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/The Conscientious Marine Aquarist

Crustaceans: An Overview

By Bob Fenner

Panulirus interruptus

Classification: Taxonomy, Relation With Other Groups

If you absolutely loathe biological classification and all of it's obscure terminology, you're going to hate the crustaceans. There are so many "split-hairs" amongst it's groups that science types have taken to making new infra-, sectional-, you-name-it categories to sub-divide all the groups. I guess that's what you get with such a collection of diverse organisms and excellent fossil history. Anyway it won't hurt my feelings if you skip on down to the next subject heading.

The Phylum Arthropoda (="jointed legs").

With the insects, trilobites, spiders, horseshoe crabs, scorpions, ticks, mites, sea spiders, centipedes, millipedes, (whew!) and a few other various and sundry groups, the subphylum Crustacea helps make up the largest phylum of life (Arthropoda); with some three quarters of a million described species. They all share 1) Metamerism: a division of the body into definite segments. 2) A pair of appendages per segment (at least in the primitive condition). 3) A nervous system with a dorsal, anterior brain, ventral nerve cord and ganglionic swellings in each segment. 4) Similar embryology/development. 5) Chitinous exoskeletons that cover the entire body; growth through periodic molting. 6) A muscular system that moves the animal by pulling on the exoskeleton.

Enough of the higher taxonomy of the group! A synopsis of the arthropod classes:

Subphylum Trilobita. The fossil trilobites.

Subphylum Chelicerata. Chelicerae for mouthparts...

Class Merostomata. The living horseshoe crabs and fossil eurypterids (some were big predators).

Limulus polyphemus, a/the Horseshoe Crab. This species is principally a sand-sifting filter feeder, though there are other horseshoe crabs that are more predatory. Up to a foot in diameter. Most die from a lack of nutrition in captivity, due to insufficient size, depth, culture of live sand organisms. Aquarium photo. 

Class Arachnida. The scorpions, harvestmen, spiders mites and ticks.

Class Pycnogonida. The sea spiders.

Subphylum Uniramia

Class Insecta. The insects.

Class Chilopoda. The centipedes.

Class Diplopoda. The millipedes.& finally (Yay), the:

Subphylum Crustacea:   

About 42,000 species of some of the most familiar arthropods; crabs, shrimps, lobsters, crayfish, wood-lice (sow-bugs, rolly-pollies, you know, terrestrial isopods, and aquatic, even parasitic marine ones). Many small members in fresh and marine habitats of importance in aquatic food chains. Primarily aquatic, mostly marine.

Some common characteristics of the crustaceans: 1) Their heads are more or less uniform with five pair of appendages: they have two pair of antennae (this feature is distinctive within the phylum); the third pair as opposing, biting, grinding mandibles. Behind the mandibles there are two pair of accessory feeding appendages, the first and second maxillae. 2) Their bodies trunks are composed of distinct segments covered by a chitinous exoskeleton strengthened by deposition of calcium salts. 3) Crustacean appendages are typically biramous (two major elements). 4) They typically have a carapace covering the trunk of their bodies. Enough of this detail. We'll cover this stuff in more general survey pieces of the mega-groups. On toward the lobsters.

A Systematic Resume of the Crustacea is necessarily large and complex. Allow me to semi-skirt around a full discussion here, giving just a systematic outline of Crustacean systematics and focus more fully on the individual groups of interest/use by aquarist at lower levels (on pages, linked elsewhere). Numbers in parentheses are current species counts.

Subphylum Crustacea (45,000+)

Class Cephalocarida (9), most primitive. Shrimp-like, less than 4mm long.

Class Branchiopoda (800), fairy shrimps, tadpole shrimps, water fleas (Daphnia)

Class Ostracoda (7500+), the Mussel or Seed Shrimps.

Class Copepoda (8000+), mostly marine, but some freshwater, some parasitic. Anchor Worm, Lernaea

Classes Mystacocarida (9)

Class Branchiura (130), ectoparasites on marine and freshwater fishes, e.g. Fish Lice, Argulus.

Class Tantulocarida (4)

Class Remipedia (130)

Class Cirripedia (900), barnacles, many parasitic forms. Little shrimps that have settled down and attached themselves to the substrate with cement gland material, built a calcium carbonate house around themselves and kick food into their mouths. 

Representative of the Barnacles, below, Lepas anatifera (Smooth Goose-Neck Barnacle) in St. Thomas, U.S.V.I., some sessile barnacles in the Bahamas and Pollinices polymerus off of San Diego, California. 

Class Malacostraca: (29,000+)

Subclass Phyllocarida

Order Leptostraca (20)

      Subclass Eumalacostraca

Order Anaspidacea (10)

Order Bathnellacea (25)

Superorder Hoplocarida

Order Stomatopoda (350), the Mantis "Shrimps"

Superorder Eucarida

Order Euphausiacea (90)

Order Decapoda (10,000+)

Superorder Peracarida

Order Spelaeogriphacea (1)

Order Thermosbaenacea (6)

Order Mysidacea (780)

Order Cumacea (800)

Order Tanaidacea (550)

Order Isopoda (10,000+)

Order Amphipoda (5,500+)

Class Malacostraca : Comprises almost three-fourths of all described species of crustaceans and most of the larger forms, such as crabs, lobsters, and shrimps. Characteristics: Trunks typically composed of 14 segments plus the telson ("tail"); the first 8 segments form the thorax, the last 6 the abdomen; all segments bear appendages. Four Superorders: Syncarida, Hoplocarida, Peracarida, and the one we want to talk about, the Eucarida.

Superorder Eucarida contains many of the large malacostracans. They have highly developed carapaces displaying fusion of all thoracic segments (the cepahalothorax). Eyes are stalked... Two living orders; the Euphausiacea (krill) and the:

Order Decapoda includes the familiar shrimps, crayfish, lobsters and crabs. This is the largest order of crustaceans with @10,000 species. Decapods are distinguishable from euphausiaceans and other malacostracans in that their first three pair of thoracic appendages, The remaining five pairs are legs (Decapoda= "ten feet"). Decapods are divided into two Suborders, the Dendrobranchiata, with "tree-like" branched gills, body laterally compressed..., eggs planktonic, nauplius as the first larval stage (as in Artemia, our brine shrimp), and the Pleocyemata.

Suborder Dendrobranchiata. Shrimps (but not all of the shrimps). Gills of a type termed dendrobranchiate (tree branch like). Bodies laterally compressed. Their first three pairs of legs are chelate, but not as enlarged chelipeds. Eggs in these shrimps are not carried by the female but are planktonic. Nauplius as first stage of larvae.

Infraorder Penaeidea. Shrimps with  well-developed rostrums. Lack overlapping pleura of second abdominal segment over first. 

Suborder Pleocyemata (Lobsters, Crabs, Hermits and more): with phyllobranchiate and trichobranchiate gills, eggs carried by the female on pleopods and hatch as zoeae. Divided into six Infraorders on the basis of structural differences in the cephalothorax and legs, & development; 

Infraorder Stenopopidea. Shrimps. Cylindrical cephalothorax. First three pair of legs chelate; one of last pair enlarged. Gills trichobranchiate. Lack overlapping pleura of second abdominal segment over first. e.g. Stenopus (The Coral Banded, Boxer Cleaner Shrimp)

Infraorder Caridea. Shrimps. Cylindrical cephalothorax. First two pairs of legs chelate or subchelate and cylindrical. Third pair chelate. Possess overlapping pleura of second abdominal segment over first. Gills phllyobranchiate. Sand Shrimps (Crangon), Snapping Shrimps (Alpheus, Synalpheus), fresh, brackish and marine Palaemonids (family Palaemonidae), Macrobrachium, Periclimenes and Hippolysmata ( Cleaner Shrimps).

Thor amboinensis, the Sexy Shrimp

Infraorder Astacidea. Crayfish and Large Clawed Lobsters. Cylindrical cephalothorax and dorsoventrally flattened abdomen. First three pairs of legs chelate, the first greatly so as claws. Includes Homarus, Nephrops, freshwater "crawdads": Astacus Cambarus, Procambarus

Infraorder Palinura: has members with the cephalothorax more or less cylindrical, abdomen well developed, flattened dorsoventrally. Legs may be chelate or subchelate, but the first pair in most species is not enlarged. These are the "spiny" (versus the Infraorder Ascidia's "clawed") lobsters including (Yay!) the

Superfamily Palinuroidea , with the family of Spiny Lobsters, Palinuridae 

At right, a notorious non-scavenger amongst aquarium lobsters, the Blue, Panulirus penicillatus, typically collected out of Hawai'i for the trade.

Family Scyllaridae , the Slipper, Spanish, Locust, Shovel-Nosed Lobsters

A pair of Scyllarides squamosus  

Infraorder Anomura, families of  Burrowing Shrimp, Hermit Crabs, Sand or Mole Crabs, true Crabs. Depressed carapaces, third pair of legs never chelate, fifth pair reduced.... 

Superfamily Thalassionoidea, Marine burrowing Shrimps/Ghost Shrimp. Have compressed carapaces, first pair of legs as chelipeds, third pair not chelate. Well-developed abdomens that are long, flattened. 

Superfamily Paguroidea, Hermit Crabs and more. Have oval carapaces, usually asymmetrical. Live either in shells or with abdomen tucked underneath. First pair of legs as chelipeds. Includes the Hermit Crab genera: Pomatocheles, Petrochirus, Clibanarius, Coenobita (land Hermit Crab), Pagurus, Pylopagurus, Birgus (the Coconut Crab), Stone Crabs like Lithodes, Paralithodes (commercial King Crab of the North Pacific).


Superfamily Galatheoidea, Family Galatheidae. Genera Cervimunida, Pleuroncodes. Crab/Lobster-like crustaceans with well-developed tail fans. First legs as chelipeds. Superfamily  includes the Porcelain Crabs of the genera Petrolisthes, Pachycheles, Porcelana, Polyonyx, and the freshwater Aegla.


Superfamily Hippoidea, Sand or Mole Crabs. Have symmetrical abdomens that are flexed beneath the thorax. Cephalothorax flattened to cylindrical. First legs chelate or subchelate (never chelipeds). Fifth pair greatly reduced. Common in sandy surf zones.


Infraorder Brachyura, the true Crabs, marine, freshwater and terrestrial. Have broad carapaces which are fused with their epistomes. First legs as heavy chelipeds, third pair never chelate. Have symmetrical abdomens which are tightly held against the cephalothorax. Further divided into five Sections...


I'll never complain about the taxonomy of the  cichlids, characins, basses or wrasses again!

Bibliography/Further Reading:

Anon. A look at lobsters. Modified trap catches both spiny and "slipper" lobsters. Natl. Fisherman.; vol. 64, no. 7, pp. 64-67; 1983.

Baensch, Hans & Helmut Debelius. 1994. Marine Atlas, v.1. MERGUS, Germany. 1215pp.

Barnes, Robert. 1987. Invertebrate Zoology. Saunders. 893pp.

Choat, J.H.; Barnes, D; Borowitzka M.A.; Coll J.C.; Davies P.J.; Flood P.; Hatcher B.G.; Hopley D. et al (eds.) Proceedings of the sixth international coral reef symposium, Townsville, Australia, 8th-12th August 1988. Vol. 2. Papers Rudloe A. Preliminary studies of the mariculture potential of the slipper-lobster Scyllarides nodifer.

Debelius, Helmut. 1999. Crustacea of the World. Atlantic, Indian, Pacific Oceans. IKAN, Germany 321pp.

Friese, U. Erich. 1984. Crustaceans for the home aquarium, part 2- lobsters and crayfish. FAMA 11/84.

Gosliner, Terrence M, David W. Behrens and Gary C. Williams. 1996. Coral Reef Animals of the Indo-Pacific. Animal live from Africa to Hawai'i exclusive of the vertebrates. Sea Challengers, Monterey California. 314pp.

Headstrom, Richard. 1979. All about lobsters, crabs, shrimps and their relatives. Dover Publ.

Williams, Austin B. 1988. Lobsters of the World- An Illustrated Guide. Osprey Books.

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