Invertebrates

Major Animal Groups

• Major animal groups we will discuss:

  • Porifera: sponges 

  • Ctenophora: comb jellies 

  • Cnidaria: jellyfish, sea anemones, coral

• Bilateria: animals with bilateral body symmetry

• There are ~ 33 animal phyla

Phylum Porifera: Sponges

• Least complex animals (no true tissues)

• Traditionally thought to be sister group to all other animals

• Most sponges are monecious (hermaphroditic): same individual makes sperm and eggs (opposite is diecious/gonochoristic) 

  • Spawn sperm to water, captured in another’s water current

  • Captured by choanocytes

• Body organized like a perforated vase that water flows through

  • Many small pores (ostia) leading to a big open internal space (spongocoel), with a big top opening (osculum)

  • Spongocoel lined by flagellated collar cells (choanocytes): beat to generate a current

  • Water flows in ostia and out the osculum 

• Suspension feeders: filter/capture food particles from the water 

  • Food particles captured by choanocytes 

• Inner and outer layers of cells, separated by mesophyll

  • May have hard (calcium carbonate or silica) skeletal elements secreted  by a special cell type (sclerocytes)

  • May have proteinaceous skeleton made of spongin (like collagen)

  • Amoebocytes move food around 

Phylum Ctenophora: Comb Jellies

• Characteristics of Ctenophores

  • Mostly pelagic; some benthic

  • Marine (surface to deep sea)

  • Fused rows of cilia called ctenes

  • Sticky structures for prey capture on tentacles called colloblasts

  • Have nerves and muscles 

Phylum Cnidaria: Fancy Gastrulas

• Radially symmetrical 

• Two tissue layers: diploblasts

  • Ectoderm

  • Endoderm

• Mouth but no anus: gastrovascular cavity 

• Alternation of Generations

  • NOT alternation of 2n/n generations (like plants)

  • Alternation of sexual and asexual generations

  • Different taxa emphasize different phases

• Two phases

  • Medusa: mouth-down, simming, sexual stage (i.e.jellyfish)

  • Polyp: mouth-up, sedentary, sexual stage (i.e. coral)

• No mesoderm, so no true muscles

  • Just weak contractile cells

  • Nervous system a diffuse nerve net, “Thus, the animal can detect and respond to stimuli from all directions.” 

Cnidarians Have Stinging Cells

• Cnidarians have stinging cells called cnidocytes

• These cells make a unique class of stinging organelle

• Most common type is called a nematocyst 

• Some are dangerous to humans!

Diversity of Cnidaria

• Class Hydrozoa: alternates between medusa and polyp phases 

  • I.e., mostly small colonies like Obelia 

• Classes Scyphozoa and Cubozoa: jellyfish and box jellies 

  • Polyp stage small relative to medusa

• Class Anthozoa: sea anemones, corals

  • Medusa stage lost

  • Solitary or colonial 

Climate change is killing coral reefs: roughly ¼ of the Great Barrier Reef died in 2016

Most Animals Belong to Bilateria

• Bilateria

  • Clade of animals with bilateral symmetry

  • Have a head (cephalization)

  • Triploblastic: three tissue layers (ectoderm, endoderm, mesoderm)

  • May or may not have a coelom 

  • Three major clades

    • Lophotrochozoa (Spiralia): most with a lophophore (ring of tentacles) or trochophore (larval type) 

    • Ecdysozoa- animals that shed their cuticle

    • Deuterostomia- “mouth second” 

  • Lophotrochozoa and Ecdysozoa make up a clade called Protostomia

  • Protostomes and deuterostomes distinguished by: 

    • Fate of blastopore (mouth v anus)

    • How mesoderm form

    • Embryonic cleavage pattern

Lophotrochozoa is one Group of Bilaterians 

• ~15 phyla grouped by molecular phylogenetics

  • Some taxa have ciliated trochophore larvae 

    • Phylum Annelida: segmented-worms

    • Phylum Mollusca: mollusks

  • Some have a feeding structure called a lophophore (horseshoe-shaped ring of tentacles)

    • Phylum Ectoprocta: bryozoans 

    • Phylum Brachiopoda: lamp shells

  • Some phyla lacking both

    • Phylum Platyhelminthes: flatworms

    • Phylum Rotifera: rotifers (“wheel animals”)

Platyhelminthes Includes Free-Living & Parasitic Flatworms 

• Flatworms generally long and flat acoelomates

  • Many species are parasites

• Like cnidarians, lack through-gut: gastrovascular cavity

  • Digestion AND circulation

• Have organs to maintain osmotic balance: protonephridia with cells called flame bulbs

• Most hermaphrodites (monecious), simultaneously male and female

  • Many also capable of asexual reproduction

Annelids are Segmented Coelomate Worms 

• Long and cylindrical coelomates

  • Coelomic spaces divided by septa 

  • Body made of repeating segments

    • Organs repeated in each segment

• Have a through-gut (complete gut): mouth and anus

• Circulatory system

• Some hermaphrodites (monecious), some gonochoristic (diecious; separate sexes)

• Hydrostatic skeleton: skeleton supported by hydrostatic fluid pressure 

Diversity of Annelida

• Three traditionally recognized classes

  • “Polychaeta”

    • Mostly marine, also freshwater

    • Have lateral parapodia for locomotion

    • Many chaetae (hairs)

  • Oligochaeta: earthworms 

    • Mostly terrestrial and freshwater

    • Few chaetae

  • Hirudinea: leeches 

    • Mostly freshwater

    • Predators and blood parasites 

Mollusca Include Diverse Taxa such as Clams, Snails, & Octopuses

• Phylum Mollusca has 8 “classes” including 

  • Polyplacophora: chitons

  • Gastropoda: snails, slugs

  • Bivalvia: clams, mussels, oysters, & scallops

  • Cephalopoda: squid, octopus, nautilus

Characteristics of Mollusca

• Many traits are shared among molluscs, but few are found in all taxa

  • Calcium carbonate shell (exoskeleton; missing in some)

    • Secreted by dorsal epidermis: mantle

  • Radula: toothed tongue-like organ used for feeding (except in bivalves)

  • Foot for crawling (chitons, snails) or burrowing (bivalves)

  • Organs concentrated into visceral mass

    • (not stretched over whole body, like worms)

  • Mantle cavity: space under mantle, behind visceral mass

    • Houses gills, openings of gut, gonads, excretory system

Ecdysozoa is Another Bilaterian Group Discovered by Molecular Phylogenetics

• Ecdysozoa is a grouping of 8 phyla 

  • All taxa molt their outer covering (ecdysis)

• Includes two very diverse phyla (and some other small ones that we won’t discuss)

  • Arthropoda: >1 million species known!!!

    • E.g.: insects, spiders, crabs

    • Also extinct lineages; around since Cambrian Explosion

  • Nematode: ~ 25,000 known, but many waiting to be discovered

    • Unsegmented roundworms

    • Free-living parasitic

Arthropods are Covered by a jointed Exoskeleton 

• Arthropods covered by a cuticle: non-living organic layer made of protein and chitin that covers the epidermis 

  • Thick in some places: rigid, hardened with calcium carbonate

  • Thin and flexible in other places

  • Provides points for muscle attachment: exoskeleton

  • Exoskeleton: layer of proteins and chitin

• Arthropods are segmented like annelids

• Each segment has a pair of jointed appendages

• Appendages diversified for a variety of functions - locomotion, sensory, feeding, etc.

Arthropods are Covered by a Jointed Exoskeleton

• The hard, waterproof exoskeleton of arthropods facilitated the invasion of land

• Exoskeleton creates a few challenges

  • Need specializes structures for gas exchange

    • Gills in aquatic taxa

    • Book lungs or trachea in terrestrial taxa

  • Need to molt (ecdysis) to grow

• Generally gonochoristic (separate sexes)

Major Groups of Arthropoda

• Trilobitomorpha

  • Extinct; Paleozoic trilobites

• Chelicerata

  • Arachnids: spiders, ticks, mites, scorpions

  • Horseshoe crabs

    • Have open circulatory systems with no adaptive immune response 

    • Has “innate immunity” baked on granular amebocytes 

      • Granular amebocytes can come into contact with endotoxin and release defense molecules, coagulate, and neutralize the pathogen

  • Sea spiders

• Myriapoda

  • Centipedes and millipedes

• Pancrustacea

  • Crustaceans

    • Decapods: lobsters, crabs, shrimp

    • Isopods: pill bugs/rolly-pollies

    • Copepods: tiny, planktonic 

    • (and many more)

• Hexapoda

  • Insects 

Most Species on Earth are Insects

• Crustaceans dominant in ocean, but insect dominate land

  • > 1 million species divided among ~30 orders

• Wings are not appendages: extensions of dorsal cuticle 

  • Originally served for thermoregulation? Gliding?

• Some insects tremendously helpful

  • Pollinate crops: e.g. honey bees

  • Eat pests: e.g. ladybugs, mantids

• Others are less helpful

  • They eat our crops: costs billions of dollars per year

  • Spread diseases: e.g. malaria 

Nematodes are Tiny but Diverse RoundWorms

• Long and cylindrical but unsegmented

  • Pseudocoelomates

• Best known species are parasites or agricultural pests

  • E.g.: Trichinella spiralis in undercooked pork 

  • E.g.: Necator americanus American hookworm 

• Know much less about all the free-living species

Vertebrates and Some Invertebrates Form the Clade Deuterostomia

• Deuteristinua has two phyla we will cover:

  • Phylum Echinodermata

    • Starfish, sea urchins, sand dollars, etc

  • Phylum Chordata

    • Vertebrates (fish, frogs, chickens, humans) and related invertebrates

Echinoderms have Secondary “Radial” Symmetry

• Phylum Echinodermata: “spiny skin”

• Quasi-radial symmetry as adults

• Bilateral larvae

• Unique water vascular system with tube feet (sensory, locomotion, and feeding)

• No organsfor water balance: exclusively marine

• Asexual (regeneration) & sexual (broadcast spawning) reproduction

Diversity of Echinodermata

• Five major lineages: 

1. Asteroidea: sea stars

2. Ophiuroidea: brittle stars

3. Echinoidea: sea urchins & sand dollars

4. Crinoidea: sea lilies & feather stars

5. Holothuroidea: sea cucumbers 

All Chordates Share Four Unique Traits (at Least at Some Point)

1. Notochord: long flexible rod of cells

• Provides skeletal support

• Something for muscles to pull against for swimming

• May be replaced by other skeletal elements

2. Dorsal, hollow nerve cord

• Dorsal to notochord

• Was hollow in early chordates but secondarily solid in some taxa

3. Pharyngeal slits

• Pharynx: posterior to mouth

• Water enter mouth passes thru slits: suspension feeding 

• In more derived chordates, modified into gills, jaws, etc. 

4. Post-anal tail

• Muscular tail extends past anus

There are 2 Invertebrate Chordate Clades 

• Lancelets (Cephalochordata)

  • Small suspension feeder capable of swimming

  • Adults live partially buried

• Tunicates (Urochordata)

  • Difficult to see how adult tunicates are chordate (missing most traits); presents in larvae 

  • Sedentary suspension feeders: basket-like pharynx with mucus that traps food