INTRODUCTION TO THE CRABS, SHRIMPS AND CRAYFISH OF BRITISH COLUMBIA
Pacific Red Hermit Crab (Elassochirus gilli), photo by Aaron Baldwin
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The order Decapoda is a group of crustaceans characterized by five pairs of walking legs on the thorax (Deca- = 10, -poda = legs). Perhaps the most familiar group of crustaceans, decapods include crabs, shrimp, lobsters, crayfish, and a host other less familiar forms. The presence of five pairs of walking legs is distinctive but not taxonomically diagnostic as some decapods may have one or more pairs secondarily greatly reduced or absent and other groups of crustaceans may have five pairs of legs as well.
Higher Level Classification
To understand what makes this order distinct I find it is easier to place it in the context of higher taxonomic levels and the lower levels contained within the order. The Phylum Arthropoda are the jointed-legged animals. The subphylum Crustacea includes those arthropods, which have five pairs of appendages on the head, two in front of the mouth called antennae and three behind the mouth called mandible and maxillae 1 & 2. One rule of arthropod nomenclature is that every somite (~segment) has only one pair of appendages. If we see two pairs of appendages we can infer that two somites or segments have fused. So the head of most arthropods looks like a single complete unit but is really multiple fused segments.
Within the crustaceans are several classes, usually characterized by the number of thoracic and abdominal appendages (remember the head appendages are fixed at five for all crustaceans). The class Malacostraca, which includes the decapods, nearly always has 19 segments bearing 19 pairs of appendages. In particular, they have five head appendages, eight thoracic appendages, and 6 abdominal appendages. There is one notable exception, the primitive Phyllostraca that have seven abdominal segments. To include these they are classified as a subclass contrasted with the typical members of the class, the Eumalacostraca. Another unifying feature of the Malacostraca is that the gonopore (where sperm and eggs are extruded) is always on the base of the sixth thoracic appendage in females and on the eighth in males. As we will see below these correspond to the third and fifth walking legs in the decapods.
Members of the order Decapoda are distinct from other Malacostracans by having their gills enclosed by the carapace (called a branchial chamber) and by having the first three pairs of thoracic appendages modified as mouthparts called maxillipeds. Like all Malacostraca they have eight pairs of thoracic appendages, but with the first three pairs used as mouthparts they are left with five pairs for locomotion (i.e. ‘Walking legs’). Some groups closely related to decapods such as Euphausiids (krill) have distinct gills outside of the carapace. Another common but not diagnostic characteristic is that most decapods have chelae (claws) on the first walking leg and sometimes on the second and third.
Lower level Classification
In the past decapod crustaceans were divided into two large groups, the Natania (= “swimmers”) and the Reptania (= “crawlers”). The former group included the shrimps and the latter the crabs, lobsters, and similar forms. This changed dramatically in 1963 (although it took decades longer to infiltrate popular classification) when Martin Burkenroad classified all decapods that brood embryos on or under the abdomen as Pleocyemata and those that don’t Dendrobranchiata. What this did was to unite crabs, lobsters, and some shrimp together while grouping other groups of shrimp as distantly related. The Dendrobranchiata, poorly represented in the north Pacific, include the commercially important tropical Gulf and tiger prawns and includes decapods that extrude eggs freely into the water column. The Pleocyemata includes nearly all decapod crustaceans likely to be encountered in British Columbia. The north Pacific dendrobranchiates are most often offshore species encountered in mid-water to deep water plankton tows.
The Pleocyemata are divided into a number of infraorders based on features of the carapace and number and position of claws on the walking legs. British Columbia has representatives of all of the major infraorders except Palinura (spiny lobsters, slipper lobsters, and blind lobsters) and the Stenopodidea (coral shrimps and glass sponge shrimps). This may change, as it is likely that a deep sea blind lobster may occur in BC as specimens of Polycheles sculptus have been found in Washington State and in the Northwestern Pacific.
The five remaining infraorders with representatives in BC are:
- Anomura - The porclain crabs, squat lobsters, heremit crabs, king crabs, and more crabs.
- Astacidea - Crayfish and true lobsters
- Brachyura - The true crabs
- Carida - The true shrimps
- Thalassinidea - The ghost and mud shrimps
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