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 (1 several 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 1 st 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 molt inhibiting 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