Week 4: Echinoderms

How many species of echinoderms are there?

• 7300 extant species

• 15,000 fossil species

What characteristic of echinoderms is different to any other invertebrate group?

all marine

Where do echinoderms live?

• shallow + deep water

• adults mostly benthic - don’t swim with a few exceptions

• sessile, embedded in mud as suspension feeder

• larvae pelagic

• constitute 95% of biomass in deep ocean → cosmopolitan in deep ocean

Echinoderms diverse lifestyles

• predators

• detritus feeders (dead organic matter)

• filter-feeders

• scrape algae from rocks

What clades is the phylum Echinodermata in?

• Echinoderms are bilaterians despite pentaradial symmetry

• They are also deuterostomes

• A part of ambulacraria with hemichordates

• sister group to chordates

Characteristics of Deuterostomes + Echinoderms

Deuterostoma	Echinodermata
Bilateral symmetry	Secondary pentraradiate symmetry
Complete gut	yes
Coelomates	yes
Gill slits	lack of gill slits (some fossils do have primordial gill slits)→ lost in modern Echinoderms
Post anal appendage (tail)	lack (some fossils do have it)

What is a coelom?

• body cavity

• similar to a peritoneum in humans

When did modern echinoderms with mineralised skeletons arise?

present in the early Cambrian (543-490 Mya)

Molecular data strongly support Deuterostomia clade

• only ~5000 genes in common with protostomes (D.melanogaster and C. elegans)

• Materna et al., Dev Biol. 2006

• using Reciprocal BLAST - basic local alignment tool → uses an algorithm to find parts in common

Ambulacra

holes where tube feet get out

Echinodermata systematics

• 5 major classes of echinoderms

• Pelmatozoa (Crinoidea)= outgroup of all other classes

• This is not disputed→ positions based on fossil record and some molecular data

Evidence

• Position based on fossil record and some molecular data (18S and 28S, Littlewood et al. 1997)

• Development and other characters (Swalla and Smith, 2006)

• Two hypothesis on how Eleutherozoa are related: Asterozoan vs. Cryptosyringid hypothesis

Asterozoan hypothesis

• proposes that sea stars (Asteroidea) and brittle stars (Ophiuroidea) are sister groups, forming a clade called Asterozoa, within the broader group of echinoderms

Cryptosyringid hypothesis

• suggests ophiuroids are more closely related to sea urchins and sea cucumbers, forming the clade Cryptosyringida

• asteroidea as an outgroup

Asterozoan vs. Cryptosyringid hypothesis

• Phylogenomic studies supports Asterozoa clade (starfish and brittle stars as a group together)

• Telford et al 2014 → study comprises 219 genes from all echinoderm classes

Echinoderm fossil record

• only 5 crown groups

• many species completely extinct

• end Permian mass extinction → bottleneck seen in surviving animals→ only 5 classes we have survived

• excellent fossil record due to mineralised skeleton

• entire rock formations made of echinoderm fossils in Carboniferous and Jurassic

  • encrinite = rock made of crinoids

Symmetry in echinoderms

• pentaradial symmetry

• evolved from triradiate and (Helicoplacoids) and bilateral (cinctans, solutes) forms

Calcitic Skeleton

• composed of ossicles called stereom

• endoskeleton (not a shell→ different to molluscs)

• endoskeleton arising from mesodermal tissues and covered by epidermis + grows internally

• cells reside in the holes and secrete the skeleton

• stereom like a single calcite crystal (CaCO3 + 5% of MgCO3) → percentage of MgCO3 depends on where the animal lives

• The pores are populated by dermal cells and fibers (stroma).

• 0.1% organic matrix protein.

• Birefringent optical properties.

• embedded in soft tissues or fused together

Birefringent optical properties of ossicles

double refraction of light

Pentaradial body organisation in adults

• Central disc and five set of body parts.

• made by 5 radii - bifurcated/ branched

• Five-fold organization of skeleton and most organ systems

• Unique motility despite the radial symmetry→ directional movement

• Body orientation: Madreporite (opening of water vascular system) and radii.

Orientation of features of body plan

• Ambulacral A = opposite to Madreporite

• top - aboral → opposite to mouth

Functions of the water vascular system

locomotion, respiration, feeding, sensory perception- can function as a hydrostatic skeleton

Is the water in echinoderms affected by its surroundings?

liquid in water vascular system is in equilibrium with sea water (not that it is sea water) i.e if salinity of seawater changes, as does the salinity of water in the system

Features of a water vascular system

• Fluid-filled canals branching from a ring canal.

• The canals lead to podia (tube feet) arranged along branches (ambulacra)

• Tube feet are sucker-like appendages

• Tube feet are extended and retracted by hydraulic pressure

• Embryological origin from coelom (left mesocoel)

• Fluid similar to sea water and cells (coelomocytes) circulated by cilia

• stone canal = completely sclerotized

Madreporite

• also called a sieve plate

• in communication with external part

Sea Urchin Water Vascular System

• tube feet→ movement/ attachment to substrate

• movement by hydrostatic pressure

• extension of tube foot → muscle contraction

• thousands of tube feet can be coordinated for movement

Hemal and excretory system

• Derived from coelom.

• Fluid moved by cilia and muscle pumping.

• Oral and aboral ring connected by axial sinus.

• Axial gland which produces some coelomocytes.

• Extend to the gonads.

• Gas exchange occurs in the dermal gills or papullae.

• Excretion and ions exchange through podia and papullae.

• Epithelia super-ciliated → cilia allows water circulation (like how we have hearts)

• Hemal system more linked to immunology

• ^ runs next to stone canal

Nervous System

• NS organised with pentaradial symmetry

• no clear anteroposterior axis

• lack of cephalisation - cephalisation = common of bilaterians

• Central (CNS) and peripheral (PNS) nervous system

• Radial nerve cords (RN) run under each of the ambulacra (cell bodies of motor neurons and interneurons) (nerve cord = like our spinal cord)

• A nerve ring (NR) connects the radial nerves.

• Radial nerve cord underneath water vascular system

• Radial nerve cord has a system similar to our spinal cord → interneurons, neuron junctions etc.

Sensory Organs

• Sensory neurons respond to touch, chemicals, light and water current.

• Located primarily within the ectoderm of podia and send axons to the radial nerves. Mostly a nerve-net.

• No centralized sensory organ

• Diffuse light sensing

• Skeleton important part of the visual system→ skeleton that forms as a lens

• tube feet essential for light sensing → have photoreceptors (sea urchins → Ullrich-Lüter et al 2011)

• ‘eye’ at end of arms

Pedicellariae

Pincer-like structure produced by the skeleton

• Responds to stimuli independently of main nervous system

• Neuromuscular reflex

• Defense, predation, hold object for camouflage

• have different forms

• aboral side- exposed to outside world

• suicidal? structures → some contain venom

Reproduction and Life Cycle

• No sexual dimorphism

• Separate sexes → very rarely are they hermaphrodites

• External fertilization → shed (many) gametes in water column

• Synchronize the reproductive activities

• Indirect development.

• Bilaterally symmetric ciliated larva (pluteus, dipleura etc.)

• rapid development

• larvae part of zooplankton

How long does the larvae state last in echinoderms?

• tropical water species can have adult in a few weeks

• temperate areas → larvae state can in the ocean for a year → can divide and produce more larvae (asexual reproduction)

What happens to the bilateral symmetry of larvae?

young adult emerged only from left rudiment (part) of larvae → changes the axis

Sea Urchin Development

• radial cleavage

• first opening = anus

• second = mouth

• larvae= bilateral symmetry

• Only left coelum → adult (in outgroup Hemichordates- adult from both left and right)

• Left and right plane of symmetry changes due to this→ new body plan

• Rapid metamorphosis

Metamorphosis in sand dollar, Dendraster excentricus

• develop inside -out

• absorbs larva

What differs the 5 classes of echinoderms?

• Orientation to the substrate and disposition of ambulacrial surface

• where the ambulacra (where tube feet get out) is in relation to the substrate defines each class

What animals are crinoids?

Sea lilies and feather-stars

Features of stalked crinoids

• mostly live in the deep ocean (more than 500 m below sea level) attached to the bottom

• Living fossils.

• All crinoids are passive suspension feeders.

• Direct development.

• U-shaped gut - mouth and anus on same surface

Pelmatozoa

crinoidea

Eleutherozoa

• asteroidea

• ophiuroidea

• holothuroidea

• echinoidea

Crinoidea fossil record

• Most abundant and important of fossils of Paleozoic and Mesozoic.

• 5000 fossil species.

• 85% of extant crinoids (540 species) are unstalked feather stars → probably just a small percentage of reality

• From Articulata

• modern sea lilies from bottleneck at end-Permian

• Crinoids divided into stalked and un-stalked

• diversity drop (ME) across Permian Triassic boundary

• mostly sessile crinoids in the past

• now can attach to substrate + release themselves

Anatomy of stalked crinoids

• Cuplike body bearing five usually branched and commonly featherlike arms → helps with suspension feeding

• Stalk made of discoidal skeletal pieces columnal. Central canal containing coelomic and neural tissue.

• Small side branch (pinnule) on alternating sides of successive ossicles along the arms.

• Comatulida have hook-like cirri.

• Comatulidae is the most common and the only shallow water

• Modern comatulids reduced the stalk

• stalk can be re-absorbed - holding apparatus - cirri

• arms are mobile - contain ossicles and muscles

• crinoids can swim by moving arms different directions

How many extant sea stars are there?

1600

Features of Asteroids

• Central disc and multiple (typically 5) radiating arms.

• No sharp demarcation between arms and central body → sometimes can’t identify arms

• Extension of a large coelomic cavity from the central disc into the arms (the gonads and pyloric caeca).

• Move using tube feet.

• Most are predators. Feed on sessile/ benthic or slow-moving prey (e.g. mollusks, corals).

• Many are able to extrude their stomachs out and digest food outside of the body→ digestion in situ

• problem for coral reefs

• when sea stars metamorphose they have 5 arms - then they can branch into more or more are developed- arms can change after regeneration→ still pentaradial

Asteroid phylogeny

• still no clear understanding on how the families relate to each other→ species + family phylogeny not well resolved

• new phylogenies every year show different relationships

• problematic - hard to identify appearance/ disappearance of important characters without phylogeny

What animals are ophiuroids?

• brittle stars

• basket stars

Features of ophiuroids

• Ophiuroids rapidly fall to pieces after death, and are rarely preserved whole (i.e brittle) → arm ossicles just attached by muscles

• ^ slightly worse fossil record/ less articulated fossil record

• Five long, flexible arms and a central, armored, disk-shaped body, up to 60 cm- strong demarcation of central disc from arms

• Arms do not have gonads, no stomachs

• Basket stars arms are very highly forked and branched.

• Dominant in many parts of the deep sea.

• Are scavengers and detritus feeders, prey on small live animals such as small crustaceans and worms, filter-feed on plankton with their arms (basket stars)

• move very fast

Anatomy of ophiuroids

• The disk contains all of the viscera.

• Calcite ossicles are fused to form armor plates, tests.

• Gas exchange and excretion occur through cilia-lined sacs called bursae.

• Move rapidly by wriggling their arms.

• The arms are supported by an internal skeleton plates that look like vertebrae (vertebral ossicles) - resemble our vertebrates- convergent evolution - allows for freedom of movement

• These are moved by a system of muscles and linked together by ball and-socket joints.

Bursa- ophiuroids

• out pocket of skin

• gas exchange organ - different to starfish

• Bursa lined with cilia

• In the bursa- opening of the gonad

What is unique about ophiuroid offspring?

ophiuroids produce fewer offspring but offspring produced are adults

Which animals are echinoids?

• sea urchins

• pencil urchins

• sand dollars

• heart urchins

How many species of echinoids are there?

940

How many species of ophiuroids are there?

1600 species

When did echinoids first appear

in the Ordovician (~450 Mya)

Features of echinoids

• Regular echinoids, with nearly perfect pentameral (five-fold) symmetry; and irregular echinoids with altered symmetry (secondary bilateral)

• Skeleton is almost always made up of tightly interlocking plates that form a rigid structure or test

• Covered with movable spines

• tube feet made of soft tissue so is degraded quite quickly → just holes left

What is unique about sand dollars?

the mouth is at the surface but the anus is lateral

What is unique about pencil sea urchins

spines remain when they die- in regular sea urchins they are movable

Echinoid anatomy

• Five conspicuous gonads arrayed interambulacrally

• Mainly herbivores (algae), but can graze on anything: plant, animal, rock

• Parallel to the intestine is the siphon

• Only soft part of skeleton is the peristomial membrane

• Unique jaw apparatus known as Aristotle's Lantern that can protrude from the mouth.

Aristotle’s Lantern

A lantern is composed of:

• five pyramids in the interambulacral space

• each pyramid is formed of two hemi-pyramids which support an elongate tooth.

• Five teeth constantly growing (like rodent teeth!)

• A number of smaller structures (Epiphysis) and muscle

• tooth attached to muscles

• peristomal membrane = only soft part of the animal

Irregular echinoids

• Includes sand dollars and sea potatoes (heart urchins)

• Secondary plane of bilateral symmetry

• Anus has migrated in evolution such that anterior posterior axis is oriented parallel to the ground

Which animals are holothuroids?

sea cucumbers

How many species of holothuroids are there?

1400 species

Features of Holothuroids

• The oldest sea cucumber spicules are from the Ordovician (~460 million years ago).

• Nearly every marine environment.

• Generally long and wormlike.

• Five rows of tube feet running from the mouth along the body.

• Skeletal plates are reduced to microscopic spicules (softbodied).

• Several species can swim

• sea cucumbers used in food + pharmaceutical industries → anticoagulant produced by sea cucumbers

Holothuroid Anatomy

• Calcareous ring that encircles the pharynx or throat.

• This ring serves as an attachment point for muscles.

• Circlet of oral tentacles.

• Madrepore opens into the coelom.

• Respiratory trees used in gas exchange are attached to the rectal area- specialised organ- attached to aboral/ anus → only class with specific gas exchange organ

• madreporite mostly internalised but still there

• convoluted + well-developed intestine

How did holothurians evolve?

from an echinoid-like ancestor

Echinoderm Regeneration

What can echinoderms regenerate?

• Complete regeneration of missing body parts

• Regenerate missing limbs, arms, spines - even intestines → matter of days/ weeks months → depends on cold/ warm water

• Some brittle stars and sea stars can reproduce asexually by breaking a ray or arm or by deliberately splitting the body in half

• Each piece then becomes a whole new animal.

• Even the larva has regenerative properties and can reproduce asexually

• Regenerate: intestine, skeleton, nervous system, gonads …..

• Generation of a wound, attraction of coelomocytes, proliferation, patterning of the new structure.

Defense mechanism: brittle stars + sea stars

can easily break their arms to escape predation (autotomy= self amputation)

Regeneration: sea urchins

regenerate skeleton and pedicellariae (Suicide structures)

Sea cucumber defense mechanism

when attacked they eviscerate the so called Cuvierian threads which are toxic (holothurin)

Sea Urchin Immune System

• Recognition of non-self and damage.

• Extensive cell-cell communication.

• Innate immunity.

• Formed by many different cell types (Coelomocytes).

• Respond rapidly to pathogen and wound.

• Sophisticated repertoire of immune molecules and large. expansion of pathogen recognition molecules. They also have the machinery for adaptive immunity.

• EchinochromeA (thought to be related to its ability to scavenge free radicals and reduce oxidative stress), antibacterial.

Hibino et al, 2006

Echinoderm Life Span

• Red sea urchin (Strongylocentrotus franciscanus) can live up to 200 years (estimate on the jaws (Ebert 2004)).

• No reduction in growth.

• No reduction in reproductive capacity (increase!)→ other organisms have peak maximum growth/ reproductive ability

• “Negligible senescence” Senescence: change in survival probability, physiological (reproduction capacity), reduced cellular function.

• Better maintenance and repair mechanisms, both at cellular and organismal level.