Aquatic Mandibulates
Chapter 19
I. Characteristics
Crustaceans and insects compose over 80 % of
all named species
Head: 2 pairs antennae, pair mandibles (jaws),
2 pairs of maxillae (handle food)
Usually 1 pair appendages/segment
Tagmata either cephalothorax & abdomen or
head (5 fused somites), thorax (8 somites), and
abdomen (6 somites)
Rostrum—anterior end
Telson & uropods form tail—posterior end
II.
Body Plan
A. External Features
Dorsal covering is carapace; may cover most of
body or just cephalothorax
Somites without carapace have tergum(dorsal)
or sternum (ventral) plates which are heavy and
have more calcareous deposits
Cuticle made of protein, chitin, and calcareous
deposits
Joints soft & thin for flexibility
Telson analogous to pygidium
B. Appendages
Specialization of appendages; all show serial
homology (derived from same biramous type)
All variations of biramous (two jointed) plan with
a basal portion called the protopod (1-2 joints)
and a side portion called an exopod (1-several
joints), and a mid portion called a endopod (1several joints)
Joints will have further modifications called
endites or exites
C. Body Cavity
Metamerism but no intersepta
No coelom, this is lost during development;
blood-filled hemocoel instead with no peritineal
lining
D. Muscular System
Striated muscle forms most of body
Antagonistic pairs; flexors and extensors
Flexors in abdomen allow crayfish to swim
backward
Muscles alongside stomach control mandibles
E. Respiratory System
Small crustaceans
exchange gases
through cuticle on thin
areas of body like legs
Large crustaceans use
feather-like gills
The gills may be in
thorax cavity or on
thoracic legs
F. Circulatory System
Open circulatory system: no veins to separate
blood from interstitial fluid
Hemolymph (blood) leaves dorsal heart (1
chamber) by arteries but washes through tissue
sinuses; goes to sternal sinus and then on to
gills for gas exchange before returning through
pericardial sinus
Hemolymph colorless; hemocyanin and/or
hemoglobin are respiratory pigments
G. Excretory System
Antennal or maxillary glands at base of those
structures; in decapods these are green glands
Connects to bladder then excretory pore; filters
body fluid, reabsorbing salts and amino acids
Freshwater crustaceans constantly get rid of
excess water; gills reabsorb Na and Cl
Ammonia is excreted by gills
H. Nervous and
Sensory Systems
More fusion of ganglia than other arthropods
Supra-esophageal ganglia connect eyes and
antennae
Neurons join this to sub-esophageal ganglion that
controls mouth, appendages, esophagus, and
antennal glands
Double ventral nerve cord with ganglion/somite to
control appendages
Eyes-compound and made of many
units(ommatidia); restricted to mosaic vision in
bright light; in dim light sees continuous image
Statocysts located at base of 1st antennae
Tactile hairs on chelae, mouthparts, and telson
III. Life Cycles
A. Types of
Reproduction
Separate sexes
Brood eggs in brood chambers, brood sacs
attached to abdomen, or attached to abdominal
appendages
Larvae go through metamorphosis starting as
naupilus; planktonic
Crayfish develop without larval form
Appendages and somites are added in a series
of molts
B. Ecdysis
Molting is necessary for crustacean to grow in size;
exoskeleton does not grow
Cuticle composition like that of previous arthropods
Enzymes released by newly forming epicuticle
dissolve old endocuticle
When only the old exocuticle and epicuticle remain,
animal swallows water and splits old cuticle
Soft new cuticle stretches and hardens as inorganic
salts (stored in gastroliths of stomach) are
deposited
Molting occurs often in young animals but may
cease in adults
C. Hormone Control
Temperature, day length, or other stimuli may
trigger ecdysis
Nervous system decreases production of moltinhibiting hormone by X-gland
This promotes release of molting hormone from
Y-glands, leading to ecdysis
Removal of eyestocks accelerates molting and
prevents protective color changes
IV. Class Brachipoda
Fairy shrimp and brine shrimp (no carapace)
Tadpole shrimp (carapace forms large shield)
Clam shrimp (bivalved carapace)
Water fleas (carapace encloses body but not
head)
Flattened leaf-like legs used for respiration
V. Class Maxillipoda
5 cephalic, 6 thoracic, 4 abdominal somites
plus telson although reductions common
No appendages on abdomen
Maxillopodan eye on nauplius larvae
A. Subclass Copepoda
Group is second to
Malacostraca in
number of species
No carapace
Nauplius eye in adult
Marine copepod
Calanus most abundant
organism in
zooplankton by
biomass
Cyclops and Diaptomus
important freshwater
plankton
B. Subclass Cirripedia
Barnacles
Adults are sessile and
attach directly (acorn)
or by a stalk (goose)
Carapace surrounds
body and is secreted as
calcareous plates
Head reduced,
abdomen absent, legs
(cirri) long with setae
for filter feeding
VI. Class
Malacostraca
A. Order Isopoda
Sow bugs and pill bugs
Only truly terrestrial crustaceans; also marine
and freshwater forms
Dorsoventrally flattened, lack carapace, and
have sessile compound eyes
Abdominal appendages bear gills
Cuticle lacks protection of insect cuticle so must
live in moist conditions
Some are modified parasites of fish or
crustaceans
Isopods
B. Order Euphausiacea
Krill
Important ocean
plankton, forming
major component of
baleen whales’ diets
and of many fishes
Most have
bioluminescent
organs
C. Order Decapoda
Crayfish, lobsters,
crabs, and true shrimp
3 pairs of maxillipeds, 5
pairs of walking legs
with the first forming
pincers
Crabs have broader
cephalothorax and
reduced abdomen
compared to crayfish or
lobsters
Decapoda