New Print and eBook on Amazon Marine Aquarium Algae Control by Robert (Bob) Fenner

To place the "real" crabs and their relatives in taxonomic perspective let's do a/the usual rundown on their systematics starting from 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. The nine Classes that don't include our family of interest enclose the primitive cephalocaridans (C. Cephalocarida), the Class Branchiopoda (fairy shrimps, tadpole shrimps, water fleas (Daphnia); the Class Ostracoda, Class Copepoda (Anchor worm, Lernaea), Classes Mystacocarida, Branchiura, Tantulocarida, Remipedia, Cirripedia (barnacles), whew! & finally, our:

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 further divided into two Suborders, the Dendrobranciata, with "tree-like" branched gills, body laterally compressed..., eggs planktonic, nauplius as the first larval stage (as in Artemia, our brine shrimp), Infraorders, Sections, Superfamilies... See Barnes re their higher taxonomy.

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

Superfamily Galatheoidea, Family Galatheidae: Squat Lobsters. 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 (Not able to "flap" quickly). Further divided into five Sections.

Family Calappidae, the Shame-Faced or Box Crabs. Mostly hide in the sand by day. Great eaters of snails.  To eight inches in diameter. Hepatus and Calappa regularly imported. 

Family Cancridae; Cancer or Dungeness Crabs. Oval bodies. 

Family Capillidae;

Family Dromiidae: Sponge Crabs. Have hairy, spherical bodies. Pinchers generally with light colored tips. Carry a sponge colony or mussel shell on their backs.

Family Epialtidae: Imitator Crabs

Bigger PIX: The images in this table are linked to large (desktop size) copies. Click on "framed" images to go to the larger size.

Family Ethusidae: Sumo Crabs

Family Eumedonidae: Urchin Crabs

Family Grapsidae; Marsh, Rock, Spray Crabs. Have flat, quadratic shaped bodies. Many amphibious, and need to be able to get out of the water (keep tank covered).