Everything: Origins of Life (PDF)

Everything Origins of life Phylogeny = evolutionary tree Biodiversity = diversity of life Can refer to genetic diversity, taxonomic diversity, and ecosystem diversity Morphology = shape of a species What is a specie...

Everything Origins of life Phylogeny = evolutionary tree Biodiversity = diversity of life Can refer to genetic diversity, taxonomic diversity, and ecosystem diversity Morphology = shape of a species What is a species? And how many? It is difficult to unambiguously define a species Biological Species Concept - “a species consists of populations of organisms that can reproduce with one another and that are reproductively isolated from other such populations” Problems with BSC - Often its hard to find out whether 2 individuals can reproduce Some clearly distinct species can reproduce e.g. Liger Doesn't apply to fossil species It is particularly difficult to define species for prokaryotes (Archaea and Bacteria) because they regularly exchange DNA with other species of prokaryotes To define different bacteria species they are defined based on overall similarity of DNA If they are 97% similar, they are classed as the same species Eukaryotes = organisms with complex cells e.g. plants, animals and fungi Theres two estimates 8.7 million 5 - 3 million Everything 1  2 million living eukaryotic species described so far which is less than 50% of the total It is estimated that 99.9% of all species that have ever live are extinct Total living species = 2-20 million How can we assess diversity? Taxonomy = Identification, description, naming and classification of organisms Systematics or classification = organisation of organisms into groups according to some system Phylogenetics = a branch of biology studying evolutionary relationships between organisms Carl Linne (1707-1778) - Father of modern taxonomy Was the first person to attempt to class organisms Introduced binomial nomenclature when classing organisms Proposed ranks for classifying species Every species has a two part name Genus name first (capitalised) Species name second (not capitalised) Full name should be in italics or underlined 7 main ranks Kingdom (Most inclusive) Phylum Class Order Family Genus Species (Least inclusive) Everything 2  An additional rank has been proposed above the level of Kingdom - Domain Taxonomic ranks above the level of species are arbitrary Phylogenetics Study of the evolutionary relationships of organisms Outcomes of evolution Better adapted to the environment Speciation - development of new species Extinction Tree of life gives an underlying framework for Linnean classifcation Eukaryotes Oldest evidence of eukaryotes is 1.65 billion years old Oldest multicellular eukaryotic cell is 1.635 billion years old Are diverse Have complex cells Have complex life cycles Include multiple groups that have evolved multicellularity Most distinctive feature of eukaryotes is the presence of a nucleus Genome sizes of eukaryotes are larger than prokaryotes 0.5mbp (Million base pairs) vs 2mbp Engage in sexual reproduction Which promotes genetic variation Major processes Meiosis - haploid cells formed from diploid cells Fertilisation - two haploid cells fusing to form a diploid cell Cells are either haploid or diploid Everything 3  Compartmentalisation of the cell Nucleus Mitochondrion Nucleus Separates the processes of DNA replication and transcription from translation which allows for extra regulation of gene expression Nuclear DNA is organised into multiple, linear chromosomes Mitochondrion Present in most eukaryotes Some eukaryotes may have lost them (some parasites) Site of cellular respiration Lyn Margulis proposed that mitochondria and plastids were once free-living bacteria that became incorporated into eukaryotic cells Mitochondria have their own genomes which are small and circular - it is most similar to proteobacteria which supports this theory Form through a process similar to binary fission Chloroplasts Site of photosynthesis Present in plants and various algae Origin of the main eukaryote cell Sediment samples found that a group of Archaea that had genes that were very similar to eukaryotes Over time other similar Archaea have been found They contain proteins that are otherwise only found in eukaryotes Everything 4  When a phylogenetic study was done it showed that eukaryotes are closely related to these Archaea Diversity of Eukaryotes Most eukaryotes are single-celled called protists They are spread throughout the evolutionary tree Some are close relatives of animals, or plants, or fungi Excavates Include a number of important parasites (Giardia and trichomonas) Don’t have obvious mitochondria Have reduced/lost mitochondria Giardia Extracellular gut parasite of mammals Has a sucker which allows it to attach to the gut wall Was discovered in 1681 by Antonie Van Leeuwenhoek Trichomonas Moves by undulating membrane and flagellae Extracellular parasite of the reproductive tract of humans Alveolates Dinoflagellates Mainly marine plankton Is photosynthetic and consumes other organism Can cause red tides Huge overgrowth Can lead to a build up of toxins in the water that can be detrimental to wildlife Some produce bioluminescence Everything 5  Produce flash of blue light Is an anti-predator function Highly diverse groups Characterised by presence of multiple cilia Mainly freshwater Apicomplexans Intracellular parasites Specialised organelle called apical complex Used to penetrate host cells Includes Plasmodium - causes malaria Toxoplasma - causes toxoplasmosis Slime moulds Usually live as single-celled organisms but when food is scarce they form a single multicellular body This can then develop into a fruiting body that releases spores into the air Colonies Vs Multicellular organisms Colony - two or more individuals of the same species living in very close proximity Advantages More efficient feeding Defence against predation Multicellular organisms Advantages Specialised for different functions Disadvantages Everything 6  Cannot survive as a single cell Algae Most are photosynthetic Some are unicellular but some are multicellular (Seaweeds) (Macroalgae) Different algal groups can be distinguished based on presence of particular photosynthetic pigments Green (Chlorophyta) Most are unicellular Some are colonial e.g. Volvox Some are multicellular e.g. sea lettuce Some live symbiotically with fungi to form lichens Chlorophytes are closest living relatives of land plants Red (Rhodophyta) Most are multicellular Red algae include several edible seaweeds Brown (Ochrophyta) Photosynthetic Includes many seaweeds Bladderwrack Giant kelp Gel-like cell walls to cushion and prevent dehydration Resembles land plants in structure This resemblance is convergent - the features evolved independently from each other in the two groups Algae don’t form a single group - spread throughout the evolutionary tree Plants play a fundamental role in the Earth’s ecosystems Everything 7  They produce much of Earth’s oxygen They can convert CO2 into organic molecules that they can use Heterophs can then consume the plant tissues to obtain organic molecules Green plants contain Cellulose in their cell walls Chloroplasts with green chlorophyll Land plants (Embryophyta) Multicellular Most live in terrestrial Nearly all obtain their energy from photosynthesis Complex reproduction with alternating haploid and diploid stages Fertilised egg begins development inside parent plant i.e. forms an embryo Reproductive cycle Alternation between haploid and diploid stages Diploid stage = sporophyte Haploid stage = gametophyte Origins of embryophytes Living relatives of embryophytes are several groups of green algae Closest relatives are stoneworts (Charales) Plantlike Live in freshwater Oldest known embryophyte is cooksonia 430 million years old Small No leaves, flowers or roots Simple branching structure Everything 8  Had cuticles Thin waxy layer covering the outer surface Had stomata Specialised pores that control entry and exit of gases into plant interior Had to prevent water loss Plants role in the development of terrestrial ecosystems Growing roots and acids secreted by plants help break down solid rock into smaller particles = biological weathering Dead plants add organic matter Result of these processes is soil Soil can support complex ecosystems/food webs with many species Main division in embryophytes Non vascular plants Xylem is absent 3 main groups Liverworts Mosses Hornworts Sometimes referred to as bryophytes Live in moist environments Small due to lack of xylem Don’t have roots so they can grow on bare rock Dominant stage is gametophyte Vascular plants Xylem is present Everything 9  Xylem transport water and nutrients from a plants roots to shoot and leaves via specialised tube-like cells. Is strengthened by lignin and can act as support for the plant Dominant stage is sporophyte Main groups Ferns Complex leaves No seeds or flowers Spores develop into free living gametophyte Gametophyte produced eggs and sperm Fertilised eggs develops into sporophyte Reproduces vid spores that are spread through the wind/air currents Gymnosperms Confiers, cycads, gingkos Have naked seeds e.g. pine cones seeds Angiosperms Flowering plants Have seeds that develop inside an ovary after fertilisation, forming a fruit Important to humans Nearly all agricultural plants are angiosperms Their origin is controversial Oldest definitive angiosperms - 130 million years old Diversification of angiosperms may have driven the “cretaceous terrestrial revolution” Reproduce via multicellular seeds Everything 10  Known as seed plants Earths terrestrial biomes are recognised largely on their plant communities Biome = major community of organisms e.g. Tundra Low temperatures and short growing season — few trees Main plant types Shrubs Grasses Mosses Lichens Grasslands Oldest fossil evidence of grasses is from the Cretaceous however grass- dominated biomes did not develop until much later 25 million years ago Development of grasslands has had major impact on many groups of animals Success of some groups of herbivorous mammals probably connected with spread of grasslands Opisthokonts = fungi, animals and a few closely related protist groups Opisthokonts Animals and fungi are closely related One feature that unites opisthokonts is the presence of a single, posterior flagellum Fungi Heterotrophs Obtain nutrients by secreting digestive enzymes and absorbing dissolved molecules Major component of the cell wall is chitin (unlike plants where it is cellulose) Everything 11  Some fungi are moulds Multicellular Grow as filaments or strands called hyphae. Together they form a mycelium Some are yeasts Single-celled >2,000,000 species of fungi - less than 10% described so far Roles of fungi Decomposers Pathogens - mostly of plants but can be in some animals e.g. chytrid fungus Symbionts Importance to humans Food and drink (mushrooms, quorn, bread production, fermentation of beer, wine, soy sauce) Production of antibiotics Disease Mushrooms and toadstools are fruiting bodies of multicellular fungi They release spores, allowing the fungi to spread These structure are temporary and are usually short lived Part of the life cycle of the fungi Kingdom Animalia (Metazoa) Eukaryotic - unlike bacteria and archaea Multicellular Heterotrophic Need to get their carbon from other organisms Lack a cell wall Everything 12  Are motile (capable of active movement at some point in their lifecycle) Origins The closest living relatives of animals are called choanoflagellates Can be unicellular or colonial Give an indication of what the last unicellular ancestor of animals looked like Sometimes the Kingdom Animalia is called Metazoa Parazoa = sponges Eumetazoa = Everything else Sponges Simplest living animals No body symmetry Embryo develops from a single primary layer of cells (Monoblastic development) No true tissues (A collection of cells performing a single function) Can regenerate from just a few cells Oldest known fossil sponge is 600 million years old (potentially) Have “choanocyte” cells lining internal cavities These generate a flow of water by moving their flagella Mainly marine Most are filter feeders Water drawn through multiple tiny pores into a central cavity lined by choanocytes Water leaves via a larger hole (osculum) Everything 13  Sponges have internal skeleton - can be composed of different material Radiata 2 phylum within radiata Cnidaria - Sea anemones, jellyfish, corals Ctenophora - comb jellies Radial symmetry (unlike sponges) Animal has a top and bottom but no left or right True tissues (Again, unlike sponges lol) Embryo develops in 2 germ layers (Diploblastic development) Two layers of cells (ectoderm and endoderm) give rise to the body tissue The outer layer of cells becomes the epidermis Everything 14  Contains contractile fibres to allow movement Can extend to form tentacles which can bear stinging cells (cnidocytes) Can also give rise to nerve cells that form a simple radial nerve net that coordinates movement The inner layer becomes the gastrodermis which is specialised for uptake of food The two layer are separated by jelly-like mesoglea Have a nervous system but no brain (No nervous system in sponges) Have an equal number of nerve cells throughout their body (Nerve net) More complex compared to sponges Ctenophora (Comb jellies and sea gooseberries) Marine Much less diverse than cnidarians (Only 100 species known) Radial symmetry Nerve net No stinging cells Everything 15  Phylum Cnidaria Two basic body types Polyp - usually sessile (fixed) with mouth and tentacles pointing up Medusa - free-swimming, jellyfish-like, mouth and tentacles pointing down Some cnidarians can switch between the polyp and medusa stage Some exist as only one or the other Polyp only - corals, hydras Medusae only - jellyfish Stinging cells are characteristic of cnidarians Often used for catching prey Major subgroups include Hydrozoa Scyphozoa Cubozoa Anthozoa Hydrozoa (Hydras) Everything 16  Some form large colonies of polyps e.g. Portuguese man of war Colony of polyps Tentacles covered in stinging cells Scyphozoa (True jellyfish) Everything 17  Mesoglea well-developed which gives the jellylike structure Mostly marine Cubozoa (box jellyfish) Everything 18  Marine Flat or cubical shape Relatively complex nervous system (Including true eyes) They are active swimmers that can avoid obstacles Highly venomous injected by stinging cells Can cause severe pain, difficulty breathing, cardiac arrest, and even death Anti-venom available for some but not all species Anthozoa (Corals and relatives) Marine Sedentary (don’t move very much) Colonial polyps No medusa stage Includes Everything 19  Sea anemones Sea fans Corals Have single celled photosynthetic dinoflagellates (Protists) living inside their tissues Provide corals with 90% of their energy Corals provide nutrients to the dinoflagellates When stress, they expel the dinoflagellates = coral bleaching Coral will starve to death if the dinoflagellates don’t return 1 line of symmetry = bilateral symmetry More than 1 line of symmetry = radial symmetry Cnidarians are radially symmetrical Ctenophore are often called bi-radially symmetrical - largely radially symmetrical but with some bilaterally symmetrical features e.g. tentacles Bilateria - bilaterally symmetrical animals Usually have a distinct head Embryo has three germ layers (triploblastic) True tissues present Often have a body cavity (coelom) Often show segmentation (recognisable repeated units e.g. backbone) Most have bilateral symmetry Except starfish Cephalisation - presence of a head-like structure Radially symmetrical animal typically have a “nerve net” Everything 20  Bilaterally symmetrical animals usually have nerve cells concentrated at the “head” Germ layer Primary layer of cells that forms during embryonic development Sponges have one germ layer = monoblastic Radiates have two germ layers = diploblastic Bilaterian’s have 3 germ layer (endoderm, ectoderm, and mesoderm) = triploblastic Originate from a blastula - a simple, hollow ball of cells The ball of cells gets pushed in which is called gastrulation (invagination of the blastula to form gastrula). This gives the balls layers. Everything 21  After gastrulation The germ layers that are formed give rise to all the tissues in the body Gastrointestinal tract - endoderm Respiratory tract - endoderm Nervous system - ectoderm Epidermis - ectoderm Coelom - mesoderm Deuterostomes and Protostomes Deep spilt within bilateria Division reflect fate of the blastopore blastopore is the opening formed by gastrulation In most deuterostomes, blastopore becomes the anus In protostomes is more complex: blastopore generally forms either the mouth, or the mouth and the anus Everything 22  Nearly al bilaterians can be confidently placed in either group with the exception of a few marine worm-like animals Protostomes Within protostomes, there are two major groupings Lophotrochozoa Ecdysozoa Lophotrochozoa Some have a feeding structure called a lophophore lophophore = a ring of tentacles surrounding the mouth and used in feeding. It looks similar to the ring of tentacles seen in polyps however this has an independent origin. Some have a distinctive type of larva called a trochophore. This larva has a cilia that helps it swim. Last common ancestor of Lophotrochozoa may have had a lophophore and a trochophore-type larva. Members of the Lophotrochozoa Platyhelminthes = flatworms Rotifera = rotifers Lophophorates = aquatic organisms with lophophores Mollusca = molluscs (slugs, snails, squid) Annelida = annelid worms (earthworms, leeches) Platyhelminthes = flatworms Everything 23  No body cavity No circulatory or respiratory organs Flattened to increase surface area which allows diffusion of gases and nutrients through its skin Only one opening in the digestive system - both ingestion of food and removal of waste in the middle of the body Cephalisation and sense organs (can sense light, movement and touch) Many species of flatworms are free living carnivores or scavenger Many species of flatworms including Flukes Tapeworms Schistosoma manisoni - a flatworm parasite Snail and human hosts Adult worms in human blood vessels cause a serious disease called schistosomiasis Eggs pass out in urine and faeces Hatch in water to produce larvae which infect the snail Asexual reproduction and development of different larval type in snails Everything 24  Larvae leave snails and burrow through human skin Lophophorates = aquatic organisms with lophophores Mainly marine filter feeders with lophophores Most well known Bryozoans (Moss animals) Brachiopods (Lamp shells) - look similar to bivalve molluscs and are ecologically similar too but they are not closely related Fossil record of brachiopods shows a big loss of diversity at the end Permian mass extinction - 250 million years ago - they appear to have been largely replaced by bivalves Rotifera Sometimes called wheel animals Tiny under 2mm long - made up of a few hundred cells Use cilia to move and feed Have a simple digestive system Two light sensitive spots Some species have all females - reproduce by parthenogenesis Annelida = annelid worms (earthworms, leeches) Segmented worms Include Earthworms - reduced head and appendages Polychaete worms - marine segmented worms often tube dwelling or sand burrowing Leeches - flattened body with reduced coelom and appendages with suckers at anterior and posterior ends Segmentation = body organised in repeated units that are similar or identical Everything 25  Externally they look like rings and internally they are divided by septa (walls of tissue) Each segment have the same set of organs Polychaetes Some are active predators Some build tubes to live in and use a “crown” of tentacles for filter feeding Mollusca Very diverse - large number of species (75,000 different living species), built very differently (morphotypes), huge size variety (0.7mm to 14 m) Ecologically widespread Members of marine, freshwater and terrestrial habitats Fossil record Many molluscs have a hard calcareous shell which fossilises well. This means that the mollusc groups are very well represented in the fossil record. However the animal inside the shell is relatively unknown. Soft tissues are almost never preserved meaning they have a very poor fossil record. Molluscs (e.g. octopi) without a shell are made of these soft tissues. Mollusc fossils can be used to correlate the ages of rocks in different locations Basic body plan of molluscs Everything 26  Three main features Mantle - a specialised organ adapted from the dorsal surface of the body wall Secretes the shell in molluscs that still have one Usually encloses a cavity which houses one or more gills and also exits for excretory and reproductive organs Shell - Calcareous (made of calcium carbonate) - has three distinct layers Inner nacreous layer (flat crystals of calcite) is known as the mother of pearl Radula Foot Muscular underside of the animal Used for movement Has been specialised in different ways e.g. Burrowing, tentacles Nervous system Everything 27  made up of pairs of nerve cords running through the foot In snails the oesophagus runs through the middle of the brain Parasites can affect the nervous system Radula A rasping tongue with chitin teeth Found in most molluscs Cephalopods (squid, octopus, cuttlefish) have chitin cutting plates that form a beak as well as a radula Digestive system Mouth secretes mucus Cilia found in the oesophagus and stomach move the mucus into the stomach which forms a long string called a food string Within the stomach a rodlike structure called a protostyle makes the food string wind around it. It also releases digestive enzymes Also a digestive gland or hepatopancreas Make enzymes for extracellular digestion Store glycogen Absorption of nutrients Intestine - adsorption of nutrients Anus - opens into mantle cavity Respiration and circulatory system Usually one or two gills houses in the mantle cavity (aquatic) Land snails lack gills but have modified the mantle cavity into a primitive lung Most molluscs have a heart which connects to an open circulatory system However cephalopods have a closed circulatory system with blood vessels - able to send oxygenated blood to tissues faster, allowing higher activity Blood Everything 28  Contains an oxygen transport pigment called haemocyanin which is blue when oxygenated (contains copper instead of iron) Mollusc blood also contains haemocytes - phagocytic cells (engulf and digest foreign material) that are important in immune defence - like white blood cells in vertebrates Excretion of waste products The heart of molluscs also acts as a filter Chambers of the heart filter out waste products Nephridia (structures equivalent to kidneys selectively reabsorb some molecules and release further waste products) Urine excreted into mantle cavity Reproduction Some individuals produce either eggs or sperm Some molluscs are hermaphrodites Fertilisation is internal in some molluscs but external in others In cephalopods males have a modified arm to transfer sperm to females The primitive larval stage of molluscs is a trochophore Major classes of molluscs Polyplacophora (Chitons) Everything 29  900 living species Chitons have lots of primitive features (Plesiomorphies) Fossil record of polyplacophora extends back at least 350 million years Flattened body Dorsal shell made of eight plates rather than a single unit (primitive feature) Body has typical mollusc features Muscular foot Visceral mass Mantle Gastropods (snails) Most species rich class of molluscs 60,000 living species Everything 30  Over 80% of living mollusc species are gastropods Includes snails, slugs, nudibranchs, limpets, cowries and abalones Slug is an imprecise term that refers to a gastropod without an obvious shell During larval development gastropods undergo torsion Distinctive share specialisation of gastropods Visceral mass, mantle and shell rotated through 180 degrees relative to head and foot Mantle cavity now above and just behind the head May have evolved to allow head to be withdrawn into the shell Some nudibranchs feed on cnidarians and store the functional nematocysts (stinging cells) at the tips of their cerata in cnidosacs Characterised by a Prominent head Well developed sensory structures Shell (when present) single, centrally placed and usually coiled Have colonised Marine benthic zones Marine Pelagic zones Freshwater benthic zones Terrestrial habitats Bivalvia (clams. mussels) Everything 31  Clams, oysters, mussels Shells in two halves (valves) hinged at the dorsal line No radula and no distinct head Most are filter feeders, trapping particles in mucus covering the gills Powerful muscles close shell Some can swim Some foot may protrude for digging Cephalopoda (Octopus, squid, cuttlefish) 800 living species Head surrounded by tentacles Mantle adapted to form siphon —- move by jet propulsion Everything 32  Shell external, internal or absent Carnivores Active predators Highly developed sensory and nervous system To detect prey Able to change colour Use specialised cells called chromatophores Each chromatophore contains a sac of pigment Contraction of muscles around the chromatophore alters the shape of the sac, altering the colour Intelligence Most complex nervous system and most complex behaviour of any non- vertebrate Octopus and cuttlefish have largest brains relative to body size of any invertebrates Brain is housed in a cartilaginous skull First recognised in 1997 using molecular data Key characteristics of Ecdysozoa Moulting of the cuticle Cuticle = outer, non-cellular layer of the body secreted by epidermis Which phyla are members of Ecdysozoa? At least 8 phyla are members of Ecdysozoa Some look worm like e.g. Priapulida, Nematoda, Nematomorpha Some ecdysozoa are arthropod like e.g. Tardigrada, Onychophora, Anthropoda Diversity driven by Nematoda and Arthropoda Everything 33  Worm-like ecdysozoans Lack paired appendages Priapulida (Penis worms) Marina Carnivorous 20 living species 0.5mm to 20cm Fossil record stretches back to Cambrian Nematoda (Roundworms) 0.1mm to >8m Occur in all ecosystems Everything 34  Parasites 80% if animals are nematodes C.elegans Widely used in labs Very simple multicellular organisms Transparent Simple nervous system Easy to keep in lab First multicellular organism to have entire genome sequenced Nematomorpha (Horsehair worms) 30-40 cm long 350 species described Usually described as parasitoid (kills host) - larvae parasitic on arthropods; adults free living in freshwatee Makes host drown themselves so they can live free in the water Arthropod-like ecdysozoans Everything 35  Have paired, ventrolateral (down and to the side) appendages Have a completely independent evolutionary origin from limbs of vertebrates Tardigrada (Water bears) 0.1-1.5 mm long 4 pairs of legs Usually feed on plants or bacteria Often live on mosses and lichens Can enter suspended animation by dehydrating to 1% normal water content (tun state) Can survive for 5 or more years Can survive cooling to -272 degrees, heating to 150 degrees, pressures of 6000 atmospheres, vacuum of space for more than 10 days, and 1000 times the radiation dose that kills humans Developmental studies show that most of the tardigrade body is equivalent to the head of arthropods, region equivalent to thorax and most of the abdomen of insects have been lost in tardigrades Onychophora (velvet worms) Segmented Multiple pairs of hollow, fluid-filled legs tipped with claws Terrestrial ambush predators 200 living species described In the tropics plus temperate regions of southern hemisphere Hunt using slime glands to capture prey Onychophora and Arthropoda are closely related Panarthropoda Over 1 million described species Everything 36  Hexapods and Crustaceans Current evidence indicates that Crustacea is a paraphyletic (includes a common ancestor but only some of its descendants) Pancrustacea refers to grouping of Crustacea + Hexapoda Pancrustacea is monophyletic (Common ancestor and all its descendants) Crustaceans Superdiverse Most aquatic Range in size - 1mm to 3m Show characteristic arthropod features Cuticle forms hard exoskeleton Jointed appendages Moulting Segmentation Structure Three tagmata Head Thorax Abdoment Same as in hexapods But head and thorax may be fused which forms a cephalothorax Paired jointed appendages specialised in different ways There are many parasitic crustaceans Some have very simplified bodies (often wormlike) Everything 37  Evidence from larval stages, sperm morphology and molecular data confirms that they are crustaceans Barnacles Linnaeus and other early scientists though barnacles were molluscs but barnacles larvae show crustacean features Larval barnacle attaches to suitable surfaces using cement glands on its head. It secrets hard calcium carbonate to glue themselves on to things such as whales, boats, rocks Malacostraca Extremely diverse Includes, shrimp, prawns, crabs, crayfish, lobsters, crabs, krill, woodlice. 40,000 species Diverse body plans Mainly marine Structure Head, thorax and abdomen Most have haomocyanin (copper-based) as their respiratory pigment Crabs, crayfish, lobsters, krill etc are all in the order Decapoda Meaning 10 legs Have 5 pairs of walking legs Everything 38