Ecdysozoa🪱

SKIN SHEDDERS:
Introduction to Ecdysozoa

Some characteristic features of Bilateria

  • Most have bilateral symmetry

  • Some degree of cephalisation

  • Triploblastic

    • three embryonic "germ layers": ectoderm, endoderm, and mesoderm

  • True tissues

  • Often have a coelom (body cavity)

  • Often show segmentation

Deuterostomes and Protostomes

  • Division reflects fate of the blastopore

    • in most deuterostomes (Deuterostomia), blastopore becomes the anus

    • situation in protostomes (Protostomia) is more complex – blastopore generally forms either the mouth or the mouth and anus

Within Protostomia, two major groupings:

  • Lophotrochozoa

  • Ecdysozoa

Lophotrochozoa has more phyla than Ecdysozoa

Ecdysozoa has more species – includes nematode worms and arthropods, which are very species rich

Which phyla are members of Lophotrochozoa?

  • Several different phyla with very different body plans – they don’t look very similar!

    • Platyhelminthes = flatworms

    • Rotifera = rotifers

    • Lophophorates = aquatic organisms with lophophores (e.g., bryozoans, brachiopods)

    • Annelida = annelid worms (includes earthworms, leeches, and others)

    • Mollusca = molluscs (slugs, snails, squid, etc.)

Most lophotrochozoans have either a lophophore (ring of tentacles for feeding) or a trochophore larva, or both.

Edysozoa

  • Name comes from:

    • Ecdysis = moulting of the cuticle

    • Zoa = animals

  • "Moulting Animals"

Edysozoa

  • Grouping (= CLADE) first recognized in 1997 based on molecular data (DNA sequences from the 18S ribosomal gene)

Evidence for a clade of nematodes, arthropods and other moulting animals

Anna Marie A. Aguilarado, James M. Turbeville, Lawrence S. Linford, Maria C. Rivera, James R. Garey, Rudolf A. Raff & James A. Lake

Aguilarado et al. (1997)
Nature 387: 489-493

Edysozoa

  • Ecydozoans characterized by:

    • moulted cuticle

      • cuticle = outer, non-cellular layer of the body secreted by epidermis

      • moulting (ecdysis) = old cuticle shed and replaced by new, larger cuticle — allows animal to grow

Which phyla are members of Ecdysozoa?

  • Based on current evidence, at least 8 phyla are members of Ecdysozoa – we won’t cover all of them!:

    • some ecdysozoan phyla are "worm-like", e.g., Priapulida, Nematoda, Nematorpha

    • some ecdysozoan phyla are "arthropod-like": Tardigrada, Onychophora, Arthropoda

  • diversity driven by two phyla: Nematoda (nematode worms, >25,000 species) and Arthropoda (arthropods, >1,000,000 species)

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Worm-like" ecdysozoans – lack paired appendages

-Examples of "worm-like" ecdysozoans:

  • Phylum Priapulida

  • (Ancient Greek: Priapus)

  • Priapulida (penis worms): Marine worms, burrow into sediment, with a cylindrical body and a spiny proboscis.

  • marine and carnivorous. 20 species ,0.5mm to 20cm

    • Fossil record stretches back to Cambrian e.g., Ottoia from the Burgess Shale (508 MYA)

  • Phylum Nematoda

  • Nematoda (roundworms): Very diverse, free-living or parasitic, found in virtually all environments; important in soil health and disease.

    • Nematode worms or "roundworms"

    • 0.1 mm to >8 m in length

    • Occur in all ecosystems (including 3 km under the Earth's surface!)

    • Often the single most common group in an ecosystem

    • In terms of numbers of individuals, ~80% of animals are nematodes!

    • About 25,000 species described to date, but many more undescribed

    • Maybe 1,000,000 species in total?

    • Around 50% of described living species are parasitic (host survives) or parasitoid (kill the host)

    • Different species parasitize plants or animals

Some nematodes:

Caenorhabditis elegans (C. elegans) — free-living
Roundworms (parasitic)
Pin or thread worms (parasitic)
Hook worms (parasitic)
Guinea worms (parasitic)

Elephantitis (Lymphatic Filariasis)

  • Caused by several different nematode species

  • Spread by mosquito bites

  • Worms infect lymph ducts → block flow of lymph → edema (swelling), usually in legs and genitals

  • Very long-term infection (years)


Why is C. elegans so widely used in lab studies?

  • Very simple multicellular organism

    • Developmental fate of every cell (959 in the adult hermaphrodite; 1031 in the adult male) mapped

  • Transparent

  • Simple nervous system

  • Easy to keep in lab (very important!)

  • First multicellular organism to have entire genome sequenced (first draft in 1998)

    • 20 thousand protein-coding genes ≈ same number as in humans!

  • 35% of C. elegans genes have clear human equivalents

Some fungi have evolved to feed on nemotodes

  • Worm-like" ecdysozoans

    • Phylum Nematoda:

  • Nematomorpha

  • (horsehair worms): Mostly parasitic in insects and amphibians during their larval stages

    • "horsehair worms"

    • superficially resemble nematodes but evolutionarily distinct

    • most 30-40 cm long

    • 350 species described, but probably 2000+

    • Usually described as parasitoid(= kills host)—larvae parasitic on arthropods; adults free living, usually freshwater

    • can modify behavior of hosts to make them drown themselves—adult worm leaves body and lives in water, but recent research indicates that host sometimes survives

"Arthropod-like" ecdysozoans (Panarthropoda)

- Have paired, ventrolateral (= down and to the side) appendages = "limbs"/"legs"
— these have a completely independent evolutionary origin from limbs of vertebrates!

  • Tardigrades (Water Bears): Tiny, resilient animals that can survive extreme conditions.

  • Onychophorans (Velvet Worms): Soft-bodied predators with many stubby legs.

  • Arthropods: The most diverse group, including insects, spiders, and crustaceans, with jointed limbs and exoskeletons.


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"Arthropod-like" ecdysozoans (Panarthropoda)

- Phylum Tardigrada

- Tardigrades or "water bears"

- Approximately 0.1-1.5 mm long

- 4 pairs of legs

- Usually feed on plants or bacteria

- Often live on mosses and lichens

Tardigrades are nearly indestructible!

  • Tardigrades can enter "suspended animation" by dehydrating to 1% of normal water content = "tun" state

    • Metabolism <0.01% of normal

    • Can survive for 5 years or more as a "tun"

  • In tun state, tardigrades can survive:

    • cooling to -272°C

    • heating to 150°C

    • pressures of 6000 atmospheres

    • vacuum of space for >10 days

    • 1000 times the radiation dose that kills humans

  • Tardigrades exit "tun" state when rehydrated

Tardigrades are almost all "head"

  • Developmental studies show that most of tardigrade body is equivalent to head of arthropods

  • Region equivalent to thorax and most of the abdomen of insects has been lost in tardigrades

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Arthropod-like" ecdysozoans (Panarthropoda)

  • Phylum Onycophora

    • Onychophorans or "velvet worms"

    • Segmented (although not very obvious!)

    • Multiple pairs of hollow, fluid-filled legs tipped with claws

    • Terrestrial ambush predators

    • ~200 described living species – tropics plus temperate regions of southern hemisphere

Arthropod-like" ecdysozoans (Panarthropoda)

  • Phylum Onycophora

    • hunt using slime glands to capture prey

    • undergo moulting (ecdysis) — remember, they are members of Ecdysozoa!

- Onychophora and Arthropoda are closely related:

Panarthropoda, in Ecdysozoa

- Annelida is in a completely different group, Lophotrochozoa

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"Arthropod-like" ecdysozoans (Panarthropoda)

- Phylum ARTHROPODA

- >1 million described species

Evolution of Ecdysozoa — the changing face of Hallucigenia

- Burgess Shale, Canada — preserves early Cambrian (~508 MYA) fauna

- Documents Cambrian Explosion — sudden appearance in fossil record of modern animal phyla

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"Different views of the Burgess Shale animals...

Charles Doolittle Walcott:

- discovered Burgess Shale fossil deposit

- identified most of the Burgess Shale fossils as recognizable members of known phyla"

Changing views of Hallucigenia – Walcott (1911)

- Originally described in 1911 as a polychaete annelid worm (Lophotrochozoa) by Walcott

-Hallucigenia sparsa

-*living polychaete* Eunice

Charles Doolittle Walcott (1850-1927) (with his portrait image)

:

Changing views of Hallucigenia – Conway Morris (1977)

- Restudied by Simon Conway Morris in 1977, who proposed a radical new interpretation

- Member of unknown phylum

- "Stilt-walker" with a single row of tentacles on back

- Bulbous head:

Late 1970s–early 1990s view: Burgess Shale "oddballs" as representatives of entirely extinct phyla — early "experiments" in evolution

Different views of the Burgess Shale animals...

- Stephen Jay Gould:

- many entirely extinct phyla present in Burgess Shale fauna

- Cambrian explosion = period of experimentation, with most phyla/body plans going extinct later

- survival of particular phyla to present day might partly be due to chance

The changing views of Hallucigenia—a genus of early onychophoran (velvet worm)—based on research by Ramsköld and Xianguang in 1991.

  • In 1991, Ramsköld and Xianguang proposed that Conway Morris's reconstruction of Hallucigenia was upside down. They suggested that what was thought to be a single "tentacle" was actually pairs of legs, with spines on its back that may have served a defensive purpose.

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Changing views of Hallucigenia — Smith and Caron (2015)

Martin R. Smith & Jean-Bernard Caron

- Hallucigenia has several features shared by both "worm-like" and "arthropod-like" ecdysozoans, e.g., throat lined with "teeth"

- probably an early ecdysozoan---

Different views of the Burgess Shale animals...

Current consensus view:

- most of the Burgess Shale animals (including Hallucigenia) are early branching ("stem") members of known phyla

- They only have some of the synapomorphies (shared specializations) of their phyla

- they split off before all of the synapomorphies characteristic of modern members had evolved

- they also have their own weird features (e.g., spines of Hallucigenia)

Different views of the Burgess Shale animals...

- The "Traditional" model shows a cone of increasing diversity and disparity.

- The Gould, 1989, model presents an inverted cone.

- The Forte et al., 1996, model shows early, high levels of disparity.

Definitions:

- Diversity = number of species

- Disparity = variation in the anatomy, function, and ecology

The current consensus view agrees with Forte et al. (1996) model—lots of disparity (different anatomical, functional, and ecological specializations) already in the Cambrian.