CHAPTER 20

1. CHAPTER 20 Crustaceans

3. Overview • Over 67,000 living species • Insects and crustacea compose over 80% of all named animal species • Probably most abundant animals in the world are members of the copepod genus Calanus • Divided into 5 classes • Current molecular phylogenies do not support the monophyly of all classes • Former members of phylum Pentastomida placed in class Maxillopoda, subclass Pentastomida

5. Overview • Pentastomids • Also called tongue worms • Parasites of vertebrates, living in lungs or nasal cavities • Closely related to fish lice

6. Subphylum Crustacea General Nature of a Crustacean • Main distinguishing characteristic of crustaceans • Two pairs of antennae • Head also has a pair of mandibles and 2 pairs of maxillae • One pair of appendages on each of the additional segments • Some segments may lack appendages • All appendages, except perhaps first antennae, are biramous

7. Subphylum Crustacea • Primitive crustaceans may have up to 60 segments; derived crustaceans have fewer • Tagmata are usually head, thorax, and abdomen • Not homologous across taxa • In most one or more thoracic segments are fused with the head as a cephalothorax • Arrangement of tagmata in Malacostraca is the ancestral plan • Head has 5 fused somites • Thorax has 8 • Abdomen has 6

9. Subphylum Crustacea • Anterior end is a non-segmented rostrum • Telson, with the last abdominal somite and uropods, forms a tail • Dorsal covering is the carapace • May cover most of body or just cephalothorax

10. Subphylum Crustacea Form and Function • External Features • Secreted cuticle is made of chitin, protein, and calcareous material • Heavy plates have more calcareous deposits • Joints are soft and thin, allowing flexibility • Dorsal tergum and ventral sternum are plates on each somite lacking a carapace • Telson is not a somite • Bears anus and may be homologous to the pygidium • In some species, telson may bear a pair of processes, the caudal furca • Gonopores may be at base of appendages, at the tail, or on somites without legs

12. Subphylum Crustacea • Appendages • Members of Malacostraca and Remipedia have appendages on each somite • Other classes may not bear appendages on abdominal somites • Specialization of appendages based on the basic biramous plan • Maxilliped has a basal protopod, a lateral exopod, and a medial endopod • Endites are medial processes, exites are lateral processes and epipod is an exite on the protopod • Appendages represent serial homology • Have evolved a wide variety of walking legs, mouthparts, swimmerets, etc. from modification of the basic biramous appendage

16. Subphylum Crustacea • Internal Features • Muscular and nervous systems and segmentation exhibit metamerism of annelid-like ancestors • Hemocoel • Persistent blastocoel that becomes filled with blood • Coelomic compartments remain as end sacs of excretory organs and gonads • Coelomates only in technical sense of the term

17. Subphylum Crustacea • Muscular System • Striated muscles • Make up a major portion of crustacean body • Most muscles arranged as antagonistic groups • Flexors draw a limb toward the body and extensors straighten a limb out • Abdominal flexors of a crayfish allow it to swim backward • Strong muscles located on each side of stomach control the mandibles

19. Subphylum Crustacea • Respiratory System • Smaller crustaceans may exchange gases across thinner areas of cuticle • Larger crustaceans use featherlike gills for gas exchange • Decapod carapace overlaps the gill cavity, leaving anterior and posterior openings • “Bailer” of 2nd maxilla draws water over gill filaments • Gills may project from pleural wall, the articulation of thoracic legs, or thoracic coxae

21. Subphylum Crustacea • Circulatory • Open circulatory system • No system of veins to separate blood from interstitial fluid • Hemolymph exits heart through arteries • Passes to hemocoel to return to the heart via sinuses • Dorsal heart • Single-chambered sac of striated muscle • Valves in the arteries prevent backflow of hemolymph • Hemolymph conducted to gills, if present, for oxygen and carbon dioxide exchange • Hemolymph may be colorless, reddish, or bluish • Contains ameboid cells that may help prevent clotting • Hemocyanin and/or hemoglobin are respiratory pigments

22. Subphylum Crustacea • Excretory System • Antennal or maxillary glands • Called green glands in decapods • End sac of antennal gland has a small vesicle and a spongy labyrinth • Labyrinth connects by an excretory tubule to dorsal bladder that opens to exterior pore • Hydrostatic pressure within a hemocoel provides a force for filtration of fluid into the end sac • Resorption of salts and amino acids occurs as the filtrate passes the excretory tubule and bladder • Mainly regulates the ionic and osmotic composition of body fluids

24. Subphylum Crustacea • Nitrogenous wastes excreted across thin areas of cuticle in the gills • Freshwater crustaceans constantly threatened by over-dilution with water • Gills must actively absorb Na+ and Cl- • Marine crustaceans have urine that is isosmotic with blood

25. Subphylum Crustacea • Nervous and Sensory Systems • More fused ganglia than in other arthropods • Pair of supra-esophageal ganglia connects to eyes and two pairs of antennae • Neuron connectives join this brain to the subesophageal ganglion • Supplies nerves to mouth, appendages, esophagus, and antennal glands • Double ventral nerve cord has a pair of ganglia for each somite to control appendages • Eyes and statocysts are largest sensory organs • Tactile hairs occur on the body, especially on chelae, mouthparts and telson • Chemical sensing of taste and smell occurs in hairs on antennae and mouth

26. Subphylum Crustacea • Statocyst opens at base of first antenna in crayfish • Statocyst lined with sensory hairs that detect position of grains of sand • Compound eyes, made of many units called ommatidia • Cornea focuses light down the columnar ommatidium • Distal retinal, proximal retinal, and reflecting pigment cells form a sleeve around each ommatidium • Each ommatidium detects a restricted area of objects, a mosaic, in bright light • In dim light, the distal and proximal pigments separate and produce a continuous image

28. Subphylum Crustacea • Reproduction, Life Cycles, and Endocrine Function • Diversity of Reproduction • Barnacles are monoecious but generally cross-fertilize • In some ostracods, males are scarce and reproduction is by parthenogenesis • Most crustaceans brood eggs in brood chambers, in brood sacs attached to the abdomen, or attached to abdominal appendages • Crayfishes develop directly without a larval form

29. Subphylum Crustacea • Most crustaceans have a larva unlike the adult in form, and undergo metamorphosis • The nauplius is a common larval form with uniramous first antennae, and biramous second antennae and mandibles that all aid in swimming • Appendages and somites are added in a series of molts • Metamorphosis of a barnacle proceeds from a free-swimming nauplius to a larva with a bivalve carapace and finally to a sessile adult with plates

31. Subphylum Crustacea • Ecdysis • Necessary for a crustacean to increase in size the • Exoskeleton does not grow • Physiology of molting affects reproduction, behavior, and many metabolic processes • Underlying epidermis secretes cuticle • Outermost epicuticle is made of a very thin lipid-impregnated protein • Most of the cuticle is composed of several layers of the procuticle • Exocuticle, beneath the epicuticle, contains protein, calcium salts, and chitin

33. Subphylum Crustacea • Endocuticle has a heavily calcified principal layer and an uncalcified membranous layer • Molting animals grow in the intermolt phases, or instars • Soft tissue increases in size until there is no space within the cuticle • When body fills the cuticle, animal is in the premolt phase • Epidermal cells enlarge before ecdysis

34. Subphylum Crustacea • Secrete a new epicuticle and then begin secreting a new exocuticle • Enzymes released into the area above new epicuticle dissolve the old endocuticle • When only the old exocuticle and epicuticle remain, the animal swallows water to expand and burst the old cuticle • Soft new cuticle stretches and then hardens with the deposition of inorganic salts • Molting occurs often in young animals and may cease in adults

36. Subphylum Crustacea • Hormonal Control of Ecdysis • Temperature, day length, or other stimuli trigger central nervous system to begin ecdysis • Central nervous system decreases production of molt-inhibiting hormone by the X-organ • Promotes release of molting hormone from the Y-organs which promotes ecdysis

37. Subphylum Crustacea • Other Endocrine Functions • Removing eyestalks accelerates molting and prevents color changes to match background • Hormones from neurosecretory cells in eyestalk control dispersal of cell pigment • Neurosecretions from pericardial organs stimulate increase in heartbeat • Androgenic glands in male amphipods stimulate expression of male characteristics

38. Subphylum Crustacea • Feeding Habits • Same fundamental mouthparts are adapted to a wide array of feeding habits • Suspension feeders generate water currents in order to feed on plankton, detritus ,and bacteria • Predators consume larvae, worms, crustaceans, snails, and fishes • Scavengers eat dead animal and plant matter • Crayfishes have a two-part stomach • Gastric mill grinds up food in 1st compartment

40. Brief Survey of Crustaceans Class Remipedia • Only 10 described species • All found in caves connected to the sea • Primitive features include 25–38 segments with similar, paired, biramous, swimming appendages • Antennules also biramous • Maxillae and maxillipeds are prehensile and specialized for feeding • Swimming legs are directed laterally rather than ventrally as is found in copepods and cephalocarids

42. Brief Survey of Crustaceans Class Cephalocarida • Only 9 species described • Live in coastal bottom sediments from intertidal zones to 300 meters depth • Thoracic limbs and 2nd maxillae are very similar • Lack eyes, a carapace, and abdominal appendages • True hermaphrodites and unique in discharging eggs and sperm through same duct

43. Brief Survey of Crustaceans Class Branchiopoda • Over 10,000 species, 4 orders • Order Anostraca, includes fairy shrimp and brine shrimp • Lack a carapace • Order Notostraca, includes tadpole shrimp • Carapace forms a large dorsal shield • Order Conchostraca, includes clam shrimp • Enclosed by a bivalved carapace • Order Cladocera, includes water fleas • Carapace encloses the body but not the head

45. Brief Survey of Crustaceans • In all flattened, leaf-like legs serve as respiratory organs, assist in suspension feeding, and, (except for cladocerans), locomotion • Most are freshwater • Important component of freshwater zooplankton • May reproduce by parthenogenesis to rapidly boost summer populations and then by sexual reproduction with the onset of unfavorable conditions

46. Brief Survey of Crustaceans • Fertilized eggs highly resistant to cold • Critical for winter survival of population • Cladocerans: Development mostly direct • Other branchiopods: Gradual metamorphosis

47. Brief Survey of Crustaceans Class Ostracoda • 6,000 known species • Most are dioecious • Enclosed in a bivalve carapace • 0.25–8.0 mm long • Fusion of trunk somites • Thoracic appendages are reduced to two or one • Most are benthic or climb onto plants, but some are planktonic, parasitic, or burrowing • Widespread in both marine and freshwater habitats • Development by gradual metamorphosis

49. Brief Survey of Crustaceans Class Maxillopoda • General body plan • Five cephalic, six thoracic and four abdominal somites plus a telson • No appendages on abdomen • When present, the eye is unique in structure and called a maxillopodan eye

50. Brief Survey of Crustaceans Subclass Mystacocarida • Less than 0.5 mm long • Live in interstitial water between sand grains • 10 species have been described from around the world • Primitive in several characteristics