BIO WEEK 7: Metazoans (Rise of Animals 1)

What Defines an Animal?

multicellular organoheterotrophs (get energy by eating organic matter, don’t photosynthesize)

no cell walls to allow flexibility, movement, and tissue formation

neurons + muscle cells which are specialized cells for movements and response (exception for sponges that lack true nerves & muscles)

Additional Animal Traits

Diploid-dominant life cycle:

• most of life is in 2n (diploid) form

Sexual reproduction (often asexual too)

Extracellular matrix (ECM)

• proteins outside cells

• important for cell adhesion + communication

• critical for multicellularity

Motile (moving) at some life stage

Origins of Animals: animals are?

monophyletic: all descended from one common ancestor

likely ancestor: Choanoflagellate

Homology

the similarity in characteristics, such as anatomical structures, genes, or developmental processes, between different organisms due to their inheritance from a common ancestor

Evidence of Homology in Animals

Sponges have choanocytes

Choanocytes look almost identical to choanoflagellates

Both use suspension feeding (filter feeding)

First Animals Debate 1: Porifera (Sponges) - Most Supported

Appear in fossil record >800 mya

Share traits with choanoflagellates

No true tissues

No nerves or muscles

Adults = sessile (immobile)

Larvae = motile (cilia, help organism move)

Important:

• have epithelium-like layers

• several interdependent cell types

• suspension feeders

• some have mutualistic cyanobacteria

(ancient, photosynthetic bacteria that generate oxygen)

**sponges have low oxygen needs —> could survive in early Earth conditions

First Animals Debate 2: Ctenophora (Comb Jellies) - Alternative Hypothesis

some say they evolved first because:

• they lack certain microRNAs found in sponges

  • RNA: single-stranded nucleic acid that is messenger between DNA & protein

BUT

• traits can be lost

• most molecular evidence supports sponges first

Evolution of True Tissues: Diploblasty

**two germ layers

Ectoderm

• skin

• nervous tissues

Endoderm

• digestive tract lining

• organs like liver

Phylum Ctenophora (comb jellies)

diploblastic

rows of comb-like cilia for movement

predators

stinky tentacles (not toxic)

some bioluminescent (emitting light living organism)

• they actively hunt, thus more complex than sponges

Phylum Cnidaria (Jellyfish, Corals, Anemones)

diploblastic

radial symmetry

true nerve net (no brain)

muscle-like tissue

Nervous System:

• diffuse nerve net —> allows coordinated contracting for swimming + prey capture

Two Body Forms:

• polyp (sessile, immobile)

• medusa (free-swimming, jet propulsion)

Unique Feature: Cnidocytes

• specialized singing cells

• contains toxic

• used for prey capture + defense

Coral & Symbiosis (living together of unlike species)

**most corals rely on dinoflagellates (photosynthetic protists)

Because of this

• most live in shallow water (need sunlight)

Coral Bleaching; caused by:

• warm water —> expulsion of dinoflagellates

• pollution

• over exploitation

• destructive fishing

**without symbionts —> coral turns white & may due

96% of reefs threatened

Cambrian Explosion (541 mya)

geologically rapid appearance of most animal phyla in fossil record

BUT:

• molecular data shows many phyla evolved earlier

So why no fossils before?

• early animals were:

  • small

  • soft-bodied

  • not easily fossilized

During this time period:

suddenly —> larger, complex, skeletonized animals appear

What caused Cambrian Explosion?

1. increase in oxygen

• due to cyanobacteria + algae

  • more oxygen = supports larger bodies + metabolism

2. new ecological niches

• as animals got bigger

  • new interactions formed

  • mew lifestyles evolved (paraistes, predators)

3. Coevolutionary Arms Races

**Early animals didn’t hunt

• then:

  • predators evolved

  • prey involved defenses

  • rapid evolutionary escalation

• two major consequences:

  • increased body size

  • increased skeletalization

Hox genes & Body Plans

Homeotic genes = determine body layout

Hox Genes

• specify structures along anterior-posterior axis (head-to-tail)

• control where limbs, segments form

Small Genetic changes —> huge body plan differences

Sponges

• no true Hox genes

gene duplication + divergence —> increased complexity

Deep Homology

when convergent traits evolve using homologous developmental genes

ex) tetrapod limbs & fish fins

correct logic: should have similar Hox genes with slightly modified activity

Homeotic Genes

master regulatory genes that determine the body plan of an embry

Homeosis

developmental phenomenon where one body part develops into the form of another body part due to changes in gene regulation.

Heterochrony

when something grows earlier, later, faster, or slower than it used to in evolution

Detritivore

An organism that feeds on dead organic matter, such as decaying plants and animals.

Examples: earthworms, millipedes.

They help recycle nutrients back into ecosystems.

Herbivore

An organism that eats plants only.

Example: deer, cows.

Carnivore

An organism that eats other animals.

Example: lions, hawks.

Omnivore

An organism that eats both plants and animals.

Example: humans, bears.

Parasite

An organism that lives in or on another organism (host) and benefits at the host’s expense.

Endoparasite

lives inside the host’s body (e.g., tapeworms).

Ectoparasite

lives on the outside of the host (e.g., ticks).

Suspension Feeder

An organism that filters small food particles from water.

Example: clams, baleen whales.

Deposit Feeder

An organism that consumes sediment and extracts organic material from it.

Example: earthworms, some marine worms.

Fluid Feeder

An organism that feeds by consuming liquid nutrients from plants or animals.

Example: mosquitoes, aphids.

Mass Feeder

An organism that eats large pieces of food (bulk feeding).

Example: lions, humans.

Parthenogenesis

A type of asexual reproduction where an embryo develops from an unfertilized egg.

Metamorphosis

A major change in body form during development from larva to adult.

Example: caterpillar → butterfly.

the whole process name

Larva (plural: larvae)

An immature form that looks very different from the adult and often has a different habitat or diet.

ex) caterpillar form when turning to butterfly

Viviparous

Offspring develop inside the parent and are born live.

Example: most mammals.

Oviparous

Organisms that lay eggs that develop outside the parent’s body.

Example: birds.

Ovoviviparous

Eggs develop inside the parent’s body, but offspring hatch internally and are born live.

The embryo is nourished by the egg yolk, not directly by the parent.

hatch eggs inside the mother's body, giving birth to live young without a placenta. This method combines egg-laying (oviparity) with live-bearing (viviparity)

***combination of both viviporous and oviparous

Example: some sharks and snakes.

Opisthokont

major evolutionary group (clade) of eukaryotes characterized by having a single posterior flagellum at some stage in their life cycle.

“rear flagellum”