Z202- Unit 7 Chapter 19 Aquatic Mandibulates Biramians I. General Characteristics A. Diversity - Over 67,000 living species & probably several times that number belong to Phylum Arthropoda Subphylum Crustacea shell - Together, insects & crustaceans compose over 80% of all named animal species B. Characteristics - Crustaceans & Uniramia may differ enough to separate them at the subphylum? - Basic similarities in Arthropodization a. antennae, mandibles, & maxillae on head b. body may be divided into cephalothorax & abdomen or head-thoraxabdomen c. in most, one thoracic segments fused cephalothorax d. crustacea are mostly marine w/ few freshwater while uniramians are nearly all terrestrial 1
II. Subphylum Crustacea A. General Nature of Crustacea - Main distinguishing characteristics = 2 pair of antennaes 1 pair Antenna, 1 pair Antennule - Head also has a pair of mandibles & 2 pair of maxillae - One pair of appendages on each somite; some somites lack appendages - All appendages are biramous (2 main branches) (except first antennae) - Primitive crustacea may have up to 60 somites, derived crustacea have fewer segments - Tagmata in head, thorax, & abdomen but not homologous across taxa - Caridoid facies arrangement of tagmata is ancestral a. 5 fused somites head; 8 thorax, 6 abdomen b. anterior end is non-segmented carapace & rostrum c. posterior telson w/ last abdominal somite & uropods form a tail - Dorsal covering is the carapace; may cover most of body or just cephalothorax 2
B. Form and Function 1. External Features - secreted cuticle made of chitin, protein, & calcareous material - heavy plates have more calcium deposits, joints soften & thin, allowing flexibility & respiration (in some) - dorsal tergum & ventral sternum are plates on each somite lacking a carapace (fig 19-2) - Telson is not a somite; bears the anus & flanked by the uropod - In some species, telson may bear a pair of processes = caudal furca - Gonopores may be at appendage base; at tail or on somites without legs (3 rd & 5 th ) 2. Appendages - Appendages on each somite, though some lack clear abdominal somites - Specialization of appendages is based on the basic biramous plan - Maxilliped has basal protopod; lateral exopod, & medial endopod - On crustacean limbs, endites are medial processes, exites are lateral processes & epipod in an exite on the protopod - Crayfish appendages represent serial homology, they have evolved a wide variety of walking legs, mouthparts, swimmerets, etc. from modification of the basic biramous appendage 3
3. Internal Features - muscular & nervous systems & segmentation show metamerism of annelid-like ancestor - Hemocoel a. major body space in arthropods is not a coelom but a blood-filled hemocoel b. vestigial coelomic sacs are lost w/in space between mesoderm, ectoderm, & yolk c. spaces develop as hemocoel are not lined by mesodermal peritoneum d. coelomic compartments remain as end sacs of excretory organs, heart, & gonads - Muscular System a. striated muscles make up major portion b. most muscles arranged as antagonistic groups = flexors draw limb toward body extensors straighten limb out c. abdominal flexors of crawfish allow it to swim backward d. strong muscles located on each side of stomach control the mandibles - Respiratory System a. smaller species exchange gases across thinner areas of cuticle on legs = cuticular respiration b. larger species use featherlike gills at base of walking legs (appendicular), maxillipeds or gill cavity c. the decapod carapace overlaps the gill cavity, leaving anterior & posterior openings 4
d. the "bailer" of 2 nd maxilla draws water over gill e. gills may project from the pleural wall, articulation of thoracic legs, or thoracic coxae - Circulatory System a. Open circulatory system; there is no system of veins or capillaries b. Hemolymph leaves heart by arteries but washes through hemocoel to return to heart via sinuses c. Contrasts w/ annelid's closed d. Dorsal heart is propulsive organ; single-chambered sac of striated muscle e. Hemolymph enters the heart from the surrounding pericardial sinus f. Valves in arteries prevent backflow g. Small arteries empty into tissue sinuses that discharge into large sternal sinus h. Afferent sinus chambers carry hemolymph to gills (if present) i. Hemolymph typically colorless; contains amoeboid cells that may help prevent clotting (some have copper-based blood like horseshoe crabs) j. 5
- Excretory System a. antennal or maxillary glands open at base of those structures b. Decapods antennal glands = green glands c. End sac of antennal gland has small vesicle & spongy labyrinth connected to excretory tubule to dorsal bladder that open to nephridiopore d. Hydrostatic pressure within hemocoel provides a force for filtration of fluid into the end sac e. Resorption of salts & amino acids occurs as filtrate passes the excretory tubule & bladder; regulating ionic & osmotic composition of body fluids f. Nitrogenous wastes (mostly ammonia) are excreted across thin areas of cuticle or gills g. Freshwater species constantly threatened with over-dilution w/ water, gills must actively absorb salts (Na + and Cl - ) h. Marine crustaceans have urine that is isosmotic with the blood = osmoconformer i. No Malpighian tubules in crustaceans - Nervous & Sensory System a. nervous system has more fusion of ganglia than seen in other arthropods b. pair of supra-esophageal ganglia connects to eyes & 2 pairs of antennae c. neuron connectives join this brain to subesophageal ganglion, a fusion that supplies nerves to mouth, appendages, esophagus, & antennae glands d. double ventral nerve cord has pair of ganglia for each somite to control the appendages 6
e. crustaceans have more-developed sense organs than do annelids f. eyes & statocysts are largest sensory organs g. tactile hairs occur on body, esp on chelae, mouthparts, & telson h. chemical sensing of taste & smell occurs in hairs on antennae & mouth i. statocyst opens at base of each 1 st antennae in crayfish j. statocyst is lined w/ sensory hairs that detect position of grains of sand k. eyes are compound, made of many units called ommatidia l. transparent cornea focuses light down the columnar ommatidium m. distal retinal, proximal retinal & reflecting pigment cells form a sleeve @ ea ommatidia n. each ommatidium only sees a restricted area of vision, a mosaic in bright light o. in dim light, distal & proximal pigments separate & produce a continuous image 4. Reproduction, Life Cycles, & Endocrine a. Diversity of Reproduction 1. Barnacles are monoecious but generally cross fertilize with extremely long genitals 2. Some Ostracods, males scarce & reproduction is by parthenogenesis 7
3. Most all brood eggs in brood chambers, in brood sacs attached to abdomen or attached to abdominal appendages 4. Crayfish develop directly w/out larval form 5. Most crustaceans have a larva unlike adult in form, & undergo metamorphosis 6. Nauplius is common larval form w/ uniramous 1 st antennae & biramous 2 nd antennae & mandibles that all aid in swimming 7. Appendages & somites are added in a series of molts 8. Metamorphosis of barnacle proceeds from free-swimming nauplius to larva w/ bivalve carapace & finally to a sessile adult w/ plates b. Ecdysis 1. Exoskeleton does not grow = Molting necessary 2. Physiology of molting affects reproduction, behavior & many metabolic processes 3. Underlying epidermis secretes cuticle 4. Outermost epicuticle is very thin lipidimpregnated protein 5. Most of cuticle - several layers of procuticle 8
6. Exocuticle is beneath epicuticle & contains protein, calcium salts & chitin 7. Endocuticle has heavily calcified principal layer & uncalcified membranous layer 8. Epidermis cells enlarge before ecdysis, secrete new epicuticle & begin secreting a new exocuticle 9. Enzymes released into the area above new epicuticle dissolve old endocuticle 10. Some calcium salts are stored in gastroliths in walls of stomach 11. When only old exocuticle & epicuticle remain, animal swallows water to expand & burst old cuticle 12. New soft cuticle stretches & then hardens w/ deposition of inorganic salts 13. Molting occurs often in young, may cease in adults X-Organ = molt inhibitor Y-Organ = molt stimulator c. Hormonal Control of Ecdysis 1. temp, day length or other stimuli trigger central nervous system to begin molt 2. central nervous system decreases production of molt-inhibiting hormone by the X-organ (medulla) 3. this promotes release of molting hormone from Y-organ (near mandible); leads to ecdysis 9
d. Other Endocrine Functions 1. removing eyestalks accelerates molting & prevents color changes to match background 2. hormones from neurosecretory cells in eyestalk control dispersal of cell pigment 3. neurosecretions from pericardial organs cause an increase in heartbeat 4. androgenic gland in male amphipods stimulate expression of % characters 5. Feeding Habits - same fundamental mouthparts are adapted to a wide array of feeding habits - suspension feeders generate water currents in order to eat plankton, detritus & bacteria - predators consume larvae, worms, crustaceans, snails & fishes - scanvengers eat dead animal & plant matter - Crayfish have 2-part stomach; a gastric mill grinds food 1 st III. Brief Survey of Crustaceans A. Class Branchiopoda (fairy shrimp, tadpole shrimp, clam shrimp, water fleas) - important freshwater zooplankton (second on food chain) - 4 orders; all have flattened and leaf-like legs that are chief respiratory organs - legs assist in suspension feeding & locomotion (except for cladocerans) - use parthenogenesis to boost summer populations (like rotifers), then use sexual reproduction w/ unfavorable conditions 10
- fertilized eggs highly resistant to cold & are critical for winter survival - direct development, some w/ gradual metamorphosis B. Class Maxillopoda 1. Subclass Cirripedia (Barnacles) - adults sessile & attach direct (acorn barnacle) or by stalk (goose barnacle) - carapace surrounds body & secretes a set of calcareous plates - reduced head, abdomen absent, thoracic legs long w/ hairlike setae - many-jointed cirri that bear setae extended from plates to feed on small particles - intertidal barnacles may be exposed to drying; plates close to protect them - most hatch as nauplii and become cyprid larvae w/ bivalve carapace & compound eyes - attach to substrate by 1 st antennae & adhesive glands - secrete calcareous plates, lose eyes, & change swimming appendages to filtering cirri - parasitic forms attacks hemocoel of host 2. Subclass Copepoda (Copepods) - small crustaceans that live in fresh and saltwater - biggest source of protein in oceans (*dominant zooplankton off Oregon coast) - 2 varieties: planktonic (float) and benthic (creep along bottom) 11
C. Class Malacostraca - largest class of Crustacea and is the most diverse 1. Order Isopoda - only truly terrestrial crustaceans (few marine & freshwater) - dorsoventrally flattened, lack a carapace & have sessile compound eye - 1 st pair thoracic limbs are maxillipeds, remaining thoracic limbs lack exopods - abdominal appendages bear gills, except for uropods - common land forms include sow bugs & pill bugs - cuticle lacks protection of insect cuticle; they must live in moist conditions - some isopods are highly modified as parasites of fish or crustaceans - development is typically direct (metamorphic in parasitic forms) 2. Order Amphipoda (7,000 species) - amphipods resemble isopods; lack a carapace, have sessile compound eyes, & 1 pair of maxillipeds - compressed laterally, gills are in thoracic position - abdominal & thoracic limbs grouped for jumping & swimming - many marine, others beach-dwelling, freshwater, or parasitic - direct development - one terrestrial (sand flea) 12
3. Order Euphausiacea - 90 species, includes the important ocean plankton called "krill" - carapace does not completely enclose gills - lack maxillipeds & have all limbs equipped w/ exopods - most biolumenescent w/ lightproducing organs (photophore) - form a major component of diet of baleen whales & many fish - eggs hatch as nauplii & development is direct 4. Order Decapoda - 3 pairs of maxillipeds & 5 pairs of walking legs, 1 st forming pincers - range from few mm to largest arthropod (4m Japanese crab) - 18,000 species known including crayfish, lobster, crabs, & true shrimp - crabs have broader cephalothorax & reduced abdomen compared to lobster & crayfish 13
IV. Phylogeny & Classification A. Phylogeny 1. The monophyly of Arthropoda controversial a. crustacea have been allied w/ uniramians in Mandibulata because both have mandibles b. critics consider the mandibles to be different & not homologous 2. Remipedia appear to be most primitive of crustacea 3. One theory is that each modern somite represents two ancestral somites that fused together, forming the biramous appendage B. Adaptive Radiation 1. Crustaceans are unquestionably the dominant arthropod in marine environments 2. Share dominance in freshwater w/ the insects 3. Class Molacostraca is most diverse & members of Copepoda are most abundant 4. Copepods are particularly successful as parasites of both vertebrates & invertebrates C. Classification of Classes: Class Remipedia Class Cephalocarida Class Branchiopoda Order Anostraca Order Notostraca Order Diplostraca Class Ostracoda Class Maxillopoda Subclass Mystacocarida Subclass Copepoda Subclass Tantulocarida Subclass Branchiura Subclass Pentastomida Subclass Cirripedia Class Malacostraca Order Isopoda Order Amphipoda Order Euphausiacea Order Decapoda 14