postabdomen or foot including spines, projections and claws are very important to cladoceran species identification.

Additional post-abdomen characteristics

Photo of Daphnia with the post-abdomen outlined in red.
Photo of Daphnia with the tail spine outlined in red.

The most typical cladoceran tail is a spiny projection of the carapace. A number of species have setae (hairs) projecting from the carapace or body which appear as a tail. Only a few groups have true tails (projection of the body rather than the carapace) -- these are usually segmented.

Additional Tail Forms

A quick note about sizes...Cladocerans are part of a group called Macrozooplankton. As the name implies, they are the largest of the free-floating (plankton) species ranging in size from about half a millimeter to over a centimeter. Males are typically smaller than females and juveniles (young) are of course smaller than the adults. Click here to see some photos of typical Great Lakes cladocerans scaled to size and to access tables listing maximum sizes (or adult ranges) for all Great Lakes cladoceran species.
- Cladoceran Sizes -

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