Echinoderms and Hemichordates

Echinoderms & Hemichordates

  • Most echinoderm  larvae develop a rudimentary calcareous endoskeleton and use ciliary tracts for feeding and swimming

  • Many echinoderms have jawlike pincers = pedicellariae on their body surface, often stalked and sometimes equipped with poison glands

  • Fossil record = bilaterally symmetrical → present = radial symmetry


Clade Abulacraria

  • Phylum Echinodermata

  • Phylum Hemichordata


Reminder →

  • Classical developmental characters associated with protostomes

    • Spiral mosaic cleavage

    • Formation of the mouth from the embryonic blastopore

    • Formation of a coelom by schizocoely, when a coelom is present

    • Examples: marine annelids, molluscs

  • Deuterostomes

    • Radial regulative cleavage

    • Formation of the mouth from a second opening

    • Coloem formation by enterocoely

    • All deuterstomes are coelomate

    • Example: Echinoderms


Echinoderms

  • Sea stars, brittle stars, and sea cucumbers

  • Share pharyngeal gill slits w/ chordates

  • 3 part (tripartite) coelom

  • Similar larval forms

  • Filtering structure = axial complex


Hemichordates

  • Acorn worms and pterobranchs

  • Share pharyngeal gill slits w/ chordates

  • 3 part (tripartite) coelom

  • Similar larval forms

  • Filtering structure = axial complex


Ambulacraria hypothesis: Predicts gill slits in the pharynx as an ancestral deuterostome character




Phylum Echinodermata

  • Echinodermata = derived from external spines/protuberances

  • Endoskeleton of large plates or small scattered ossicles

  • Water-vascular system

  • Pedicellaria

  • Dermal branchiae

  • Basic pentaradial symmetry in adults

  • 3 groups of echinoderms (sea cucumbers, and two groups of sea urchins) have secondarily evolved a superficial bilateral organization

  • Virtually all bottom dwellers, although a few pelagic species exist

  • No paraisitc echinoderms are known

  • Many animals made their homes in or on echinoderms, including parasitic or commensal algae, unicellular eukaryotes, ctenophores, turbellarians, cirripeds, copepods, decapods, snails, clams, polychaetes, fishes and other echinoderms

Sea stars

  • Commonly found on hard, rocky surfaces but numerous species live on sandy or soft substrates

  • Some are particle feeders, but many are predators, feeding on sedentary or sessile prey

Ophiuroids → Brittle stars/ serpent stars

  • Move by bending their jointed muscular arms, rather than by walking on tube feet

  • Some have a swimming ability and some burrow

  • Can be scavengers, browsers, deposit or filter feeders, or predators

  • Some are commensal in large sponges, where they live in their water canals

Holothurians → sea cucumbers

  • All seas

  • Many inhabit sandy/mucky substrates, where they lie concealed

  • Greatly extended in the oral-aboral axis → long body

  • Most are suspension or deposit feeders

Echinoids → Sea Urchins

  • Adapted for living on ocean floor

  • Keep their oral (mouth) surface in contact with substratum

  • Regular urchins 

    • Radially symmetrical

    • Feed mainly on algae or detritus

    • Prefer hard substrates

  • Irregular urchins

    • Secondarily bilateral

    • Feed on small particles

    • Sand dollars and heart urchins→ are usually found ON sand

Crinoids

  • Stretch arms outward and up like flower’s petals and feed on plankton and suspended particles

  • Most spend time on substrate, fastened by aboral appendages = cirri

Oldest Cambrian fossil echinoderm - Yanjahella biscarpa

  • Bilaterally symmetrical w/ two arms interpreted as feeding appendages

  • Unclear if it was a deposit-feeder or suspension feeder

  • Had a muscular stalk without skeletal plates

  • Middle Cambrian bilateral forms were benthic deposit feeders

  • Most echinoderm fossils are attached (sessile) forms w/ radial symmetry


→ Most echinoderms are unable to osmoregulate against a strong concentration gradient and rarely venture into brackish water

→ Spiny bodies = not often prey of other animals besides other echinoderms



Why are echinoderms used in developmental studies?

  • Sexes are separate

  • Both sexes release large numbers of gametes into the water for external fertilization



Class Asteroidea

  • 1500 living species

  • Shoreline → aggregate on rocks, muddy/sandy substrates, coral reefs

  • External features

    • Central disc that gradually merges w/ tapering arms

    • Covered with a ciliated, pigmented epidermis

    • Mouth is centered on the under/oral side, surrounded by soft membrane

    • Ambulacrum/ambulacral area → runs from the mouth on the oral side of each arm to the tip of the arm

    • Typically have 5 arms but can have more

    • Ambulacral groove → found along the middle of each ambulacral area, and groove is bordered by rows of tube feet (podia)

    • Tube feet are usually protected by movable spines

    • Large radial nerve can be seen in center of each ambulacral groove between rows of tube feet

    • Covered by ossicles/other dermal tissue → closed ambulacral grooves

    • Pedicellariae→ pincerlike tiny jaws manipulated by muscles

      • Keep body surface free of debris, protect papulae, and sometimes aid in food capture

    • Papulae:  = skin gills

      • Involved with respiration

    • Madreporite

      • Plate leading to the water-vascular system

      • On aboral side

  • Ossicles = 

    • Beneath the epiderms

    • Small calcareous plates bound together with connective tissue

    • Underneurological control

  • Coelom, excretion, and respiration

    • Body coelom filled with fluid

    • Exchange of respiratory fases and excretion of nitrogenous waste, occur by diffusion through the thin walls of papulae and tube feet

  • Water-Vascular System

    • Set of canals and specialized tube feet, together w/ dermal ossicles, form a hydraulic system

    • Primary functions → locomotion and food gathering, respiration, and excretion

    • Opens to the outside through small pores in madreporite

      • Madreporite leads to stone canal → to ring canal



Class Echinoidea: Irregular vs Regular

  • Regular echinoids

    • Sea urchins

    • Nearly spherical symmetry

    • Anus and mouth on opposite sides

  • Irregular

    • Sand dollars, sea biscuits, heart urchins

    • Bilateral symmetry

    • Anus posterior to mouth on edge of the test


Class Echinodermata: Class Holothuroidea

  • 1200 spp.

  • Soft bodied and vermiform

  • Ossicles highly reduced

  • Highly branched respiratory trees connected to anus

  • Oral-aboral axis elongated

  • Locomotory tube feet limited to 2-3 ambulacra

  • PEARLFISH use sea cucumbers as a home to live in

  • Trick: Evisceration and regeneration of internal anatomy

    • Evisceration → the act of removing an organisms internal organs, especially in the abdominal cavity

    • A defense mechanism to avoid being eaten by predators

    • Organs aact as a distraction and meal for predator, allowing for sea cucumber to escape

    • Organs can be toxic to predator

    • Help flush out parasites




Echinoderm Development

  • Separate sexes

  • Broadcast spawners

    • An organism that reproduces by releasing large quantities of eggs and sperm directly into the water column, where fertilization occurs externally, with no parental care involved

  • Bilateral symmetry in planktonic larval forms

  • Wide diversity in larval forms and strategies

  • Dramatic metamorphosis prior to return to benthos


Echinoderm Nervous System

  • Nervous system comprised of nerve ring surrounding gut

  • Radial nerves run along arms

  • Sensory neurons occur throughout ectoderm

  • Scattered ganglie along radial nerves, no centralized brain


Phylum Hemichordata

  • Gill slits now are deuterostome trait not just chordate

  • Rudimentary notochord but is not homologous to chordata notochord

  • Tripartite

  • Wormlife sea dwellers in shallow waters

  • Some live in secreted tubes

  • Most are sedentary/sessile

  • Share synapomorphy of stomochord

  • Stomochord formerly thought to be homologous to chordate notochord

    • Gene expression data suggest it is not

  • Stomochord projects from foregut into porboscis for support

  • Supports heart-kidney located in proboscis

  • Dorsal, sometimes hollow, nerve chord may be homologous to Chordata

Hemichordates are deuterostomes

  • Radial cleavage

  • Blastopore forms anus

  • Mouth forms secondarily

  • Coelom forms via schizocoely

    • Schizocoely = solid mass of mesodermal tissue splits to form secondary body cavities

  • Tripartite coelom

    • Protocoel

    • Mesocoel

    • Metacoel

Class Enteropneusta = acorn worms

  • Live in burrows or under stones, in mud/sand flats

  • Mucus-covered body → divided into three distinct regions:

    • Tonguelike proboscis

    • Short collar

    • Long trunk

  • Many are deposit feeders, extracting organic components of sediments while others are suspension feeders

Class Pterobranchia

  • smaller