Subphylum Crustacea
Body Structure
Series of somites (segments)
Derived forms have more fusion of segments through the process of tagmatization
Each bearing a pair of biramous jointed appendages
Tagmata typically include cephalothorax and abdomen
Cephalothorax is covered by a carapace (shield) -- unsegmented exoskeleton of CaCO3
The two somites anterior to the head have a pair of antennae used for chemosensory and tactile senses, but also for locomotion in some --- these are the ONLY pre-oral appendages
The appendages surrounding the mouth are the mandibles for chewing food
The first set of post-oral appendgaes are the maxillae used to aid in feeding and/or respiration either directly or by transferring food to the mouth or indirectly by creating water currents
Pereopods used for walking(each associated with a filamentous gill -- located on the cephalothorax and pleopods (swimmerets) on the abdomen
Which are followed by maxilli-peds (may be 0-3 pairs)
In crabs, crayfish, and lobsters --- the first pair of walking legs are equipped with powerful claws chelae and the entire appendage is considered a cheliped
The last segment of the abdomen is the telson which is often flat and may have fan-like appendages attached (uropods)
Body System
Open circulatory system
If present, the heart resides in a blood filled chamber (pericardial sinus) and conveys hemolymph through a pair of ostia to the body
Many body organs are serviced by sternal sinus which runs along the ventral surface of the body
Respiration is accomplished by diffusion in smaller crustaceans or by exchanging across the many Gills
Excretion occurs by diffusion across the Gills
In some, the legs themselves are the respiratory surface and are called phyllopodia
Some antennal glands (or maxillary glands or green glands --all the same thing) are used for excretion to rid the body of some nitrogenous waste and water
Digestion varies. -- all have mandibles to aid in ingestion.
Variety of feeding modes -- filter feeding, scavenging, predation, parasitism, and herbivory (mostly grazing) -- a few are omnivores
Food is broken down mechanically within a large cardiac stomach (grinds food) that is lined with cuticle, and subsequently chemically digested into a smaller pyloric stomach
Both stomachs are a part of the foregut --- the mostly digestd food then enter the intestine, which may have digestive glands to complete chemical breakdown of food and absorb the nutrients.
Indigestible materials are eliminated through the anus which is the last segment on the telson
Nervous system and Sensory
dorsal brain and double ventral longitudinal nerve cord that has regional ganglia**
A variety of sensory organs gather information about the body or the environment, including Two pairs of antennae for chemo- and mechanosensation
compound eyes and/or ocelli for photoreception
Statocysts for determining position relative to gravity
Reproduction and Development
Most crustaceans reproduce sexually but some can reproduce asexually through parthenogenesis
mostly Dioecious except for many barnacle are monoecious
Fertilization is mostly internal through depostion of sperm into a female -- very rare for external
Indirect development (distinct larval stages) while others have direct development (no larvae)
Indirect development happens through metamorphosis
Larval stages are variable, but most include a nauplius the first larval stage---free swimming, central eye, and fringed appendages for swimming
There are additional larval stages in the larger crustaceans, including the zoea and megalops --- each are found in different environments and may even eat different foods than adults
Taxonomic Divisions
Class Malacostraca
Shrimp-like body plan, compound eyes; two tagmata (cephalothorax and abdomen)
Class Branchiopoda
Order Decapoda
3 pairs of maxillipeds and 5 pairs of walking legs, stalked compound eyes, heavy exoskeleton due to calcium carbonate deposits
Order Isopoda
Dorsoventrally flattened with no carapace, single pair of maxillae, non stalked compound eyes
Order Amphipoda
Laterally flattened with no carapace, single pair of maxillae, non stalked compound eyes
Order Euphausiacea
carapace covers cephalothorax and has a distinct rostrum with keel, but does not cover gills; stalked compound eyes, mouthparts and thoracic limbs are ventral
Order Stomatopoda
extremely complex and stalked compound eyes with 16 different photoreceptors; carapace leaves sides of thorax and posterior 4 thoracic segments uncovered; raptorial second pair of thoracic appendages
Class Ostracoda
Class Copepod
Class Cirripedia
Small, mostly freshwater, diverse body shapes, minimal tagmatization; leaf-like and flattened legs aid in gas exchange, non hinged carapace or lacking carapace
Small freshwater and marine organisms with two-part carapace hinged dorsally (clam like) attached to body with adductor muscle
Freshwater and marine zooplankton, some parasitic, cephalothorax with 6 segments and abdomen with 5 segments; single naupliar eye, no abdominal segments
Sessile, hardened plates protecting body, numerous thoracic appendages modified for filter or suspension feeding, no abdomen