Topic 25: Deuterostomia

Echinoderms

Marine deuterostome animals with a calcareous endoskeleton, water vascular system, and (usually) pentaradial symmetry in adults

Habitat of echinoderms

Exclusively marine, found from intertidal zones to deep ocean environments

Evolutionary origin of echinoderms

Evolved from bilateral ancestors during the Cambrian explosion and later developed secondary radial symmetry

Symmetry in echinoderms

Adults exhibit radial (usually pentaradial) symmetry, while larvae are bilaterally symmetrical

Pentaradial symmetry

Body organized into five repeating parts around a central axis

Exception to radial symmetry

Sea cucumbers exhibit superficial bilateral symmetry but retain internal pentaradial organization

Echinoderm endoskeleton

Internal skeleton made of calcareous ossicles embedded in dermal tissue

Ossicles

Calcium carbonate plates or spines that form the echinoderm endoskeleton

Function of ossicles

Provide structural support, protection, and contribute to fossilization potential

Water vascular system

Hydraulic system derived from the coelom used for locomotion, feeding, gas exchange, and sensory functions

Madreporite

Porous structure that regulates water entry into the water vascular system

Ring canal

Central canal in the disk that distributes water to radial canals

Radial canals

Canals extending along each arm that connect to tube feet

Tube feet

Fluid-filled appendages used for movement, feeding, respiration, and sensory perception

Mechanism of tube feet movement

Operate via hydraulic pressure controlled by ampulla and attach using suction and adhesive mucus

Gas exchange in echinoderms

Occurs through diffusion via tube feet and simple gills

Nervous system of echinoderms

Diffuse nerve network with a central nerve ring and radial nerves, lacking a true brain

Reproduction in echinoderms

Mostly sexual with separate sexes and external fertilization through broadcast spawning

Broadcast spawning

Release of gametes into water requiring synchronization and high gamete output

Asexual reproduction in echinoderms

Occurs via fission and regeneration in some species (e.g., sea stars)

Development in echinoderms

Indirect development with bilateral, planktonic larvae that undergo metamorphosis into radial adults

Asteroidea

Echinoderm class characterized by a central disc with 5 or more thick arms and tube feet on the underside

Key feature of Asteroidea

Predatory feeding using tube feet and eversion of the stomach to digest prey externally

Regeneration in Asteroidea

Ability to regrow lost arms and sometimes reproduce asexually

Ophiuroidea

Echinoderm class with long, thin, flexible arms distinct from the central disc

Key feature of Ophiuroidea

Movement primarily via arm rowing rather than tube feet

Echinoidea

Echinoderm class including sea urchins and sand dollars with no arms and a rigid test

Test (Echinoidea)

Fused ossicles forming a rigid external skeleton

Key feature of Echinoidea

Movable spines used for locomotion and protection

Feeding in Echinoidea

Herbivory using specialized jaw structure (Aristotle’s lantern)

Holothuroidea

Echinoderm class consisting of elongated, soft-bodied sea cucumbers

Key feature of Holothuroidea

Reduced ossicles and tube feet modified into feeding tentacles

Feeding in Holothuroidea

Deposit feeding or filter feeding using tentacles

Crinoidea

Echinoderm class including sea lilies and feather stars with feathery arms

Key feature of Crinoidea

Suspension feeding using tube feet on branched arms

Sessile lifestyle in Crinoidea

Many species attach to substrate and feed passively

Body plan diversity in echinoderms

Variation across taxa arises from differences in Hox gene regulation despite shared ancestry

Key synapomorphy of echinoderms

Presence of a water vascular system

Evolutionary evidence in echinoderms

Bilateral larvae support descent from bilaterally symmetrical ancestors

Fossilization of echinoderms

Enabled by calcareous ossicles forming durable endoskeletons

Sea star is placed upside down and cannot attach to substrate; which system is most directly impaired?

Water vascular system (tube feet cannot generate suction without hydraulic function)

An organism has bilateral larvae but radial adults; what does this indicate about its evolution?

It evolved from a bilaterally symmetrical ancestor and developed secondary radial symmetry

A marine organism has spines, no arms, and a rigid fused skeleton; which clade does it belong to?

Echinoidea

An echinoderm cannot regulate internal water pressure due to damage near its aboral surface; which structure is affected?

Madreporite

A student observes an echinoderm moving by rowing flexible arms rather than using tube feet; which clade is it?

Ophiuroidea

Two echinoderms differ: one has thick arms and moves slowly with tube feet, the other has thin arms and moves quickly; what explains the difference?

Asteroidea use tube feet for locomotion, while Ophiuroidea use arm movement

A fossil shows calcareous plates arranged in a radial pattern; why is this organism likely an echinoderm?

Presence of ossicles forming an endoskeleton supports echinoderm identification

An organism releases gametes into the ocean in synchrony with others; what reproductive strategy is this?

Broadcast spawning

Why would a mutation affecting Hox gene regulation change echinoderm body form significantly?

Hox genes control developmental patterning, so changes alter body plan organization across taxa

A sea cucumber appears bilateral externally; why is it still classified as an echinoderm?

It retains internal pentaradial symmetry and echinoderm features like a water vascular system

An echinoderm lacks a centralized brain but still responds to stimuli; how?

Through a diffuse nerve net with a nerve ring and radial nerves

A predator removes sea urchins from an ecosystem; what is the likely ecological outcome?

Increase in kelp due to reduced grazing pressure

A starfish pries open a clam and digests it externally; which structure enables this feeding method?

Eversible stomach

An echinoderm’s tube feet stop functioning; which processes are directly affected?

Locomotion, feeding, and gas exchange

An organism has tentacle-like structures around its mouth used for feeding; which class is most likely?

Holothuroidea

Why are echinoderm larvae planktonic while adults are benthic?

Larvae disperse for colonization, then settle and metamorphose into radial adults

An echinoderm species begins reproducing by splitting into two individuals; what advantage does this provide?

Rapid population increase without needing mates

A researcher finds an echinoderm with highly reduced ossicles and a soft body; what does this suggest?

It is likely a sea cucumber (Holothuroidea)

An echinoderm cannot exchange gases efficiently; which structures are likely compromised?

Tube feet and simple gills

Why is the water vascular system considered a defining feature of echinoderms?

It is unique to the phylum and used for multiple essential functions (movement, feeding, respiration)

A brittle star loses its arms to escape a predator; what biological ability allows survival?

Regeneration

An echinoderm is found attached to a substrate using feathery arms to capture food; which class is it?

Crinoidea

Two species share pentaradial symmetry but have completely different body shapes; what explains this?

Differences in developmental gene regulation (Hox genes)

An echinoderm fails to metamorphose properly and remains bilateral; what stage is disrupted?

Transition from larval to adult development

A mutation prevents formation of ossicles; what is the major consequence?

Loss of structural support and reduced protection