Zoology 250 (2002) ARTHROPODA (IV): Crustacea (Crustacea study images; click on tree to see full cladogram)

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• 1) SubPh. CRUSTACEA (crabs, shrimps & relatives) >50,000 marine, freshwater & terrestrial spp.; share 3 traits: i) 2-pairs of antennae, ii) terminal moveable spine(s) on telson, & iii) a nauplius larval stage

• 2) The 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 head sheild; a carapace may extend over a few or all thoracic segments, yielding a 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)
  e) primitively had an elongate body & many similar biramous legs

• 3) Crustacean diversity
  
  a) 5 classes: 3 major (Branchiopoda, Maxillopoda, Malacostraca) & 2 small, phylogenetically important (Remipedia, Cephalocarida)
  b) habitats: most classes are marine, but one class (Branchiopoda) is primarily freshwater and three others have freshwater members
  c) lifestyles: most are free living; 2 classes have a number of parasitic species, and one subclass (Cirripidea) is entirely sessile as adults
  d) body size: generally small (<20 mm) in most classes/subclasses, but large size (>100 mm) occurs in three subclasses
  e) classes are distinguished by: number of tagmata, number of segments per tagmatum, number of thoracic segments covered by a carapace, number of segments fused with the carapace, and presence/absence of movable spines (rami) on the telson
  f) most diverse class (Malacostraca) share 3 characters: i) all have 21 somites, ii) stalked eyes in adults, iii) limbs on abdominal segments

• 4) Particle feeding in Crustacea is accomplished in many ways
  
  a) primitively, crustaceans were probably particle feeders
  b) filter feeding at small size is difficult because of high viscosity: water has no momentum, 'wall effects' are large, flow is reversible
  c) in fairy shrimp a filter box generates high pressure that forces water through a sieve of setae; other small species use similar methods

• 5) Some feeding adaptations are impressive (e.g., mantis shrimps, skeleton shrimp, stone crabs)

  For some spectacular computer animations of the form, and of the swimming, escape, and prey capture behaviors of the predatory copepod Euchaeta, see the Virtual Copepod Page.

• 6) Larval stages support the view that 'ontogeny recapitulates phylogeny'

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