Invertebrates

egan lecture 21

Where did animals likely evolve from?

single-celled eukaryotes similar to present-day choanoflagellate protists

What are ediacaran biota?

they are soft-bodied, mostly sessile animals who formed the earliest seafloor communities of large multicellular life including Porifera and cnidarians

How do sponges’ body work?

like a sac perforated with pores

it captures particles suspended in water that pass through its body — the spongocoel — and flows out through osculum

What are cnidaria?

they are carnivores and ecological engineers: think corals, hydras, sea jellies

What is the cnidarian body plan?

they are diploblastic with radial symmetry and a simple nerve net

they also evolved a gastrovascular cavity

cnidocytes make them the first active predators and engineers

cnidarians use what to capture and consume prey?

tentacles armed with cnidocytes — nematocysts are specialized organelles within cnidocytes that eject a stinging thread

What two variations form the cnidaria body plan?

polyps and medusa

What entail polyps?

they are sessile, adhering to the substrate by the aboral end of the body

What entail medusa?

a free-swimming form that has a bell-shaped body with the mouth on the underside

What happens for medusozoans?

the life cycle alternates between polyp and medusa body forms and reproduce asexually (through budding) and sexually (through production of medusae)

What happens during the Cambrian Explosion?

diversity of large animals increased dramatically

What rose from/during the Cambrian Explosion, and what is its effect?

Atmospheric O2 increased — animals can support larger, more active bodies and enabled complex tissues and skeleton to form and maintain

What evolved from the Cambrian Explosion, altering animals?

The expansion of the Hox gene complex:

• allowed precise control over body patterning along head-tail axis

• modular segments to specialize

• small changes in hox regulation generate many new body plans quickly

What relationships came out of the Cambrian Explosion?

new predator-prey relationships:

• evolutionary arms race

• selected for shells, spines, burrowing, and better senses in predators/prey

• created new ecological niches and lifestyles

What did lophotrochozoan body plans innovate?

segmentation in annelids provided repeated, modular body units that could specialize

molluscan body plan generated range

lophophore and trochophore stages represent key innovations in feeding and larval dispersal

Why are lophotrochozoans important?

They are major ecosystem engineers, important players in food webs, and help reconstruct past oceans and mass extinctions

How did nervous systems evolve?

early animals had diffuse nerve nets

early bilaterians concentrated at two ends of the body

then these centers expanded and linked along the body axis, giving rise to centralized brains and nerve cords

Ecdysozoans have an external what?

cuticle that must be shed or molted and replaced to grow (ecdysis)

What is the largest phyla out of the ecdysozoans?

arthropods

What is the arthropod body plan?

segmented body, hard exoskeleton, and jointed appendages (think trilobites!)

What and how did arthropods evolve?

segments united to form “body regions” specialized for feeding, walking, or swimming

they likely evolved due to changes in sequence/regulation of existing Hox genes

How did arthropods terrestrialize?

they were the first animals to colonize land following the Cambrian explosion (think trilobites), and “followed plants”

What are advantages and challenges to life on land?

higher O2

new sources of food and decreased competitors

scarce water

greater fluctuation in temperatures

no support against gravity

Why did arthropods colonize land first? What challenges to land did their traits have that allowed them to live on land first? (4)

Challenge: Scarce water > desiccation risk

• waterproof exoskeletons

Challenge: gravity > need support structures and movement

• exoskeleton and pre-existing jointed limbs

Challenge: protecting gametes

• early internal fertilization

Challenge: gas exchange

• tracheal breathing system that derived with the transition to land

What advances do arthropods have?

eyes, olfactory receptors, and antennae that function in both touch and smell

• open circulatory system uses a heart to pump hemolymph to cavity surrounding tissues and organs (the hemocoel)

What lineages do arthropods diverge into?

chelicerates (sea spiders, horseshoe crabs, scorpions, ticks, mites, spiders)

myriapods (centipedes and millipedes)

pan crustaceans (insects, lobsters, shrimp, barnacles, and other crustaceans)

What clade includes insects and their relatives?

hexapoda

What evolution caused rapid diversification?

flight

Why were arthropods (especially insects) so successful on land?

protective, waterproof exoskeleton

highly effective movement and flight

flexible life histories and diets

What two clades display strong co-evolutionary patterns?

insects and angiosperms

When were ediacaran biota living?

~575-542 mya