Sand Dollars – Nature’s Flattest Coinage For Sleepy Minds

Welcome to the I Can't Sleep Podcast,

Where I bore you to sleep,

One fact at a time.

I'm your host,

Benjamin Boster.

Thanks to Megan Kay for sponsoring tonight's episode about sand dollars.

Sand dollars,

Also known as sea cookies or snapper biscuits in New Zealand and Brazil,

Or pansy shells in South Africa,

Are species of flat,

Burrowing sea urchins belonging to the order Clibiasteroida.

Some species within the order,

Not quite as flat,

Are known as sea biscuits.

Sand dollars can also be called sand cakes or cake urchins.

The term sand dollar derives from the appearance of the test skeletons of dead individuals after being washed ashore.

The test lacks its velvet-like skin of spines and has often been bleached white by sunlight.

To beachcombers of the past,

This suggested a large silver coin,

Such as the old Spanish dollar,

Which had a diameter of 38 to 40 millimeters.

Other names for sand dollar include sand cakes,

Pansy shells,

Snapper biscuits,

Cake urchins,

And sea cookies.

In South Africa,

They are known as pansy shells for the suggestion of a five-petaled garden flower.

The Caribbean sand dollar,

Or inflated sea biscuit,

Is thicker in height than most.

In Spanish-speaking areas of the Americas,

The sand dollar is most often known as galleta de mar,

Sea cookie.

The translated term is often encountered in English.

In the folklore of Georgia and the United States,

Sand dollars were believed to represent coins lost by mermaids.

Sand dollars diverged from other irregular echinoids,

Namely the casiduloids,

During the early Jurassic,

With the first true sand dollar genus,

Togochaemus,

Arising during the Paleocene.

Soon after Togochaemus,

More modern-looking groups emerged during the Eocene.

Sand dollars are small in size,

Averaging from 80 to 100 millimeters.

As with all members of the order Clipiasteroide,

They possess a rigid skeleton called a test.

The test consists of calcium carbonate plates,

Arranged in a five-fold symmetric pattern.

The tests of certain species of sand dollar have slits called lunules that can help the animal stay embedded in the sand,

To stop it from being swept away by an ocean wave.

In living individuals,

The test is covered by a skin of velvet-textured spines,

Which are covered with very small hairs,

Cilia.

Coordinated movements of the spines enable sand dollars to move across the seabed.

The velvety spines of live sand dollars appear in a variety of colors,

Green,

Blue,

Violet,

Or purple,

Depending on the species.

Individuals which are very recently dead or dying are sometimes found on beaches,

With much of the external morphology still intact.

Dead individuals are commonly found with their empty test,

Devoid of all surface material,

And bleached white by sunlight.

The bodies of adult sand dollars,

Like those of other echinoids,

Display radial symmetry.

The petal-like pattern in sand dollars consists of five paired rows of pores.

The pores are perforations in the endoskeleton,

Through which bodea,

For gas exchange,

Project from the body.

The mouth of the sand dollar is located on the bottom of its body,

At the center of the petal-like pattern.

Unlike other urchins,

The bodies of sand dollars also display secondary front-to-back bilateral symmetry,

With no morphological distinguishing features between males and females.

Sand dollars can be found in temperate and tropical zones along all continents.

Sand dollars live in waters below the mean low tide line,

On or just beneath the surface of sandy and muddy areas.

The common sand dollar can be found in the northern hemisphere from the intertidal zone to the depths of the ocean,

While the keyhole sand dollars can be found on many a wide range of coasts in and around the Caribbean Sea.

The spines on the somewhat flattened top side and underside of the animal allow it to burrow or creep through the sediment when looking for shelter or food.

Fine,

Hair-like cilia cover these tiny spines.

Sand dollars usually eat algae and organic matter found along the ocean floor.

Though some species will tip on their side to catch organic matter floating in ocean currents.

Sand dollars frequently gather on the ocean floor,

In part to their preference for soft bottom areas,

Which are convenient for their reproduction.

In 2008,

Biologists discovered that sand dollar larvae will clone themselves for a few different reasons.

When a predator is near,

Certain species of sand dollar larvae will split themselves in half in a process they use to asexually clone themselves when sensing danger.

The cloning process can take up to 24 hours and creates larvae that are two-thirds their original length,

Which can help conceal them from the predator.

The larvae of these sand dollars clone themselves when they sense dissolved mucus from a predatory fish.

A larvae exposed to this mucus from the predatory fish respond to the threat by cloning themselves.

This process doubles their population and halves their size,

Which allows them to better escape detection by their predatory fish,

But may make them more vulnerable to attacks from smaller predators like crustaceans.

Sand dollars will also clone themselves during normal asexual reproduction.

Larvae will undergo this process when food is plentiful or temperature conditions are optimal.

Cloning may also occur to make use of the tissues that are normally lost during metamorphosis.

The flattened test of the sand dollar allows it to burrow into the sand and remain hidden from sight from potential predators.

Predators of the sand dollar are the fish species cod,

Flounder,

Sheep's head,

And haddock.

These fish will prey on sand dollars even through their tough exterior.

Sand dollars have spines on their bodies that help them to move around the ocean floor.

When a sand dollar dies,

It loses the spines and becomes smooth as the exoskeleton is then exposed.

Symmetry in biology refers to the symmetry observed in organisms,

Including plants,

Animals,

Fungi,

And bacteria.

External symmetry can be easily seen by just looking at an organism.

For example,

The face of a human being has a plane of symmetry down its center,

Or a pine cone displays a clear symmetrical spiral pattern.

Internal features can also show symmetry.

For example,

The tubes in the human body responsible for transporting gases,

Nutrients,

And waste products,

Which are cylindrical and have several planes of symmetry.

Biological symmetry can be thought of as a balanced distribution of duplicate body parts or shapes within the body of an organism.

Importantly,

Unlike in mathematics,

Symmetry in biology is always approximate.

For example,

Plant leaves,

While considered symmetrical,

Rarely match up exactly when folded in half.

Symmetry is one class of patterns in nature whereby there is a near repetition of the pattern element,

Either by reflection or rotation.

While sponges and placozoans represent two groups of animals which do not show any symmetry,

I.

E.

Are asymmetrical,

The body plans of most multicellular organisms exhibit and are defined by some form of symmetry.

There are only a few types of symmetry which are possible in body plans.

These are radial,

Cylindrical symmetry,

Bilateral,

Biradial,

And spherical symmetry.

While the classification of viruses as an organism remains controversial,

Viruses also contain icosahedral symmetry.

The importance of symmetry is illustrated by the fact that groups of animals have traditionally been defined by this feature in taxonomic groupings.

The radiata,

Animals with radial symmetry,

Formed one of the four branches of Georges Cuvier's classification of the animal kingdom.

Meanwhile,

Bilateria is a taxonomic grouping still used today to represent organisms with embryonic bilateral symmetry.

Organisms with radial symmetry show a repeating pattern around a central axis,

Such that they can be separated into several identical pieces when cut through the central point,

Much like pieces of a pie.

Typically,

This involves repeating a body part four,

Five,

Six,

Or eight times around the axis,

Referred to as tetramorism,

Pentamorism,

Hexamorism,

And octamorism,

Respectively.

Such organisms exhibit no left or right sides,

But do have a top and a bottom surface,

Or a front and a back.

Georges Cuvier classified animals with a radial symmetry in the taxon radiata,

Which is now generally accepted to be an assemblage of different animal phyla that do not share a single common ancestor,

A polyphyletic group.

Most radially symmetric animals are symmetrical about an axis extending from the center of the oral surface,

Which contains the mouth to the center of the opposite end.

Threefold triradial symmetry was present in Trilobozoa from the late Ediacaran period.

Fourfold tetramorism appears in some jellyfish,

Such as Aurelia marginalis.

Flowering plants show fivefold pentamorism in many of their flowers and fruits.

This is easily seen through the arrangement of five carpels,

Seed pockets,

In an apple when cut transversely.

Among animals,

Only the echinoderms,

Such as sea stars,

Sea urchins,

And sea lilies,

Are pentamorous as adults,

With five arms arranged around the mouth.

Being bilaterian animals,

However,

They initially develop with mirror symmetry as larvae,

Then gain pentaradial symmetry later.

Hexamorism is found in the corals and sea anemones,

Which are divided into two groups based on their symmetry.

The most common corals in the subclass Hexachorallia have a hexameric body plan.

Their polyps have sixfold internal symmetry and a number of tentacles that is a multiple of six.

Octamorism is found in corals of the subclass Octochorallia.

These have polyps with eight tentacles and octameric radial symmetry.

The octopus,

However,

Has bilateral symmetry,

Despite its eight arms.

Icosahedral symmetry occurs in an organism which contains 60 subunits,

Generated by 20 faces,

Each an equilateral triangle,

And 12 corners.

Within the icosahedron,

There is twofold,

Threefold,

And fivefold symmetry.

Many viruses,

Including canine parvovirus,

Show this form of symmetry due to the presence of an icosahedral viroshell.

Such symmetry is evolved because it allows the viral particle to be built up of repetitive subunits consisting of a limited number of structural proteins encoded by viral genes,

Thereby saving space in the viral genome.

The icosahedral symmetry can still be maintained with more than 60 subunits,

But only in multiples of 60.

For example,

The T3 tomato bushy stunt virus has 60 times 3 protein subunits,

180 copies of the same structural protein.

Although these viruses are often referred to as spherical,

They do not show true mathematical spherical symmetry.

In the early 20th century,

Ernst Haeckel described,

Haeckel 1904,

A number of species of radiolaria,

Some of whose skeletons are shaped like various regular polyhedra.

Spherical symmetry is characterized by the ability to draw an endless line,

Or great but finite number of symmetry axes through the body.

This means that the spherical symmetry occurs in an organism if it is able to be cut into two identical halves through any cut that runs through the organism's center.

True spherical symmetry is not found in animal body plans.

Organisms which show approximate spherical symmetry include the freshwater green alga vulvax.

Bacteria are often referred to as having a spherical shape.

Organisms with bilateral symmetry contain a single plane of symmetry,

The sagittal plane,

Which divides the organism into two roughly mirror image left and right halves,

Approximate reflectional symmetry.

Animals with bilateral symmetry are classified into a large group called bilateria,

Which contains 99% of all animals.

All bilaterians have some asymmetrical features.

For example,

The human heart and liver are positioned asymmetrically,

Despite the body having external bilateral symmetry.

The bilateral symmetry of bilaterians is a complex trait,

Which develops due to the expression of many genes.

The bilateria have two axes of polarity.

The first is an anterior-posterior AP axis,

Which can be visualized as an imaginary axis running from the head or mouth to the tail or other end of an organism.

The second is the dorsal-ventral DV axis,

Which runs perpendicular to the AP axis.

During development,

The AP axis is always specified before the DV axis,

Which is known as the second embryonic axis.

The AP axis is essential in defining the polarity of bilateria and allowing the development of a front and back to give the organism direction.

The front end encounters the environment before the rest of the body,

So sensory organs such as eyes tend to be clustered there.

This is also the site where a mouth develops,

Since it is the first part of the body to encounter food.

Therefore,

A distinct head with sense organs connected to a central nervous system tends to develop.

This pattern of development with a distinct head and tail is called cephalization.

It is also argued that the development of an AP axis is important in locomotion.

Bilateral symmetry gives the body an intrinsic direction and allows streamlining to reduce drag.

In addition to animals,

The flowers of some plants also show bilateral symmetry.

Such plants are referred to as zygomorphic and include the orchid and pea families and most of the figwort family.

The leaves of a plant also commonly show approximate bilateral symmetry.

Biradial symmetry is found in organisms which show morphological features,

Internal or external,

Of both bilateral and radiosymmetry.

Unlike radially symmetrical organisms which can be divided equally along many planes,

Biradial organisms can only be cut equally along two planes.

This could represent an intermediate stage in the evolution of bilateral symmetry from a radially symmetric ancestor.

The animal group with the most obvious biradial symmetry is the ctenophores.

In ctenophores,

The two planes of symmetry are 1.

The plane of the tentacles and 2.

The plane of the pharynx.

In addition to this group,

Evidence for biradial symmetry has even been found in the perfectly radial freshwater polyp hydra,

A cnidarian.

Biradial symmetry,

Especially when considering both internal and external features,

Is more common than originally accounted for.

Like all the trades of organisms,

Symmetry,

Or indeed asymmetry,

Evolves due to an advantage to the organism,

A process of natural selection.

This involves changes in the frequency of symmetry-related genes throughout time.

Early flowering plants had radially symmetric flowers,

But since then many plants have evolved bilaterally symmetrical flowers.

The evolution of bilateral symmetry is due to the expression of cycloidia genes.

Evidence for the role of the cycloidia gene family comes from mutations in these genes,

Which cause a reversion to radial symmetry.

The cycloidia genes encode transcription factors,

Proteins which control the expression of other genes.

This allows their expression to influence developmental pathways relating to symmetry.

Symmetry is often selected for in the evolution of animals.

This is unsurprising since asymmetry is often an indication of unfitness,

Either defects during development or injuries throughout a lifetime.

While symmetry is known to be under selection,

The evolutionary history of different types of symmetry in animals is an area of extensive debate.

Traditionally,

It has been suggested that bilateral animals evolved from a radial ancestor.

Cnidarians,

A phylum containing animals with radial symmetry,

Are the most closely related group to the bilaterians.

Cnidarians are one of two groups of early animals considered to have defined structure,

The second being the ctenophores.

Ctenophores show by radial symmetry leading to the suggestion that they represent an intermediate step in the evolution of bilateral symmetry from radial symmetry.

Interpretations based only on morphology are not sufficient to explain the evolution of symmetry.

Two different explanations are proposed for the different symmetries in cnidarians and bilateria.

The first suggestion is that an ancestral animal had no symmetry,

Was asymmetrical,

Before cnidarians and bilaterians separated into different evolutionary lineages.

Radial symmetry could have then evolved in cnidarians and bilateral symmetry in bilaterians.

Alternatively,

The second suggestion is that an ancestor of cnidarians and bilaterians had bilateral symmetry before the cnidarians evolved and became different by having radial symmetry.

Both potential explanations are being explored and evidence continues to fuel the debate.

Although asymmetry is typically associated with being unfit,

Some species have evolved to be asymmetrical as an important adaptation.

Many members of the phylum Porifera,

Sponges,

Have no symmetry,

Though some are radially symmetric.

The presence of these asymmetrical features requires a process of symmetry breaking during development,

Both in plants and animals.

Symmetry breaking occurs at several different levels in order to generate the anatomical asymmetry which we observe.

These levels include asymmetric gene expression,

Protein expression,

And activity of cells.

For example,

Left-right asymmetry in mammals has been investigated extensively in the embryos of mice.

Such studies have led to support for the nodal flow hypothesis.

In a region of the embryo referred to as the node,

There are small hair-like structures that all rotate together in a particular direction.

This creates a unidirectional flow of signaling molecules causing these signals to accumulate on one side of the embryo and not the other.

This results in the activation of different developmental pathways on each side and subsequent asymmetry.

Fluctuating asymmetry,

FA,

Is a form of biological asymmetry,

Along with anti-symmetry and direction asymmetry.

Fluctuating asymmetry refers to small random deviations away from perfect bilateral symmetry.

This deviation from perfection is thought to reflect the genetic and environmental pressures experienced throughout the human body.

This is a form of asymmetry that has been studied for a long time and is still being studied throughout development,

With greater pressure resulting in higher levels of asymmetry.

Examples of FA in the human body include unequal sizes,

Asymmetry,

Of bilateral features in the face and body,

Such as left and right eyes,

Ears,

Wrists,

And thighs.

Research has exposed multiple factors that are associated with FA,

As measuring FA can indicate developmental stability and it can also suggest the genetic fitness of an individual.

Human physical health is also associated with FA.

For example,

Young men with greater FA report more medical conditions than those with lower levels of FA.

Multiple other factors can be linked to FA,

Such as intelligence and personality traits.

Since sand dollars have a velvet-like texture,

I'm going to read a little bit about velvet.

Velvet is a type of woven fabric with a dense even pile that gives it a distinctive soft feel.

Historically,

Velvet was typically made from silk.

Modern velvet can be made from silk,

Linen,

Cotton,

Wool,

Synthetic fibers,

Silk-cotton blends,

Or synthetic natural fiber blends.

Velvet is woven on a special loom that weaves two thicknesses of the material at the same time.

The two layers are connected with an extra warp yarn that is woven over rods or wires.

The two pieces are then cut apart to create the fabric's pile,

And the two lengths of fabric are wound on separate take-up rolls.

This complicated process meant that velvet was expensive to make before industrial power looms became available.

And well-made velvet remains a fairly costly fabric.

Velvet is difficult to clean because of its pile,

But modern dry-cleaning methods make cleaning more feasible.

Velvet pile is created by cutting the warp yarns,

While velveteen pile is created by cutting the weft yarns.

Velvet can be made from several different kinds of fibers.

The most expensive of which is silk.

Much of the velvet sold today as silk velvet is a blend of silk and another fiber,

Often rayon or cotton.

Velvet made entirely from silk is rare and usually has market prices of several hundred US dollars per yard.

Cotton is also used to make velvet,

Though this often results in a less luxurious fabric.

Velvet can also be made from fibers such as linen,

Mohair,

And wool.

A cloth made by the Kuba people of the Democratic Republic of Congo,

From the Rafia palm,

Is often referred to as Kuba velvet.

Modern velvet can be polyester,

Nylon,

Viscous,

Acetate,

Or blends of synthetics and natural fibers.

For example,

Viscous mixed with silk produces a very soft,

Reflective fabric.

A small percentage of spandex is sometimes added to give the final material a certain amount of stretch,

Hence stretch velvet.

Velvet has a thick pile and can be cut pile up or pile down for more shine or more saturated color.