Crustaceans, carcinology, the insects of the seas. A large group of unsettled rank (phylum, subphylum, superclass, or class). Ancient and very diverse, but not nearly as rich in species as are the insects. Over 40,000 species occur worldwide, most are marine, about 10% are freshwater and, only a few are terrestrial. The chitinous cuticle of larger species is calcified.


Diverse! All non-insect, non-arachnid aquatic arthropods. Most have 2 pairs of antennae, and many pairs of segmented appendages, typically with gills for gas exchange. The appendages are basically biramous (protopod, endopod, exopod).

The head has 5 pairs of appendages: first antennae (antennules), second antennae (or just antennae), mandibles, first maxillae, second maxillae.

The trunk is primitively composed of many similar segments each with similar biramous appendages, more advanced groups have uniramous appendages, fewer segments that are more specialized or fused into distinct regions (thorax and abdomen), part of the thorax may be fused to the head (cephalothorax).

The trunk appendages are often highly modified (maxillipeds, gnathopods, pereiopods, pleopods, and uropods). Malacostracan pereiopods may be chelate and have 7 segments (coxa, basis, ischium, merus, carpus, propus, and dactylus), the exopodite is often loss.

Terminal telson is usually not counted as a segment.

Life Cycle

Most crustaceans are dioecious with copulation, but parthenogenesis is common and hermaphroditism sometimes occurs. Marine forms often have a series of planktonic larval forms (nauplius, zoea, megalops, etc.). Freshwater species tend to have more direct development and to brood the eggs. Females may produce a single or multiple broods per season or alternate between sexual and non-sexual (parthenogenetic) reproduction. Molting may continue after the adult stage is reached. Resistant eggs are common. Non-emergent, only resistant stages may disperse over land. Cyclomorphosis occurs in some species.

Trophic Roles

Free-swimming and filter-feeding by way of setae is primitive in Crustacea and still widespread. Others are benthic or intermittent swimming filter-feeders. Others crawl or burrow and feed as herbivores, scavengers or carnivores. A few are parasitic.


Often rather simple, at least by insect standards, with few species, though some in great abundance. Crustaceans occupy nearly all oxygenated aquatic habitats, including intermittent ponds, saline ponds, hot springs and deep lakes. The species are separated by micro-habitat, habits, body size, food size, seasonality, or time of activity. They are the dominant macroinvertebrates in the open water of large lakes and reservoirs, but shallow, heavily vegetated lentic water has the most species. Some species have considerable diurnal vertical migrations. Many species occur in interstitial areas, subterranean waters (troglobites), and groundwater.

Economic Importance

Important primary and secondary consumers in many aquatic systems, critically so in limnetic waters. Some species support direct fisheries and aquiculture for human food or fish bait, important fish and wildlife foods, some may help control aquatic vegetation, many are good environmental indicators for metals, acid rain, global warming, etc. Parasites may destroy hatchery fish, some serve as intermediate hosts to other parasites, some may out-compete fish for food or prey on the fish directly, some damage underwater wood, dense populations may clog filters, burrowing crayfish may denude farmland and destroy earthen dikes and dams.


Most major groups occur worldwide, species of small body size or those with resistant eggs may also be globally distributed. In general, most groups have a few species that are widespread and many species that are endemic to single stream systems or to caves, some of these are endangered. Many groundwater species evolved in deep refugia from surface fluctuations, some of these survived under the continental ice sheets.


Plankton nets and routine benthic samples will get many of the "important" species. Many require very fine nets. Some plankton species require large nets worked very deep or on dark nights. Crayfishes may be better trapped or picked up at night by using headlamps, interstitial and ground-water species may be filtered from pumped water or taken in springs. Lighted bottle traps take many species in abundance, even very small ones.


Small crustaceans can be adequately fixed directly in 70-80% ethanol. Larger specimens are best fixed in neutral 5% formalin and transferred to 70% ethanol within a few days. Kahle's fluid quickly decalcifies crustacean exoskeletons, but preserves the rest very well. Isopropyl alcohol is of no use with crustaceans as they become explosively brittle. Briefly boiling the specimens in water or 70% ethanol is often a suitable substitute for formalin. Most color patterns are lost during storage so notes should be taken while the specimens are fresh.


Fairly well studied at the order and family level. Higher level relationships are more difficult. Some species are well known, but many are known only from the original description, and some remain undescribed. Juvenile instars and the "wrong" sex can be very difficult or impossible to identify. Many species identifications require carefully prepared slide mounts.

Higher classification of freshwater Crustacea

  • Subclass Branchiopoda- phylopods, first antennae reduced
    • Order Notostraca- tadpole shrimps
    • Order Anostraca- fairy shrimps
    • Order Conchostraca- clam shrimps
    • Order Cladocera- water fleas
  • Subclass Ostracoda- seed or mussel shrimps, no compound eyes, carapace calcified
    • Order Podocopa- 2nd antennae with endopodite length greater or equal to the exopodite
  • Subclass Copepoda- copepods, nauplius eye, but no compound eyes, no carapace
    • Order Harpacticoida- 1st antennae with 5-9 segments, benthic and interstitial
    • Order Cyclopoida- 1st antennae with 10-16 segments, planktonic & benthic
    • Order Calanoida- 1st antennae with 16-26 segments, planktonic
    • Order Siphonostomatoida- specialized parasites
  • Subclass Branchiura- sessile compound eyes, shieldlike carapace
    • Order Arguloida- fish lice
  • Subclass Malacostraca- 8 segmented thorax, 6 segmented abdomen
    • Superorder Syncarida- freshwater, fossil marine
      • Order Anaspidacea- South America and Australia only.
      • Order Bathynellacea- minute, reduced blind interstitial forms
    • Superorder Pancarida- eggs carried in a dorsal brood pouch
      • Order Thermosbaenacea- small, subterranean forms
      • Superorder Peracarida- eggs carried in marsupium under the thorax, no zoea
        • Order Mysidacea- opossum shrimps
        • Order Isopoda- isopods or sowbugs
        • Order Amphipoda- amphipods or scuds
      • Superorder Eucarida- eggs carried under the abdomen, zoea larvae
        • Order Decapoda- 3 pairs of maxillipeds and 5 pairs of legs on thorax
          • Suborder Natantia- shrimp
          • Suborder Reptantia- crayfish

Artificial key to higher taxa of Crustacea (Thermosbaenacea and Bathynellacea omitted)

1a. Eyes stalked.................. 2
1b. Eyes sessile or absent.................. 5
2a. Carapace absent.................. Anostraca
2b. Carapace present.................. 3
3a. Carapace free posteriorly, all legs biramous.................. Mysidacea
3b. Carapace fused, some legs uniramous.................. Decapoda 4
4a. First two pair of pereiopods chelate.................. Natantia
4b. First three pair of pereiopods chelate.................. Reptantia

5a. Carapace absent.................. 6
5b. Carapace present.................. 11
6a. Abdominal appendages lacking.................. Copepoda 7
6b. Abdomen with pleopods.................. 10
7a. Highly modified parasites.................. Siphonostomatoida
7b. Not highly modified for parasitism, rarely parasitic.................. 8
8a. Abdomen not much narrower than thorax.................. Harpacticoida
8b. Abdomen much narrower than thorax.................. 9
9a. First antennae almost as long or longer than body.................. Calanoida
9b. First antennae not reaching beyond the cephalothorax.................. Cyclopoida
10a. Body dorsal-ventrally compressed.................. Isopoda
10b. Body laterally compressed.................. Amphipoda

11a. Carapace univalved.................. 12
11b. Carapace bivalved.................. 13
12a. Thorax with 4 pairs of appendages.................. Branchiura
12b. Thorax with 40 or more pairs of appendages.................. Notostraca
13a. Carapace not enclosing the head.................. Cladocera
13b. Carapace enclosing the entire body.................. 14
14a. Thorax with 10 or more pairs of appendages.................. Conchostraca
14b. Thorax with 3 or fewer pairs of appendages.................. Ostracoda

Some changes from Pennak

  • Eubranchiopoda and Oligobranchiopoda are abandoned as unnatural taxa.
  • Eucopepoda is discarded, Branchiura is elevated to subclass and the copepod suborders are elevated to orders.
  • The more specialized parasitic copepods, Caligoida and Lernaeopodoida are combined into the order Siphonostomatoida. Lernaea is now placed in the Cyclopoida.
  • The genus Hyalella is generally placed in its own family the Hyalellidae rather than the Talitridae.

Synopsis Of Neartic Orders

  • Anostraca- fairy shrimp, many pairs of thoracic appendages, no carapace, compound eyes stalked, second antennae of males modified as claspers, filter feeders, swim up side down, 180 species, none are marine, characteristic of temporary pools or extreme environments, Artemia in saline lakes.


  • Notostraca- tadpole shrimps, sessile eyes, antennae reduced, depressed body, large shield-like univalved carapace, many pairs of thoracic appendages, about 11 widely distributed species, all freshwater.


  • Conchostraca- clam shrimps, completely enclosed within a bivalved carapace, sessile eyes, second antennae used for swimming, many trunk appendages, 180 species, all freshwater.


  • Cladocera- water fleas, bivalved carapace covers only the trunk, large and sessile eyes, large second antennae used for swimming, only 4-6 trunk appendages, body segments fused, egg brooders, 450 species, mostly freshwater, planktonic and littoral.


  • Podocopa- seed shrimps, entire body enclosed within a bivalved calcified carapace, compound eyes lacking, both antennae well developed and used for swimming, three pairs of thoracic appendages, body segments fused, abdomen reduced, mostly found in vegetation, 5000 species, marine and freshwater.


  • Harpacticoida- copepods with 5-9 segments in the first antennae, abdomen not much narrower than the thorax, 2800 species, mostly marine and benthic.


  • Cyclopoida- copepods with the first antennae with 10-16 segments and not reaching much beyond the cephalothorax, abdomen (urosome) much narrower than the thorax (metasome), 450 species, planktonic and benthic.


  • Calanoida- copepods with the first antennae with 16-26 segments and about as long or longer than the body, abdomen much narrower than the thorax, 2300 species, mostly marine and planktonic.


  • Siphonostomatoida- copepods that are highly modified for parasitic existence, 1430 species, marine and freshwater.


  • Arguloida- fish lice, body flattened, carapace covers most of the body, eyes sessile, mouthparts suctorial, first maxillae forming suckers, both pair of antennae reduced, abdomen short and bilobed, four pair of thoracic appendages, 150 species worldwide.


  • Thermosbaenacea- reduced carapace fused only to the first thoracic segment and used as a dorsal brood pouch in the females, pleopods reduced or absent, 9 species, thermal water, saline wells, ground-water.


  • Bathynellacea- small (0.5 - 3 mm), elongate, blind, all 8 thoracic segments are free, pleopods reduced or absent, 100 species, interstitial and ground water.


  • Mysidacea- opossum shrimps, carapace extends over most of the thorax but not fused with the last 4 segments, eyes stalked, 1 or 2 pairs of maxillipeds, 6 or 7 pairs of biramous swimming legs, 5 pairs of pleopods the fourth is very long in males, 780 species, all but 42 are marine.


  • Isopoda- isopods or sowbugs, body flattened, no carapace, eyes sessile, first thoracic segment fused with the head, 1 pair of maxillipeds, 1 pair of gnathopods, 6 pairs of pereiopods, 5 pairs of flattened pleopods, 0-5 abdominal segments fused with the telson, 1 pair of uropods, over 4,000 species, mostly marine, also terrestrial, freshwater, many troglobits.


  • Amphipoda- scuds, body laterally compressed, no carapace, eyes sessile, first thoracic segmented fused with the head, 1 pair maxillipeds, 2 pair gnathopods, 5 pair pereiopods, 3 pair of narrow pleopods, 3 pairs of uropods, 6000 species, mostly marine, also freshwater, some terrestrial, many troglobits.


  • Decapoda- crawfishes, shrimps, crabs, eyes stalked, carapace fused with all 8 thoracic segments, 3 pairs or maxillipeds, 5 pairs of pereiopods (decapod), 5 pairs of pleopods, 1 pair of uropods, 10,000 species, mostly marine, also freshwater and terrestrial.


Montana Species list

Updated on 25 MAR 1996 D.L. Gustafson
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