Copyright The McGraw Hill Companies, Inc. Permission required for reproduction or display. Chapter 20 Aquatic Mandibulates Diversity and Characteristics Source: www.coastalstudies.org/ research/calving.htm Over 30,000 living species Together, insects and crestaceans compose over 80% of all named species Members of the copepod genus Calanus are most likely the most abundant animals in the world Characteristics Crustaceans and uniramians differ enough to separate them at the subphylum level Arthropodization may have occurred more than once, but there are basic similarities Similarities Both have pairs of antennae, mandibles and maxillae on the head. The body may be divided into a head and trunk, or into a head, thorax, and abdomen. In most crustaceans, one or more thoracic segments are fused with the cephalothorax. Uniramians are nearly all terrestrial; Crustacea are mostly marine with a few freshwater. Subphylum Crustacea Distinquishing Features 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 somites; Some may lack them. Appendages (except 1 st antennae in some) are biramous (two main branches). More Distinquishing Features Few (derived) to as many as 60 (primitive) somites. Tagmata are usually head, thorax, and abdomen not homologous across all taxa. Caridoid facies arrangement of tagmata is the ancestral plan. Dorsal covering is the carapace; May cover most of body or just the cephalothorax.
Figure 19 1 Form and Function External Features. Cuticle made of chitin, protein, and calcareous material. Joints soft and thin flexible. Telson is not a somite, bears anus. Gonopores may be at the base of appendages, at the tail, or on somites without legs. Appendages Malacostraca and Remmipedia have appendages on each somite. Other classes may not bear appendages on abdominal somites. Specialization based on basic biramous plan. Crayfish appendages represent serial homology.
Internal Features Metamerism of annelid like ancestors shown in muscular and nervous systems. Hemocoel Major blood filled body space not a coelom. Vestigial coelomic sacs are lost between mesoderm, ectoderm, and yolk. Is not lined with mesodermal peritoneum. In crustaceans, coelom remains as end sacs of excretory organs and gonads. Muscular System Striated muscles make up most of the body Most muscles arranged as antagonistic groups of flexors and extensors Abdominal flexors in crayfish allow it to swim backwards Mandibles are controlled by strong muscles located on either side of stomach Respiratory System Smaller crustaceans gas exchange occurs across leg cuticle Larger crustaceans use gills Bailer of 2 nd maxilla draws water over the gill filaments Circulatory System open system; no veins to separate blood from interstitial fluid. Hemolymph leaves the heart by arteries but washes through a hemocoel to return to the heart via sinuses. Contrasts with annelids which have a closed system. Hemolymph colorless; contains ameboid cells that may help to prevent clotting. Hemocyanin and/or hemoglobin are respiratory pigments.
Excretory System Antennal (green glands in decapods) or maxillary glands. Hydrostatic pressure provides force for filtration. Nitrogenous wastes (NH 3 ) exreted across gill cuticle. Freshwater crustaceans have problem with over dilution; gills actively absorb Na + and Cl. Marine crustacean urine is isosmotic with blood. Nervous System More fusion of ganglia than is found in other arthropods. Double ventral nerve cord has a pair of ganglia for each somite to control the appendages. Crustaceans have more developed sense organs than annelids. Eyes (compound with ommatidia) and statocysts are largest sense organs. Chemical (taste, smell) sensing occurs in hairs on antennae and mouth. Reprodution, Life Cyles, and Endocrine Function Great diversity in reproductive strategies Monoecious (barnacles) Parthenogenesis (ostracods), Dioecious (most) Development may be direct (crayfish) or indirect (most) Nauplius is the common larval form with uniramous 1 st antennae, and biramous 2 nd antennae and mandibles that aid in swimming. Molting (Ecdysis) Steps Epidermis secretes inactive form of enzymes at the base of the skeleton. Epidermis then detaches from skeleton and secretes new epicuticle. The inactive enzymes now become activated and digest the untanned endocuticle. Ca and proteins from old cuticle are reabsorbed. New procuticle secreted beneath new epicuticle. Longitudinal rupturing of old cuticle along dorsal or lateral sides of body. Animal pulls self out.
Molting Growth possible because new cuticle is soft and pliable. Animal takes in water or air to expand it. Gradually grows new tissues to fill new armor. Growth is therefore gradual. Instars or stadia are growth periods between molts. Segments are added and pairs of appendages. Metamorphosis occurs at various molts. Molting Disadvantage. Animal vulnerable. Dead white and conspicuous until the cuticle hardens (soft shell crabs). Movement restricted because of soft skeleton. Behavioral Changes Associated With Molting. Most hide. Occasionally reverse normal reflex patterns (e.g. light gradients). Land crabs need privacy or won't molt. Hormones get fouled. Sensory input from disturbing factors increases secretion of inhibitory hormone from brain and prevents secretion of molting hormone. Other crabs eat molting crabs. Can't even trust your relatives. Neuro endocrine Controls in Crustaceans. Hormonal Control Summary X organ in eye stalk produces neurosecretory hormone which inhibits molting and is stored in the sinus gland. Y gland located in antennary or 2nd maxillary segments produces molting hormone. Degenerates in animals which reach terminal stage and no further growth occurs. When the CNS receives a stimulus that it is time to molt, the sinus gland stops releasing the molt inhibitor and the Y organ is free to release its ecdysone and the molting process begins. Temperature, day length or other stimuli may act as stimulus to begin ecdysis.
Other Endocrine Functions Removing eyestalks accelerates molting and prevents color changes to match the background. Hormones from neurosecretory cells in eyestalk control dispersal of cell pigment. Functions unrelated to ecdysis Regulation of heartbeat Expression of male characteristics in amphipods Feeding Habits Wide array of feeding habits Suspension feeders generate water currents to eat plankton, detritus, and bacteria Predators eat larvae, worms, crustaceans, snails, and fish Scavengers eat dead animals and plants Class Branchiopoda Four orders includes fairy shrimp, brine shrimp, and water fleas. All have flattened and leaf like legs that are chief respiratory organs. Legs also assist in suspension feeding and locomotion (except for Cladocera). Most are freshwater; make up large portion of freshwater zooplankton.
Class Maxillopoda General body plan has 5 cephalic, 6 thoracic, and 4 abdominal somites, plus a telson. No appendages on abdomen. Eye of nauplius unique. Subclass Ostracoda Resemble tiny clams have bivalve carapace. Live on bottom or climb onto plants, some are planktonic, parasitic and burrowing. Widespread distribution, both marine and freshwater. Subclass Copepoda Source: scilib.ucsd.edu/sio/nsf/fguide/ arthropoda 1.html 2 nd largest group. Lack a carapace; single pair of uniramous maxillipeds and 4 pairs of flattened, biramous thoracic swimming appendages. Calanus most abundant organism in zooplankton by biomass. Some free living copepods are intermediate hosts of human parasitic tapeworms and nematodes. Subclass Cirripedia Source: bioweb.uwlax.edu/.../ arthropod copepods.htm Includes Barnacles Adults are sessile and attach directly or by a stalk. Head is reduced, abdomen is absent; thoracic legs long with hair like setae Many jointed cirri that bear the setae are extended from the plates to feed on small particles. Hemaphroditic, undergo metamorphosis during development.
Class Malacostraca Largest and most diverse class of Crustacea Includes Isopods (pillbugs), Amphipods, Euphausiaeans (krill) and Decapods (crabs, lobsters, crayfish, and true shrimp) Source: scilib.ucsd.edu/sio/nsf/fguide/ arthropoda 1.html
The End.