Week 10: Deuterostomes

what features of embryonic development do deuterostomes share?

the gut develops from posterior to anterior

the coelom develops from outpocketings of mesoderm

characteristics of phylum hemichordata

mostly acorn worms, mostly burrow-dwelling marine animals

adults are sessile deposit feeders/filter feeders, larva are pelagic plankton eaters

characteristics of phylum echinodermata

“spiny skin”

adults exhibit pentaradial symmetry, larval forms have bilateral symmetry

capable of regeneration, water vascular system

lack a head region, have a nerve ring

marine animals, about 7,000 species have been identified, abundant especially in deep water environments

five major lineages of echinoderms

asteroidea, ophiuroidea, echinoidea, crinoidea, holothuroidea

what is the major body plan of echinoderms?

radial symmetry in adults - capable of regeneration

an endoskeleton (hard protective and supportive structure located inside a thin layer of epidermal tissue) of calcium carbonate

a water vascular system (a series of branching, fluid-filled tubes and chambers that forms a hydrostatic skeleton) with tube feet (elongated, fluid-filled appendages, each consisting of an ampulla on the inside of the body and a tube-like podium projecting on the outside)

have a nerve ring (not central)

larvae are bilaterally symmetrical

how does the endoskeleton form in echinoderms?

during development through the secretion of calcium carbonate plates inside the skin

in some species, the plates fuse and form a rigid case

in some species, the plates remain independent and flexible (the tissue that connects the plates is reversibly stiff or flexible depending on conditions - catch connective tissue)

what forms of feeding do echinoderms use?

most methods of feeding, including mass feeding, suspension feeding, and deposit feeding

tube feet play a key role in obtaining food

keystone predator

predator that controls the populations of other species, impacting an entire food chain

characteristics of class holothuroidea (sea cucumbers)

no true brain, almost no sensory structures

modified tube feet form tentacles around the mouth

gas exchange via respiratory trees - located inside the anus/cloaca, cloaca pumps water in and out for respiration

defense mechanisms include shooting sticky tubules from their anus that typically release toxins

eviscerate themselves when stressed or threatened (digestive tract, respiratory tree, gonads)

characteristics of phylum chordata

all vertebrates are in this phylum, but not all are vertebrates

four morphological all share:

openings into the throat called pharyngeal gill slits, a dorsal hollow nerve cord that runs the length of the body, comprised of projections from neurons, a stiff and supportive but flexible rod called the notochord that runs the length of the body, and a muscular post-anal tail

what are the subphyla in phylum Chordata?

subphylum urochordata, subphylum cephalochordata, and subphylum vertebrata

characteristics of subphylum urochordata

also called tunicates or sea squirts

possess a protective “tunic” composed of cellulose secreted by the epidermis

both larvae and adults have pharyngeal gill slits that function in feeding and gas exchange

the notochord, dorsal hollow nerve cord, and tail occur only in the larvae or in sexually mature forms of motile species

mostly sexual reproduction, but some retain eggs for internal fertilization; some form large sessile colonies via asexual budding

larvae are motile, adults are sessile on the ocean floor, filter feeders

characteristics of subphylum cephalochordata

also called lancelets or amphioxus

small, mobile suspension feeders that resemble fish

adults burrow in sand in their ocean-bottom habitats

their dorsal hollow nerve cord runs parallel to a notochord which stiffens the body and muscle contractions on either side result in fishlike movement

sexual reproduction with separate sexes; external fertilization

what do the chordate characteristics become in vertebrates?

the pharyngeal pouches present in embryos develop into gills in aquatic species, but not in terrestrial species

the notochord becomes the vertebral column

the dorsal hollow nerve cord becomes the spinal cord

what are arcualia in lamprey?

cartilaginous structures surrounding the notochord

what is the role of the notochord early in development of vertebrates?

helps organize the body plan by secreting proteins that induce somite formation (segmented blocks of tissue that later differentiate into vertebrae, ribs, and skeletal muscles)

what are the two synapomorphies that distinguish vertebrates?

a column of cartilaginous or bony structures called vertebrae, which form along the dorsal side of most species (protects the spinal cord)

a cranium - a bony, cartilaginous, or fibrous case that encloses the brain (protects the brain and sensory organs)

how is the vertebrate brain unique from other brains?

divided into three regions:

forebrain which houses the sense of smell

midbrain which is associated with vision

hindbrain which is responsible for balance and hearing

how did the vertebrate brain evolve in jawed vertebrates (gnathostomes)?

the hindbrain consists of enlarged regions called the cerebellum and medulla oblongata

part of the forebrain became elaborated into a large structure called the cerebrum, especially in birds or mammals

what were the first vertebrates?

earliest fossils are in the Chengjiang formation of China and Burgess Shale in Canada (dated to the Cambrian explosion)

earliest members lived in the ocean about 520 mya

what is the hagfish hypothesis for early vertebrates?

hagfish and lamprey are thought to have branched off from the lineage that led to jawed vertebrates

these two may share a more common recent ancestor than other vertebrates

hagfish lost their vertebrae secondarily (evidence against hagfish being primitive precursors to other vertebrates)

what are the four major lineages of jawed fishes?

cartilaginous fishes (chondrichthyes), ray-finned fishes (osteichthyes), coelacanths, lungfishes

what is the fossil evidence for the origin of the jaw?

cartilaginous fishes (sharks and rays) have jaws made of reinforced cartilage rather than bone, fossil evidence disproved shark-like origin of jawed fishes, pointing instead to jawed fishes that show up early in the Silurian (~430 mya)

armored fishes, including placoderms, had heads covered with bony shields

after the appearance of jaws, teeth appear in the fossil record

evolution of jaws is significant because it improved the ability of fishes to capture and kill prey (no longer limited to suspension or deposit feeding)

other key traits are paired fins, internal fertilization

what is the gill-arch hypothesis for jaw evolution?

proposes that natural selection acted on developmental regulatory genes that determine gill arch morphology (curved regions of tissue between the gills)

mutation and natural selection increased the size of the most anterior arch and modified its orientation slightly, producing the first working jaw

expression of developmental regulatory genes (like Hox genes) are similar in jaws and gill arches

what is a protrusible jaw? (ray-finned fishes)

the jaw can be extended to bite at food

how have jaws been modified?

several lineages of ray-finned fishes have a second specialized set of jaws called a pharyngeal (throat) jaw

modified gill arches located in the back of the throat

makes food processing more efficient