1. What are cerrata?

Specialized evaginations of the digestive system that extend through the body wall and house kleptocnidae in nudibranchs.

2. What is a pneumostome?

This is the opening into the mantle cavity of pulmonate gastropods whereby the mantle cavity serves as a lung.

3. What is the difference between an iridiophore and a chromatophore?

Both are found in cephalopods. Chromatophores appear in the most superficial layers of the epidermis and give color to brown, red, black are the usual colors. Under nervous system control and direct musculature

Iridiophore are cells that lie underneath the chromatophores and reflect incidental light intercepting them. The reflector portion of the cell is oriented at about 10 degrees to parallel of the incidental light and gives cephalopods a blue, silver, or green appearance from the reflected light.

4. What is the difference between a "direct" and "indirect" eye?

Receptor cells face away from the light source, direct has better resloution and the retinal cells or rhabosomes face toward the light with the impulse conducting neurons behind them.

Mollusc Evolution and Origin

I’ve mentioned in various groups about debates as to which invertebrate phyla are related to which and that taxonomists and cladists argue about these kinds of issues. I’d like to address one of these debates in a little more detail

There are really two camps with respect to mollusc phylogenetic affinities.

One group believes molluscs are related to annelids

The other forwards the idea that flatworms are their closest relatives.

The only agreement either group has with each other is that whatever they are related to-they have evolved independently for a considerable period of time.

The diversity of form molluscs have has obfuscated which are primitive and which are derived molluscs. This has caused problems with finding an anscestral group.

First, lets consider why molluscs are believed to be derived from annelid anscestors in the first place.

1) The presence of metamerism among some primitve molluscs.

2) The presence of a true coelom

3) Protostomate condition-like annelids, including the "molluscan cross"

4) The trochophore larval form

The arguments for a flatworm relative is

1) Flatworms exhibit spiral cleavage like molluscs (although so do annelids)

2) Molluscs exhibit locomotion through ciliary gliding and use mucous-like many flatworms.

3) Both flatworms and molluscs make extensive use of body musculature to move (including dorsoventral and oblique muscles)

It seems as though the weight of evidence is in favor of annelid relations-but lets consider these arguments in more detail-

SHOW MONOPLACOPHORA and POLYPLACOPHORA (PAGE 253 WILMER)

These are considered to have metamerism-the monoplacophorans. In reality this mollusc may be regarded as pseudometameric.

Why-

Neopilina galatheae has two pairs of atria, five pairs of gills, six pairs of excretory organs, eight pairs of pedal retractor muscles, and ten pairs of nerve commissures. Other species of monoplacophorans have different repetitive sequences.

Other "metameric" groups such as the chitons always have eight shell plates covering the dorsal surface-with eight pairs of retractor muscles associated with them. The gills repeat-but are staggered and not really paired and nerve commissures repeat out of phase with the shell plates and muscles suggesting not true metamerism.

During embryogenesis, the repetition of these structures isn’t readily observable and then a reduction in number result-as is the case in arthropods and annelids. Instead, the paired structures appear de novo and are not synchronized or in phase with each other. The fact that the nerve commissures don’t match up with anything-also suggests not true metamerism-but convergence with annelids-and a derived, not primitive condition.

Also if you consider how most molluscs move-through ciliary gliding, a divided segmented body is not conducive to this locomotory pattern.

The other argument is that they are protostome coelomates like annelids and arthropods.

They do have spiral determinate cleavage, a primary mouth formed for the blastopore, a schizocoelic coelom, and a trochophore larva.

Since nemerteans and flatworms also have spiral cleavage-this alone doesn’t constitute evidence for phylogenetic relatedness

SHOW OVERHEAD PAGE 256

Some embryological arguments are for the presence of a molluscan and annelid "cross" by micromeres during development.

The cross is derived from different micromeres in each case however and has a different position.

As for schizocoely, yes molluscs do have a split in the mesoderm but unlike annelids and arthropods-this isn’t derived from a broad and large open coelom. that is then reduced in the case of arthropods.

Instead, only a part of the mesoderm, much later in development, splits and gives rise to the pericardial space. Thus the annelid and mollusc embryology is not homologous.

Flatworm relative proponents assert that molluscs are coelomate, but instead of being derived from the hydrostat of molluscs, have had the solid mesenchyme of flatworms undergo cavity formation as a means of providing sinuses for fluid transport.

The pericardial sinus, although surrounded by mesoderm, did not arise through the same pattern as is found in annelids and arthropods-but differently-suggesting convergent evolution and a functional necessity to have a space around a heart in order for it to beat properly. In this sense-there is no embryological evidence to presume common anscestry. Excretory structures generally demand being surrounded by a fluid filled cavity of some sort-either blood or coelomic fluid-since they need to filter something liquid-so it isn’t surprising that coelomic cavities surround the excretory structures as well.

Spiral cleavage, observed by molluscs, is also observed in some flatworms, nemertineans, and other groups.

There is also a functional reason for considering flatworms as mollucan ancestors-

They both move by cilia and mucous secretions.

What about the trochophore larvae that is shared with annelids-this is dismissed as convergent evolution based on similar selection pressures for dispersing forms. Also, the molluscan trochophore lacks one of the bands of cilia, apical ocelli, and protonephridia found in annelid trochophores.

Of course this means the through gut, fully functional kidneys, heart, blood vessels, and other internal organs must have arisen through convergent evolution-a rather tough nut to swallow.

And it is parsiomony that annelid phylogeny is based.

I suggested that the process of arthropodization-or the formation of suites of adaptive characteristics are the result of a single adaptation-the exoskeleton-and subsequent diversification.

So it may be said with molluscs, that the formation of the shell gave rise to a suite of adaptive characteristics and changes to accomodate this functional shift.

No matter what the reason, it is very likely that animals with shells had sizable advantages over those without.

What are the functional consequences of having a shell?

1) If respiration occurs through the general body surface, then gills are necessary since the shell reduces available surface area.

They would also probably be situated posteriorly or laterally to take advantage of hydrodynamic properties as the mollusc moves through the environment.

Given the size of the mollusc, the tissue beneath the shell, still wouldn’t be provided enough oxygen-thus necessitating additional circulating fluid.

If molluscs come from a flatworm ancestor, then they must acquire a through-gut and a means of distributing the foodstuffs to the rest of the body-thus a circulatory system of some type is a necessity.

From these evolutionary hagglings of taxonomists emerged four general theories-

The turbellarian theory, just discussed, suggests that acoelomate turbellarian flatworms were the ancestors of modern molluscs.

The modified turbellarian theory suggests that annelids and molluscs were derived from a common stem with its root in the turbellarian taxon.

The coelomate theory suggest that annelids and molluscs share a common coelomate ancestor

And the annelid theory that they were derived from annelids themselves.

I believe the modified turbellarian, or coelomate theory to be the most tenable of the four.

SHOW FIGURE 10.1 WILLMER PAGE 250

Whatever the pattern of evolution-it is clear that the shell has been an impetus for diversification and evolutionary success among molluscs as the exoskeleton has been for arthropods.

SHOW OVERHEAD PAGE 762 BRUSCA-PHYLOGENY

1) Pentaradial symmetry with oral and aboral surfaces

2) Coelomate Deuterostomes

3) Water vascular system

4) Circulatory system of coelomic sinuses or absent

5) No excretory structures

6) Calcareous endoskeleton with living tissue surrounding individual calcite crystals

7) Complete throughgut, but secondarily lost in some

8) Nerve system diffuse nerve net or centralized nerve ring

9) Mutatability of connective tissue

10) Mostly dioecious, some asexual reproduction

Echinoderms represent a smaller group than the molluscs, only about 7,000 species total. They are the sea stars or starfish, sea cucumbers, sea urchins, sand dollars, the feather stars and basket stars, and brittle stars.

All are marine, although a few are found in brackish water. They are common and widespread in the world’s oceans however. The vast majority are benthic with only a few pelagic varieties. They are predators-especially the sea stars, herbivores-especially the sea urchins, deposit feeders-sea cucumbers, and filter feeders, the crinoids. Various other groups may be scavengers as well.

They are all strictly marine.

Every phylum is unique, but echinoderms are downright bizarre compared to most other phyla. There are very few features that appear to link them easily to any other taxonomic group.

1) The fact that "star" is in so many of the names of these animals belies there unique form of symmetry-pentamerous or pentaradial symmetry. Five points or areas extending from a central axis. This pattern is superimposed on an oral and aboral axis-with most echinoderms having the mouth oriented downward. Some echinoderms like the sea cucumbers have undergone a secondary bilateral symmetry. It is strange that sea stars represent one of the few predatory and mobile animals that are exceptions to the rule when it comes to radial or bilateral symmetry. They feed on slow moving prey like bivalves and cnidarians-but there symmetry patterns may represent a movement from some filter feeder.

Some starfish have more than five stars-so pentaradial symmetry is not a strict requirement of the phylum.

2) They are deuterstomes, and for this reason, are frequently believed to be more closely related to ourselves than most other invertebrates. This means the anus arises from the blastopore in early development, it is enterocoelous, has radial cleavage, and indeterminate cell fate.

3) Echinoderms have a unique water vascular system that represents a well modified portion of the coelom. It serves as a hydraulic mechanism for appendages-called podia or tube feet-another unique feature of echinoderms. The water vascular system also serves an internal transport function. The tube feet or podia are variously modified for a locomotory function, respiratory function, feeding structures, attachment structures, and sensory reception.

4) The coelom is large and spacious in most echinoderms. They have exploited this space as well. The water vascular system makes up most of the coelom. Another portion of the coelom The circulatory system makes up another portion serves as a circulatory system since the water vascular system is constantly moving pressurized water in and out of the entire body there is no need for more highly developed circulatory system.

5) Echinoderms also lack an excretory system. The flow and removal of water for hydraulic purposes, and the permeability of the body surface itself, is sufficient for water removal. This means that echinoderms are osmoconformers, water and ions pass freely through the general body wall. Salinity fluctuates with the surrounding water. They are rather tolerant of internal fluctuations in salt content

Of course the heavy reliance on the water vascular system and lack of specialized excretory/osmoregulatory structures means that echinoderms are restricted to marine environments-some have tolerated brackish water, but certainly freshwater-and especially terrestrial environments are out of the question.

6) Echinoderms get there name- "spiney-skin" from the fact that they have an internal skeleton composed of calcite crystals embedded within the body covering. This is derived from mesoderm and is surrounded by tissue. The calcite crystals are called ossicles and have various conformations in different echinoderms. Some appear as a solid mass of fused plates called a test with an epidermis that covers them like in sand dollars and sea urchins. Other echinoderms have them diffuse throughout the body wall-as in sea cucumbers.

7) The digestive system is highly variable. It is rarely a straight tube except in sea cucumbers-mostly because the echinoderms themselves aren’t bilaterally symmetrical. It may terminate in an arm as is the case in crinoids, the aboral surface on sea stars, or may lack an anus and intestines altogether as is the case in brittlestars. Echinoderms are unusual in many ways.

8) With a lack of bilateral symmetry, comes an absence of cephalization. The nerve cords are either diffuse throughout the animal or are arranged radially from a nerve ring surrounding the central area of the echinoderm

9) One of the most interesting features of echinoderm biology is mutable or catch connective tissue. What does this mean? It means that echinoderm connective tissue can rapidly and voluntarily change in stiffness. This tissue can transform from a tough rigid state to a near liquid. If you have the chance to pick up a sea cucumber-you should-it will usually go from one state to another soon after touching it. When it is in its softest form-it may almost poor between the fingers. As a frame of reference-it is the consistency of gak (or slime-depending on what generation your from).

Currently there has been some study of this mutability property. It is the extracellular matrix that changes in solidity-and it is not the muscles, or inflating with water or air.

Nerves are involved with the process. There are two types. One nerve type hardens the matrix, the other softens it.

It appears that an increase in the calcium ion concentration may form links between macromolecules-and harden the matrix.

Different echinoderms take advantage of this property of their connective tissue in different ways.

Sea urchins use it to crawl into tight spaces and then harden-and wedge themselves in place to avoid predators.

Sea cucumbers also use this ability to wedge itself in crevices that it otherwise could not-since it can conform to the shape of the crevice.

Sea stars use mutable connective tissue to stiffen. This provides a scaffolding while the tube feet are used to exert pressure on the sides of bivalves that they are attempting to eat- that is the sea star hardens its connective tissue to serve as a lever.

Recently (1995) Crinoids, which are filter-feeding echinoderms, have been found to have mutable cirri-which may be collapsed singly or in groups to modify the currents for capturing prey. Also stiffening of cirri of the attachment stalk helps anchor the crinoid to the substratum. When it becomes soft-it allows it to loosen its grip from the substratum.

Brittlestars, or ophiuroids use this characteristic to reproduce asexually. They soften areas near the arms and break them off. The arms then regenerate into new animals.

10) Most echinoderms are dioecious and have broadcast spawning fertilization with various planktonic larvae. Sea urchin development has been a staple and tradition withn developmental biology classes for many years.

Since they free spawn, the sexes tend to look the same externally and there are no copulatory organs.

After fertilization,

Echinoderm larvae are bilaterally symmetrical-even though the adults rarely are. This suggests strongly a bilateral ancestor. The larvae of each class have distinctive names associated with them.

Echinoderms have well-developed powers of regeneration. Some voluntarily break themselves into pieces and regenerate into complete clones. This process can occur at various stages of development. In some species, the larval forms divide and give rise to new individuals.

Before going through each class of echinoderm, I’d like to mention why they are an important group. Arthropods and Molluscs warrant studying by their extreme diversity and species number alone.

Since echinoderms are a smaller group-I’d like to provide some justification for their study-

Some are of great economic and ecological importance. Sea urchins have been known to decimate vast tracts of seaweed and seagrass. They have been likened to locust on agricultural systems-stripping all vegetation and depleting fish spawning areas and sea otter feeding grounds. Starfish may do the same to coral reefs. The infamous coralivore Acanthaster planci has caused over 90% loss of the coral in some reefs. Considering the biodiversity present in some reef systems, this has a tremendous ecological impact.

Although the crown of thorns starfish has been blamed for destruction of coral reefs. Sea urchins are responsible for over 90% of bioerosion in carribean reefs.

Starfish are also a major "crop" pest in oyster beds. They decimate populations of scallop and other commercially available shellfish. Before starfish biology was known, fishermen used to cut up the starfish and throw them back overboard the ship-not realizing that the regenerative power of starfish and other echinoderms is so great that they were multiplying their numbers. Although echinoderms may not be diverse in species number-their biomass and individual numbers can be astounding. They often cover vast areas of abyssal regions of the oceans. In high fishing areas, there numbers have reached 150,000 per hectare (2.5 acres).

Many Starfish are keystone predators. That is they are of fundamental importance in determining the makeup of the invertebrate communities in which they live. Sea urchins are keystone species as well.

Although sea urchins cause the destruction of many reefs, some are important to coral reef survival.

The importance of a single species - Diadema antillarum was proved by a 5 fold increase in algal biomass on coral reefs after an epidemic loss of the species.

The algae removal allowed for larval settlement of coral planulae so without it-coral reef building almost ceased. Usually there are about 10-50,000 individuals per hectare-but after some bacterial pathogen-97% of this species was wiped out and there was nothing to stop algae growth on coral reefs.

SHOW FIGURE PAGE 65 BRUSCA?

Some fishermen have gotten sweet justice. Asterias species have been harvested (1,100 metric tons annually) for purposes of feeding to poultry. Often the ingredient that says "fishmeal" on chicken feed should read "echinoderm meal".

At the same time, some species of sea cucumber and starfish are collected as a delicacy in parts of the world.

Although we don’t consider sea cucumbers as anything edible, at least 13,000 metric tons of sea cucumbers are harvested every year-especially in Korea and Japan. They are eaten raw or dried. Dried cucumber or Trepang, are gourmet fare.

In many countries they are sold dried and salted in bags like potato chips.

They are even reared in aquaculture for harvesting.

The ecological impact of sea cucumbers isn’t known, but some species dredge about 50 kg/square meter of dry sediment from the bottom every year (10 lbs./square foot). They may be found at densities of 35/square meter-It has been suggested that they do for marine sediment what earthworms do for terrestrial soil since they are deposit feeders in most cases.

Other echinoderms also have economic importance

Brittlestar eggs have been eaten in Indonesia, Sea urchin eggs are consumed in South America and Japan.

So although echinoderms are not very species rich, they do represent significant biomass to be important ecologically and economically.

Now that you have some idea of their importance to us and to marine systems, I’ll go through the biology of each class starting with the Asteroids or starfish and then comparing them to the other classes

I’ll start with starfish or sea stars because the 1,800 or so species are the most recognizable members of the phylum and their image is almost emblematic of the world’s oceans.

SHOW OVERHEAD OF EXTERNAL ANATOMY ORAL AND ABORAL SURFACES.

Pentaradial symmetry is also most apparent in this group. This body form is often referred to as stellate.

There is a central disk with symmetrical projecting arms or rays.

For most echinoderms there are ambulacra and areas between them known as interambullacral areas.

The ambulacrum correspond to the arms in a starfish and the interambullacral areas are between the arms or rays. This is a significant anatomical feature since you will find homologous structures in the other echinoderms to compare it to.

The ambullacrum is an area where the tube feet or podia extend. And you’ll see modifications in the position of this ambulacral groove.

Sometimes the ambullacrum is referred to as an arm furrow.

The aboral surface has an anus that is situated just off from the center of the central disk. It is usually difficult to locate without a dissection and may be absent in some starfish and other echinoderms that have secondarily lost a throughgut.

There is a much more conspicuous structure on the aboral surface- the madreporite. This is an exceptionally large perforated ossicle that connects to the water vascular system. This is where water is taken in (and sometimes out) for regulation of water pressure in the coelomic cavity of the water vascular system.

SHOW OVERHEAD OF EPIDERMIS

With closer inspection, the epidermis has many weird structures and projections from it.

The outer body covering of starfish is one indication of the unique features of this phylum

Picking up a starfish you notice that it is hard yet flexible. The ossicles make it feel like sandpaper. Some of these ossicles may extend out of the epidermis and be visible on the body surface-but they are produced from mesodermal tissue of the dermis

Cells produce them in the dermis and as the crystal becomes larger, it gets surrounded by a large number of daughter cells of the original cell. Shape and size of the the ossicles is determined by the shape and size of dermal cells. The ossicles may be up to 95% calcite. But they are often composed of magnesium carbonate and other material such as trace metals.

This is very similar to spicule formation by amoebocytes in sponges.

Numerous kinds of variations of ossicles, also in spines exist.

There are paxillae of Astropecten-the burrowing sea star.

The ossicles are elevated off the epidermis in a mushroom shape. This gives a smooth appearance to the starfish. This special spiney ossicle and associated ossicles is called a paxilla.

The paxilla forms a roof over the top of the epidermis and keep sediment from smothering the papulae for respiration.

Ossicles are fused together in some areas such as the ambulacral grooves on the oral surface. Here they provide support surrounding the tube feet. Many of the ossicles are perforated with spaces-presumably this reduces the weight of the ossicles.

There is considerable turnover in the mineral constituents of the ossicles-much like bones in vertebrates.

The outer body wall has many unique features to echinoderms-there are papulae that are thin walled areas of the epidermis that extend out and increase surface area for respiration.

SHOW OVERHEAD OF PEDICELLARIA

There are strange accessory structures on the epidermis of two orders of starfish and other echinoderms called pedicellaria.

These are weird pincer-like things that have tiny muscles associated with them. They are small and jawlike.

The pedicellaria are so weird that when they were initially discovered they were thought to be ectoparasites on starfish and sea urchins, or perhaps commensals. They were given scientific names and cataloged as separate animals. There are many different types of pedicellaria even on a single starfish-and the various species names have remained and are still found in much of the literature when the morphotypes of pedicellaria are referred to.

It appears that their primary function is to remove debris and prevent larvae from settling on the surface.

Some are specialized for capturing prey-like fish.

Some have poison glands associated with them.

When fish or some other cue is presented near a starfish-the pedicellaria become very active and begin snapping. Some types that are on stalks may begin to move around. The activity of the pedicellaria often give a temporary "fuzzy" appearance to the starfish-although this may be due to papulae too.

One of the questions with the pedicellaria is that they appear to lack any connections to the gut or nervous system.

So how do they move-respond to stimuli and gain nutrients?

Apparently they have their own neuromuscular responses. This suggests that they must at least be linked to some mechanoreceptors and/or chemoreceptors. It is believed that they gain their nutrition through dissolved organics or perhaps some ingestion method that is unknown at present.

In any regard, they are weird. They are largely metabolically independent from the rest of the starfish or sea urchin-but are produced by them.

SHOW OVERHEAD OF WATER VASCULAR SYSTEM

A hallmark of all echinoderms, but particularly well developed in starfish-the asteroidea, is the water vascular system.

This is a series of fluid filled canals derived from one of three coelomic compartments that form during embryogenesis.

The canals lead to thin walled tubular structures-the podia or tube feet.

Think of them as part of the water vascular system that penetrate the body wall through various areas called ambulacral grooves.

A sieved plate-the madreporite-a perforated ossicle regulates water flow into the water vascular system. This functions as a screen keeping even small particles from entering the water vascular system. Connecting to the madreporite is an ossicle lined tube-called the stone canal.

This canal leads to a ring tube around the mouth and has radiating tubes that lead out to the end of each arm.

There are bulb shaped pockets that connect to these radial canals and are very important in functioning of the tube feet. There is usually one bulb or ampulla for each tube foot.

The tube feet do not usually have circular muscles associated with them.

Instead there are muscles in the ampullae that contract-shrink the size of the ampulla, and force water into the podia. This causes the tube feet to expand.

There is a one-way valve that keeps the fluid from backing up into the radial canal.

There are longitudinal muscles in the podia that contract to retract to shorten the tube foot. Fluid then gets backed up into the ampulla again.

The longitudinal muscles may contract independently allowing the podia to bend during retraction or extension.

The podium secretes a substance that adheres to the substratum and another that releases it-much like the duoglands in turbellarian flatworms. If pressure is exerted while it is attached, a suction may be formed too.

This is possible due to the flattened end of the podium.

For me, the obvious question was does this fluid get replaced and if so how?

Yes, the podium leaks because it is semipermeable. This more water to leak in via the radial canal to replace this loss. Since the podia are semipermeable-it provides a surface for gas exchange too.

The peritoneum lining the podia are well ciliated so that some coelomic fluid circulates.

Sort of

It takes advantage of differences in pressure with a cavity that houses an incompressible fluid.

The mutability of the connective tissue, and or the ossicles provides the structural support for muscles to act against-but not in the ampulla and podia-here it is coelomic fluid that antagonizes the muscles.

Requirements of a hydrostatic skeleton

2) that this cavity be surrounded by a flexible outer body membrane, permitting deformations of the outer body wall to take place and is elastic enough to return to its previous form after muscles are relaxed (a rare condition in animals and explains the evolution of the cuticle found in pseudocoelomates except rotifers and acanthocephalan worms-one lost its pseudocoel as a byproduct of small body size, the other turned it into a giant gonad and doesn’t require much in the way of pressure)

3) that the volume of fluid in the cavity remain constant

There are accessory organs associated with the water vascular system.

These are the Tiedemann’s bodies that surround the ring canal

and the Polian vesicles.

It is believed that the polian vesicles regulate fluid pressure within the water vascular system.

The Tiedemann’s bodies produce coelomocytes that have a variety of functions including immunological and excretory functions.

The coelomocytes are very important cells and are found in all echinoderms.

They recognize and phagocytize foreign substances.

They synthesize pigments and collagen which makes up most of the connective tissue. Some of the coelomocytes contain hemoglobin and transport oxygen.

They digest food particles and transport nutrients.

Coelomocytes also plug up holes when starfish (or other echinoderms) lose an appendage.

Did I leave any functions out?

I pretty much explained the excretory and circulatory system in echinoderms when I talk about coelomocytes.

Waste is carried to the papulae and sometimes the podia where the ends of the coelomocytes pinch off and release the waste to be diffused across the body wall.

Echinoderms aren’t known for their intelligence, but coordinating up to 2,000 tube feet requires a bit of nervous system integration.

Given the pentameric body plan, the nervous system is structured as a ring surrounding the esophagus and the nervous system has a tendency to be diffuse.

There are three nerve networks that have different functions.

Ectoneural (oral)-sensory integration

Hyponeural (deep oral)-motor function-integrating all the podia

Endoneural (aboral)-is reduced or absent in most groups except crinoids where it is the main nerve network for sensory and motor integration.

The sensory system of starfish-and other echinoderms is simple.

The epidermis has various sensory neurons that respond to:

touch

chemicals

water currents

light-pigment cup ocelli at the tips of the arms

These various sensory structures have not been explored much.

Starfish may possess statocysts or other analogous structures since they right themselves if turned upside down. The mechanism involved in this process is unclear.

Starfish have many interesting behaviors they exhibit.

DIGESTIVE SYSTEM IN STARFISH

Feeding is one of them. Starfish have a throughgut and large digestive cecae that occur as paired organs running into the arms.

They have two stomachs a pyloric one that is aboral.

And an oral cardiac stomach.

To feed they extrude the thin cardiac stomach out of the mouth and feed extraorally.

Starfish are ravenous predators but they are known to be non-selective-and selective deposit feeders as well as scavengers too- glooping up organic material with mucous on the podia and wading them up into balls and passing them to the mouth.

Many starfish specialize on molluscs.

The collagenous connective tissue is rendered stiff. The tube feet are extended on the sides of the bivalve and they begin to pull. At this point it is a matter of time to fatique the adductor muscles of the bivalves before a tiny gap is opened up. The cardiac stomach can fit within a space of 1/10th of a millimeter. Once inside, the stomach releases digestive enzymes that break down the adductor muscle and the rest of the mollusc and it finishes digestion.

Often starfish can take advantage of irregularities in the shell and slip its stomach inside without having to try to pull the valves open.

Coralivores like the crown of thorns, lays its cardiac stomach over a patch of coral until the soft polyps are dissolved, then it moves on to the next patch.

When they capture gastropods-especially opistobranchs-they attach their tube feet and pull on the mollusc in all directions until it the tissue loosens up. It may do this until the mollusc becomes a soupy mash.

Reproduction in starfish

Starfish reproduce by broadcast spawning. This means that eggs and sperm are shed into the water, and fertilization occurs externally.

Broadcast spawning requires several adaptations on the part of the animal because the selective pressures acting on reproductive biology is very different.

1) Small size of eggs

2) More abundance of eggs

3) Gonads are proportionally larger

4) Aggregations of groups, synchronized spawning (sperm limitation)

5) Chemotaxis of sperm

Sperm is limited in echinoderm fertilization-a fact that it reversed for terrestrial or internal fertilizing animals.

Notice that fertilization success was generally low-but always highly variable. The one species of sea cucumber had the highest fertilization success.

A consequence of sperm limitation, is a general absence of female choice in mating. That is females are unselective about which males fertilize their eggs-since not all of them will be fertilized anyway. This contrasts with terrestrial animals where males may fight for copulations with females or females may choose among different males.

In this case, the products of sexual selection seen on male morphology such as decorative ornaments to attract females, or fighting structures like horns, antlers, or other structures are generally absent.

Males and females tend to look much more alike.

There are a number of trade-offs echinoderms have

Ways to increase fertilization success is to increase sperm and egg output-for females this means smaller eggs and reduced survivability of each offspring.

SHOW OVERHEAD OF BARRY SINERVO AND LR MCEDWARD FIG 3 ‘88 PAPER

Barry Sinervo did a number of elegant studies on sea urchins by experimentally reducing the size of the egg and examining the growth patterns and morphology of the larvae.

Barry found that much of the morphology of the larvae is dictated by size.

As you know the larval forms of echinoderms are variable and complex-he suggests that much of the variation is a by-product of different egg nutrient allocation strategies for different echinoderms.

There is a cost to higher sperm production beyond the direct energetic costs as well.

Levitan has shown that sperm size and motility are generally inversely related.

A way to get around this, is to increase size before reproducing-that way more eggs and sperm may be shed.

Usually this means delaying reproduction which increases the chances of mortality prior to reproduction. Thus echinoderms have a series of cost-benefit trade-offs to contend with -as do other broadcast spawning invertebrates.

There are other unusual strategies as well Eckelbarber et al. (1989) found dimorphic sperm in a abyssal sea urchin. Some of the sperm have two tails and tangle another type of sperm. This has been suggested to function by clumping sperm and preventing premature diffusion of sperm. This may serve as a functional "spermatophore" for some sea urchins.

Many species have behavioral responses to low fertilization success. Waiting until a large aggregation of individuals is present. Spawning only during calm waters is another.

I’d like to go over some of the other classes of echinoderms-and contrast the general biology of starfish with these other taxa

Brittle Stars-

Ophiuroidea

These are named brittlestars because of their penchant for breaking off legs easily if mishandled. This is a form of reproduction that starfish are also capable of.

SHOW OVERHEAD BRITTLESTAR-STARFISH EVOLUTION BARNES 944

One of the primary morphological differences between ophiuroids and asteroids, is the modifications of the ossicles and water vascular system.

The ossicles tend to be fused and larger to a greater extent in brittlestars than in starfish. In brittlestars, the water vascular system is mostly surrounded by only a few large ossicles, instead of many smaller ones.

As a result, the brittlestars are more solid and hard than starfish.

Brittlestars are usually easy to distinguish from starfish because

the arms are more set off by the central disk, which is proportionately larger.

Many brittle stars have five arms like starfish, but in basketstars, the arms may branch at the base or more distally-giving rise to multiple tentacle-like branching.

The arms are the principle means of locomotion in brittlestars-rather than the podia found in starfish.

There is also no ambulacral groove in brittlestars.

There are large flattened ossicle plates called shields that are found both orally and aborally.

There are two lateral shields, an oral, and aboral one. Inside is a large ossicle called a vertebrate. The body of brittlestars is mostly ossicle as a result-and the coelom is greatly reduced.

The lateral shields often have spines associated with them.

There are five oral shields that serve as jaws-these two often have spines on them that work as teeth.

One of these oral shields is modified as a madreporite-so the madreporite is on the oral rather than aboral surface as in starfish.

The mouth leads into a single stomach that does not evert. The digestive system is very simple-just a infolded sac. No intestines or anus.

Despite a simple digestive system, brittle-stars are very successful generalists in their diet and feeding mode.

Filter feeding

Selective and non-selective deposit feeding

Scavenging

Predation may be seen within a single individual.

The podia are frequently involved in filter feeding-extruding a mucous for catching plankton. They may use them to pick up bits of organic material from the substratum and move them toward the mouth in mucous balls.

They will also capture prey with their arms-arthropods, molluscs, and fish.

Basket stars are filter-feeders-but can capture some prey up to 3 centimeters in size.

The coiled arms wrap around the prey and move to the mouth for feeding-the prey are passed through oral papillae. The podia do not move prey to the mouth.

Excretion in ophiuroids is through bursae-these are invaginations of the oral surface and function as gas exchange structures too.

SHOW ECHINOIDEA

Echinoidea-meaning "hedgehog like" -the sand dollars and sea urchins are divided into Regular urchins (sea urchins) and irregular urchins-sand dollars and heart urchins.

The sea urchins almost have a superficial spherical symmetry, but their pentaradial symmetry is apparent by the ambulacral grooves that roll up the sides of the animal. The podia are extremely long often longer than the spines.

The spines themselves are moveable and, in the case of sea urchins, allow them to wedge into tight crevices and help them flip over. The long podia are used to flip over if they get tossed on their aboral surface.

There is a ciliated epidermis, and nervous and dermal tissue below that. Under these tissue layers, lies the fused ossicles into a solid test.

Sea urchins have a pentaradial feeding structure called aristotle’s lantern. This is really five plated ossicles oriented in a vertical position and shaped like arrowheads. They grow down and out into the mouth and the structure is bounded by several different obliquely arranged muscles.

Sea urchins are mostly grazers but will scavenge and eat crustaceans, annelids and other small invertebrates if they can. Some feed on coral, kelp and other abundant lifeforms that make up ecosystems. The sea urchins may reach such high densities that they largely dictate the makeup of the community in which they live. They may be likened to the crop pests of the oceans but are beneficial as well. They are often used in environmental monitoring due to their importance in trophic interactions.

Many species burrow into hard rock-or rather chew there way into it. They may remain there permanently in some cases.

Irregular urchins are adapted to burrowing in sand and soft sediment. The oral and aboral surfaces are covered with short, broad spines for digging.

Some common sand dollars have notches or gaps in the test called lunules.These are evenly arranged around the periphery.

The lunules may be a by-product of test formation, but they often close up with increasing growth in some irregular urchins. It is believed that these holes reduce lift and prevent them from being flipped over in strong currents.

If they are flipped over, they usually burrow into the sand anterior downward and push their posterior up in the water current-where they are flipped back over.

These urchins have tertiarily acquired a tendency toward bilateral symmetry. The mouth is situated near the middle in most. The anus is also on the oral surface at one end.

Respiration

SHOW HOLOTHUROIDEAN OVERHEAD

Sea cucumbers are the strangest members of the living echinoderm classes.

They have a tertiary bilateral symmetry too-although some argue that they were never quite pentaradially symmetrical to begin with.

The morphological evidence points to the contrary. Many sea cucumbers have 5 rows of tube-feet-representing the ambulacra of other echinoderms. They have 5 retractor muscles and

In most cases, three of the rows lie on a ventral surface and provide a creeping sole or trivium. The other two rows are scattered on the dorsal surface.

The podia are usually reduced in size-or even absent in some sea cucumbers.

The exception are the podia around the mouth. These have been modified greatly into long dendritic tentacles for deposit or filter feeding.

The tentacles are still controlled by ampullae like in sea stars, but the water vascular cavity is somewhat reduced overall. The ring canal surrounds the esophagus, but has a calcareous ring running through it. Some echinodermologists believe this is derived primitively from the same ossicles found in aristotle’s lantern.

Also the madreporite floats suspended in the coelomic cavity -so it doesn’t appear to connect directly to the outside.

Sea cucumbers have a very large coelom. This is the medium of transport. A second portion of the coelom, the hemal coelom, transfers some oxygen and waste as well.

Some sea cucumbers even have a heart of sorts pushing fluids through the body. This may be an adaptation for marine sediment.

Sea cucumbers also have much more developed respiratory structures than other echinoderms. They have a respiratory tree that attaches to their anus and they must constantly ventilate it by sucking water in and out of their butt.

Weird huh?

Holothuroideans differ from other echinoderms in other ways too. They lack the pedicellaria found in sea urchins and starfish.

The ossicles are very much reduced, usually microscopic in size.

This is variable however. Some sea cucumbers still have up to 80% of the dry weight being composed of ossicles. Other species lack them entirely.

The dermis is thick and muscular but most of the body wall is connective tissue. This is the main reason why sea cucumbers change body shape so readily. There are no ossicles to interfere with transformation to a near liquid state.

Within the body wall, they have circular and longitudinal muscles for peristaltic contractions. Some of them live in a manner similar to lugworms with a U shaped burrow and a ventilating current.

It has all the characteristics necessary for a hydrostatic skeleton too. -perhaps providing support for the idea that a hydrostat was originally an adaptation for burrowing.

If sea urchins are the locust of the oceans, then sea cucumbers are the earthworms. A single sea cucumber may pass up to 130 kg of sediment through its digestive system in one year.

Up to 90% of the biomass in some abyssal regions are composed of sea cucumbers.

One other interesting aspect of sea cucumbers-they have a couple of unique predator avoidance behaviors.

If greatly disturbed, they may expell very sticky and/or toxic structures called Cuvierian tubules from its anus. These are associated with the respiratory tree and apparently have no other function.

Another, more extreme option, is they eviscerate themselves.

They turn themselves inside out by spewing their internal organs out of their mouth. This includes the entire digestive system, gonads, and sometimes the respiratory tree too. The predator may feed on these organs and the cucumber may regenerate an entirely new set of internal organs. I’d like to know how development works in sea cucumbers.

Holothuroid immune responses from pollutants

SHOW CRINOIDEA OVERHEAD

The sea lillies and feather stars are the oldest of the echinoderm classes. They have a fossil record dating back 600 million years. There are about 80 species of sea lillies, believed to be the more primitive of the two, and 550 species of feather stars alive today. All the stalked varieties are found in deep ocean water.

All crinoids are suspension feeders.

The sea lillies stalk is composed of columnals or disks stacked on each other-basically fused ossicles connected by mutable connective tissue.

Both types of crinoids are really inverted compared to the other echinoderms. The mouth is oriented upward instead of against the substratum.

There is a calyx, where the body organs reside, and a tegmen-the membrane that covers the digestive system.

There are between 5 and 200 arms-usually in multiples of five.

Pinnules extend in two rows from the arms via the ambulacral groove.

Podia also extend from these grooves. There are no ampullae to regulate podial movement however. They move the tube feet by retracting muscles in the radial canal instead.

Crinoids also lack a madreporite. The water vascular system penetrates the outside through numerous ciliated tubes in the tegmen.

The stalked forms lack an ability to move. There is no musculature in this stalk either. The connective tissue is differentially hardened or softened to allow bending-and appears to work as efficiently as muscles-or more so.

The feather stars can however relocate via cirri, which are crawling tentacles that can turn into a holdfast when the mutable connective tissue hardens.

Some feather stars can even swim and relocate.

SHOW OVERHEAD OF WATER CURRENTS GENERATED FROM CRINOIDS

DRAW CONCENTRICYCLOIDEA-SEA DAISEY

In the mid 1980s small creatures were hoisted up from 1000 meters off the Pacific Ocean. These were taken out of bacteria-rich decaying wood.

The aboral surface is covered with plate-shaped ossicles.

They have two ring canals instead of one and the podia arise out of one of the pair.

No mouth-instead a membrane covers the oral surface and this probably is the diffusion surface for ammonia and oxygen.

Since the discovery of the first species, Xyloplax medusiformis (literally "medusa-like plated animal found on wood"), a second species was found at 6800 ft. This second species lacked a stomach and the second species produces eggs. The first species seemed to be viviparous.

I’d tell you more about them-but that is most all that is known.

Now, the dreaded phylogeny of these groups

I know it is bad to do this-but invertebrate poetry seems to follow me around.

This poem by Craig Young (1995) (not Garstang) show the history of echinoderm phylogeny: Dave Nichols

Darwin started the debate

about the true ancestral state

of all the spined and spineless creatures

based on anatomical features

And so it wasn’t very long

before the starfish joined the throng

of groups with forebearers enigmatic

discussed by scholars, non-pragmatic.

Bury, Semon, and McBride

were clever men, but sometimes tried

To foist wild beasts imaginary

on phylogenists, unwary.

Thus dipleurula was born

in someone’s brain, one foggy morn

with structures found in every class-

a compromise that would not pass.

It seemed, to some, a strange contortion

to derive pentamery from torsion.

So dipleurula was replaced by a larva actual;

A neotonous holothuroid, pentactual.

Bather thought that creatures spiny

came from hydroids in the briny.

For decades, theories came and went

but strange ideas were far from spent!

Nichols came, with Oxford learning,

put pen to page, his neurons burning,

and what he wrote made colleagues squirm-

Echinoderms were sired by worms!

Not polychaetes, platyhelminths or nematodes,

Not echiurans, dicyemids or trematodes,

but peanut worms! the group though real

Lacked both spine and hydrocoel.

Jefferies stirred the fossil pot

Concocting thence a theory, hot,

that showed echinoderms regressing

from the chordates (backbones missing).

So here we are in ninety-five.

With several theories still alive;

the great debate continues still

a match of wits and strength and will.

Soon someone must capitulate;

no longer must we speculate,

for Dave’s disciples are now sequencing

endless genomes, the old profs wincing.

I shall not be too surprised

if all the theories of the wise

Are foiled, and DNA shows worms

to be the stem echinoderms.

The jury is still out on the evolutionary relationships between echinoderms-morphological and molecular data do not agree very well.

Here are some possible evolutionary relationships.

SHOW EVOLUTIONARY RELATIONSHIPS BETWEEN ECHINODERM CLASSES.

Stellate with five or more arms. Arms not set off from central disc with own articulations. Open ambulacra, tube feet with internal ampullae. Madreporite aboral

Five unbranched or branched articulated ambulacral arms. Closed ambulacral grooves. No suckers on ampullae. No anus. Madreporite on oral surface.

Discoid body. Ring o ff marginal spines. No radiating arms. Concentrically arranged skeletal plates. Suckerless podioa arranged in a ring. Two ring canals.Gut present or absent, no anus.

Body discoid or round. Skeletal plates fused as solid test. Movable spoines. Ambulacral grooves closed.