Biodiversity – Oct. 17/12
- The phylum arthropoda includes crabs, lobsters, shrimps, spiders, ticks, mites, scorpions, horseshoe
crabs, millipedes, and centipedes.
- What is an arthropod?: Arthropods are animals with bilaterial symmetry. They have n exoskeleton
(synapomorphy). They have a segmented body with segments grouped into functional units. They
have jointed appendages and many pairs of jointed limbs. They undergo metamorphosis with
different stages – egg, immature (nymph/larva), adult. They are the most diverse group of organisms
on the planet.
- Exoskeleton: Is an external skeleton and is the entire outer surface of the animal. Muscles are
attached on the inside. It is chitinous. It is formed of chitin (a structural polysaccharide) and sclerotin
(proteins in a matrix). This makes it strong and flexible. The exoskeleton is continuous, but
segmented – hard rigid plates are connected by softer, flexible material (joints). It is periodically shed
and renewed as the animal grows (metamorphosis).
- Metamorphosis in arthropods is a change in form during postembryonic development and growth.
When an animal outgrows the current exoskeleton, first the new one forms underneath the existing
exoskeleton. Then molting occurs – a process of shedding the old outer exoskeleton. There can be no
change or very little change in form from one molt to the next (ex. millipedes), or there can be drastic
change in form, as is the case with most insects. Growth occurs just after molting but before
hardening. It is a hormonally-controlled process of growth. Due to the exoskeleton, arthropods
cannot grow without shedding the old exoskeleton and growing a new one. The new exoskeleton is at
first soft and flexible and expands before it hardens. The exoskeleton becomes hardened when
hormones are released.
- Animalia (metazoan) evolution major events: Eukaryotic cells were present 2BYA (Proterozoic).
Animal fossils were present 630MYA and they are relatives of today’s jellyfish and sea anemone.
Green algae and probably fungi were present (late Proterozoic). During the Cambrian period (began
550MYA), the Cambrian explosion saw a relatively rapid appearance of most major phyla (fossil
record) in oceans (early Paleozoic). Arthropod fossils from Trilobites were also present at this time.
Ordovician period (began 480MYA) saw animal diversification with vertebrates in oceans and
invertebrate animals (ex. arthropods) colonizing land (along with plants and fungi). Silurian period
(began 440MYA) – On land, invertebrate animals diversified (along with plants and fungi). Devonian
period (began 420MYA) – There was continued diversification on land, fins evolved into four limbs,
amphibians were the first land vertebrates but they were dependent on water for reproduction.
Carboniferous period (began 360MYA) – The amniotic egg and amniota developed, allowing land
vertebrates to become totally adapted to life on dry land. Permian period (began 300MYA) – The
End-Permian mass extinction resulted in 95% of life going extinct. This was mostly marine
invertebrates and insects, and species and higher taxa, but not phyla (ex. arthropoda). Trilobites
went extinct. Mesozoic period (began 250MYA) – There was animal diversification in oceans and on
land, and coevolution with angiosperms (flowering plants).
- Trilobites: Are the only extinct major group of arthropoda. They became extinct during the End-
Permian mass extinction. They were the most successful of early animals and were present for 270
million years in oceans (shown by fossil record).
- eukaroytes – animals (metazoan) – bilateria – arthropoda
- Major groups of extant arthropoda:
- Millipedes and centipedes: Retain basal characters – have head with one pair of antennae and an
undifferentiated body. The antennae are sensory appendages of the head and their primary function is olfaction. Millipedes are scavengers. They have basic chewing mouthparts (mandibles). Centipedes
are predaceous. Their mouthparts are modified into fangs with venom.
- Crustaceans: They have two pairs of antennae and two body regions (abdomen and cephalothorax).
They have mouthparts and mandibles. They have five pairs of walking legs and legs modified for
swimming. There are 50,000 species of crustaceans described worldwide. They are mostly marine
and are essential in marine food webs (ex. krill). Human consumption of crustaceans is 10 million
- Horseshoe crabs: They have blue blood because they contain hemocyanin. Hemocyanin carries
oxygen in their blood. It contains copper and it turns their blood blue. Horseshoe crabs are found in
shallow ocean waters. There are four extant species, but they are in decline. They are called liv