Zoology 250 Lecture 25 ARTHROPODA (III):
Subphylum Crustacea
- 1) Although tremendously diverse (>50,000 spp.), crustaceans share three distinctive traits: i) 2-pair antennae, ii) terminal moveable spine(s) off the telson, and iii) a nauplius larval stage
- 2) Body plan is surprisingly stereotyped for such a diverse group:
- a) head has 5 segments with limbs: 2 antennae, 1 mandible, 2 maxillae
- b) head is typically covered by a carapace; the carapace may extend over a few to all of the thoracic segments (-> cephalothorax)
- c) some thoracic segments may fuse with the head and their limbs modified as maxillipeds for feeding; walking legs= periopods
- d) abdominal segments may or may not have limbs (pleopods)
- 3) Crustacean diversity
- a) 5 classes, 3 major (Branchiopoda, Maxillopoda, Malacostraca) & 2 small, phylogenetically important (Remipedia, Cephalocarida)
- b) most are marine, but one class (Branchiopoda) is primarily freshwater and three others have freshwater members
- c) most are free living, 2 classes have a number of parasitic species, and one subclass (Cirripidea) is entirely sessile as adults
- d) body size is generally small (<20mm) in most classes/subclasses, but large size (>100mm) occurs in three subclasses
- e) classes are distinguished by: number of tagmata, number of segments per tagmatum, number of anterior segments covered by a carapace, number of segments fused with the carapace, presence/absence of movable spines (rami) on the telson
- f) primitively, crustaceans were probably particle feeders; however the most successful class (Malacostraca) is mainly raptorial
- 4) Primitive crustaceans probably had an elongate body with many similar pairs of biramous limbs; most have a carapace that covers the head and part or all of the thorax
- 5) Particle feeding in Crustacea is accomplished in many ways
- a) filter feeding at small size is difficult because of high viscosity: water has no momentum, 'wall effects' are large, flow is reversible
- b) in fairy shrimp a filter box generates high pressures that force water through a sieve of setae; other small spp use similar methods
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(revised Jan. 14, 1999)