Image from An Image-Based Key To The Zooplankton Of North America.

The crustacean class Branchiopoda is represented by six orders, all of them restricted to inland, continental waters: Anostraca, Notostraca, Laevicaudata, Spinicaudata, Cyclestherida, and Cladocera. Taxonomically, the class is divided into two subclasses consisting of 1) Anostraca and 2) everything else. But, ecologically, we recognize two groups: 1) cladocerans and 2) the phyllopodous branchiopods. The ecological division is easier to recognize and more useful for discussion. The taxonomy is listed below.

The phyllopodous branchiopods consists of the five non-cladoceran orders: Anostraca, Notostraca, Laevicaudata, Spinicaudata, and Cyclestherida. These taxa represent a “grade” (rather than a clade) sharing some key characteristics: phyllopodous thoracic appendages, visible segmentation, and absence from permanent water-bodies. There are roughly 500 species in 43 genera worldwide, with more than 90 species (20 genera) in North America (Brendonck et al., 2008). The richest order is the Anostraca (fairy shrimp), with more than 300 total species. The Notostraca, or tadpole shrimp, has only 15 species. The Laevicaudata, Spinicaudata, and Cyclestherida comprise the clam shrimp (formerly the “Conchostraca”). There are fewer than 200 species of clamp shrimp.

Cladocerans or water fleas differ from the other branchiopod orders in their lack of conspicuous segmentation and their inhabitance of permanent water bodies. There are c. 620 species (in 4 suborders) recognized worldwide, although it is estimated that the true richness may be 4x that. In North America, there are 189 species in 52 genera (Forró et al., 2008).

The branchiopod orders exhibit a wide range of morphological diversity. Anostracans have three distinct body tagmata: head, thorax, and abdomen. The head has stalked compound eyes and large fleshy antennae. The thorax bears 11 pairs of phyllopodous appendages used for locomotion, and the abdomen only has a pair of short, terminal cercopods. Fairy shrimp generally swim ventral side up. Notostracans have a large carapace covering their cephalothorax. The number of thoracic phyllopods varies from 35-71. Tadpole shrimp lack abdominal appendages except their terminal cercopods. The clam shrimps (Laevicaudata and Spinicaudata) have a bivalved carapace the encloses the body. Like notostracans, the compound eyes are sessile, clam shrimp have large, setose antennae used for swimming. The thoracic phyllopods vary in number from 10-32, and the abdomen is much reduced. The cladoceran body plan is similar to that of the clam shrimps. However, there are important differences. The cladoceran carapace is not bivalved, although it covers the thorax and reduced abdomen. The head is distinct, with a pair of large biramous antennae for locomotion and a single compound eye.

The phyllopodous branchiopods are also known as the “large” branchiopods. While cladocerans are typically μm-mm in length, the other orders are in the mm-cm range.

In contrast to these morphological differences, these taxa share the ability to produce resistant eggs that can withstand harsh environmental conditions. The large/phyllopodous branchiopods occupy ephemeral bodies of water, and their resting eggs allow populations to survive prolonged desiccation, freezing, etc. These orders generally have indirect development with a nauplius larval stage. Cladocerans are well known for their alteration of asexual and sexual reproduction, with the latter producing resistant eggs known as ephippia. Development is direct.

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Bibliography

• Kotov, A., L. Forró, N.M. Korovchinksy & A. Petrusek. 2013. World checklist of freshwater Cladocera species. World Wide Web electronic publication. Available online at http://fada.biodiversity.be/group/show/17. Generated 28 June 2013. 38 pp. [Available online]
• Dodson, S.L., C.E. Cáceres, & D.C. Rogers. 2010. Cladocera and other Branchiopda. [in] J.H. Thorp & A.P. Covich (eds.). Ecology and Classification of North American Freshwater Invertebrates, 3rd edition: 773-827.
• Forró, L. N.M. Korovchinsky, A.A. Kotov & A. Petrusek. 2008. Global diversity of cladocerans (Cladocera; Crustacea) in freshwater. Hydrobiologia 595: 177-184. [Available online]
• Brendonck, L. D.C. Rogers, J. Olesen, S. Weeks, & W.R. Hoeh. 2008. Global diversity of the large branchiopods (Crustacea: Branchiopoda) in freshwater. Hydrobiologia 595: 167-176. [Available online]
• Smith, D.G. 2001. Phyllopodous Branchiopoda (fairy, tadpole, and clam shrimps). Pennak's Freshwater Invertebrates of the United States, 4th edition: 427-452.
• Smith, D.G. 2001. Cladoceran Branchiopoda (Water fleas). Pennak's Freshwater Invertebrates of the United States, 4th edition: 453-488.
• Underhill, J.C., W.D. Schmid & D.E. Gilbertson. 1982. Taxonomic Keys to the Common Animals of the North Central States, exclusive of the parasitic worms, terrestrial insects, and birds, 4th edition. 205 pp.
• Mattox, N.T. 1959. Conchostraca. [in] W.T. Edmondson (ed.). Ward & Whipple Fresh-Water Biology, 2nd edition: 577-586.
• Linder, F. 1959. Notostraca. [in] W.T. Edmondson (ed.). Ward & Whipple Fresh-Water Biology, 2nd edition: 572-576.
• Dexter, R.W. 1959. Anostraca. [in] W.T. Edmondson (ed.). Ward & Whipple Fresh-Water Biology, 2nd edition: 558-576.
• Brooks, J.L. 1959. Cladocera. [in] W.T. Edmondson (ed.). Ward & Whipple Fresh-Water Biology, 2nd edition: 587-656.

Web Pages

• Animal Diversity Web
• Integrated Taxonomic Information System
• Tree of Life
• WikiSpecies
• Wikipedia
• World Register of Marine Species
• Zooplankton of the Great Lakes