Chapter 32Pages 724-748

Protostome Animals

Protostome AnimalsOne of two monophyletic groups of

bilaterally symmetric, (mostly)coelomate animals that share a pattern of embryological

development, including spiral cleavage, formation of the mouth earlier than the anus and includes

arthropods, mollusks, and annelids.

And that means????• Monophyletic groups:

– refers to any group of organisms that includes the most recent common ancestor of all those organisms and all the descendants of that common ancestor

Monophyletic, paraphyletic, & polyphyletic

Bilaterally Symmetric

• In bilateral symmetry (also called plane symmetry), only one plane, called the sagittal plane, will divide an organism into roughly mirror image halves (with respect to external appearance only).

• Often times leads to cephalization (head and body)

• Bilateral symmetry permits streamlining, favoring the formation of a central nerve center

• Bilateral symmetry is an aspect of both chordates and vertebrates

Bilateral symmetry

sagittal plane

Coelomate Animals

• Possess a body cavity between the gut and the body wall that is completely lined by mesoderm tissue.

• The coelom serves several important functions, such as:– providing space for the development and

function of complex internal organs

– providing a hydrostatic skeleton to facilitate the movements of soft-bodied animals.

• ***Not all protostomes are coelomates

Acoelomate and Pseudocoelomate

• There was a reversion of protostomes to complete lack of a body cavity by flatworms (Platyhelmenthes). This is called acoelomate.– Flukes, tapeworms, and other types of flatworms are

all acoelomate animals but ARE protostomes.

• Some protostomes have a false body cavity that forms between the mesoderm and the endoderm layers called a Pseudocoelomate– Because the cavity is not within the mesoderm and it

does not support organs it is not a real body cavity

– Protostome examples of pseudocoelomates are roundworms (Nematoda) and rotifers (Rotifera)

What is an Animal?

• An animal:

– Eukaryotic

– Heterotrophic

– Multicellular

– Generally mobile at some stage

– Highly evolved

– complex

Spiral Cleavage

• Spiral cleavage is characteristic of protostomes.

• Found in EARLY embryonic development of organisms

• A developing embryo has spiral cleavage if:– it undergoes cell division (cleavage)

– changes from a four-cell embryo to an eight-cell embryo, with the cells divide at slight angles to one another

– none of the four cells are in one plane

• Caused because mitotic spindles are at angles

Spiral cleavage

Spiral vs. Radial cleavage

Mouth First Development• During the gastrulation of protostome organisms,

the initial invagination that forms within the embryo becomes the mouth

• The coelom forms later in development when openings form within the blocks of mesodermal (middle) tissue

• This mouth first development leads into a head and brain development

• This leaves little time for advanced development of cerebral structures– This is theorized to cause lower cerebral functioning

and smaller brains all together

The winning group???

• The question may have came to you, “Why study protostomes?”

• Protostomes are widely the champions of the theory of evolution– Maybe not the champions, but they definitely took

the idea natural selection by adaptation to a new level by rapid levels of speciation

• There is remarkable number of different protostome species– 925,000 different arthropod species have been

formally identified (80 percent of the world's species)• It has been hypothesized that the actual # may be as high as

10 mil

WHY???• This remarkable monophyletic group not only has

easily the most species rich clade, but its overall numbers on this planet surpass even the imagination– Within one single acre of rural land in England an

estimated 17, 825, 000 beetles will reside

– At any time, it is estimated that there are some 10 quintillion (10,000,000,000,000,000,000) individual insects alive.

– Recent figures indicate that there are more than 200 million insects for each human on the planet!

– A recent article in The New York Times claimed that the world holds 300 pounds of insects for every pound of humans

• How is this even possible???????????????????????

One in a Quintillion

• Having the ability to not only have a tremendous number of offspring

• But also to have those offspring reach maturity rapidly

• Natural selection speeds up to a rapid pace comparable only to prokaryotes and viruses

• The rate of possible mutation increases as well allowing for overall speciation to increase

Tube Within a Tube• Most members of the protostome super-phyla

have a wormlike bodies – These bodies contain a basic tube within a tube

setup

– The outside is the skin which is derived from the ectoderm

– The inside tube is the gut, derived from the endoderm

– Muscles and organs derived from the mesoderm located between the two tubes

– In some organisms this becomes the hydroskeleton

Importance of Protostomes as Food?

• Shellfish such as clams, mussels, and oysters are consumed throughout much of the world

• Crustaceans such as shrimp, lobsters, and crabs make up some of the highest priced meet in the world. (26% of all marine fishery harvests)

• Snails are considered a delicacy in some countries, where they appear on menus as "escargot”

Protostomes in the Medical World• Leeches have been used for a wide variety of medicinal

purposes as far back as 2,500 years ago– Leeches secrete a substance that prevents blood from clotting,

and they can consume five times their own body weight in blood

– Currently they are being used in plastic and reconstructive surgery

• Studies of horseshoe crab eyes have led to treatments for human eye disorders

• Chitin forms the chemical chitosan that removes metals and toxins from water, and its fat-absorbing properties help remove fat and cholesterol from human bodies

• Mollusks extract compounds have promise as anticancer drugs.

• Compounds in the venom of cone shells are being considered as potential drugs for treating neurological disorders or acting as painkillers.

Medical leaches

Ecological Value of Protostomes

• Some species of protostomes, particularly those that burrow and feed in soft sediments, exert a major influence on the structure and functioning of their habitats

• These organisms have remarkably individual niches

• They make up the largest biomass on the planet and exist as primary, secondary, and tertiary consumers

• They make up the largest group of prey on the planet

Model Organisms

• Two of the most famous protostomes in the field of science are:

– Drosophilia melanogaster the fruit fly

– Caenorhabditis elegans the round worm

• Pronounced see-no-rab-DIE-tiss

• D. melanogaster and C. elegans are exceptional model organisms for a science lab because of slightly different reasons…

Drosophilia melanogaster• Since the early 1900s D. melanogaster has been a

key experimental subject for genetics beacause:– Its genome has been fully sequenced

– It is easily reared in a laboratory

– It is an inexpensive model organism

– Mating can be arrange within a controlled setting

– The life cycle is completed within two weeks

– Females lay large numbers of eggs

– Genetic defects can be seen in early embryonic development

– Insight into eukaryotic gene development and influence

Drosophilia melanogaster

Caenorhabditis elegans

• C. elegans emerged as a model organism in developmental biology in the 1970s because of Sydney Brenner and colleagues.

– Its cuticle is transparent, allowing for easy viewing of individual cells

– Adults have exactly 959 nonreproductive cells and

– The fate of each of those cells can be predicted

• If researchers examined a 64 celled embryo of C. elegans, they can predict the fate of EVERY one of those cells

» AND THERE IS MORE…

Caenorhabditis elegans and apoptosis

• Apoptosis is the term for programmed cell death– We all have (nonfunctioning) gills and webbing on our

hands and feet during late embryonic development within the womb

– The cells in this webbing and gill material go through apoptosis before birth, and we are born without them

• C. elegans has 959 adult cells… but 1090 embryonic cells… the 131 cells that are missing are programmed to die

• However… if a C. elegans is born with more than 959 cells it generally leads to cancer in adulthood…

• So this means cancer now has two definitions…

Caenorhabditis elegans

Lophotrochozoa and Ecdysozoa

• Molecular biology suggests that a major branching occurred within the protostome lineage creating the groups:

– Lophotrochozoa

• Mollusks, annelids, rotifers, and flatworms– All members grow by increment additions to their bodies

– Ecdysozoa

• Roundworms (Nematodes) and Arthropods– All members grow by molting

Lophotrochozoans

• Mollusca:

– Bivalvia (2 shells)

• Clams, Mussels, Scallops, Oysters

– Gastropoda (belly-feet)

• Snails, slugs, nudibranchs

– Polyplacophora (many-plate-bearing)

• Chitons

– Cephalopoda (head-feet)

• Squid, Nautilus, Octopus

Edysozoans

• The lineage Edysozoa is named for the distinctive molting done by all members of its group– Ecdysis is the Greek root which means to escape or

slip out of

• During a molt, individuals split their exoskeleton or cuticle and then slip out of it

• Their bodies are allowed to then expand, and a new cuticle or exoskeleton forms– Organisms are INCREDIBLY vulnerable to attack and

damage during a molt

Edysozoa

• Nematoda– Roundworms

• Arthropoda– Myriapods (millipedes and centipedes)

– Chelicerata (Spiders, ticks, mites, horseshoe crabs, daddy longlegs, scorpions)

– Insecta (insects)

– Crustaceans (Shrimp, lobsters, crabs, barnacles, isopods, copepods)