Jellyfish (Remastered) Sleep Facts

Jellyfish drift through the oceans like translucent ghosts with questionable decision-making skills. Their entire vibe is vaguely threatening but mostly just slow, making them oddly perfect for a bedtime story that induces drowsiness and existential apathy in equal parts.

Transcript

Welcome to the I Can't Sleep Podcast where I help you drift off one fact at a time.

I'm your host Benjamin Boster and today let's fall asleep learning about jellyfish.

Jellyfish,

Also known as sea jellies or simply jellies,

Are the medusive phase of certain gelatinous members of the subphylum Medusazoa,

Which is a major part of the phylum Cnidaria.

Jellyfish are mainly free-swimming marine animals,

Although a few are anchored to the seabed by stalks rather than being motile.

They are made by an umbrella-shaped main body made of mesoglia known as the bell,

And a collection of trailing tentacles on the underside.

Via pulsating contractions,

The bell can provide propulsions for locomotion through open water.

The tentacles are armed with stinging cells and may be used to capture prey or to defend against predators.

Jellyfish are found all over the world,

From surface waters to the deep sea.

Siphozoans,

The true jellyfish,

Are exclusively marine,

But some Hydrozoans with a similar appearance live in freshwater.

Large,

Often colorful jellyfish are common in coastal zones worldwide.

The Medusae of most species are fast-growing,

And mature within a few months,

Then die soon after breeding,

But the polyp stage attached to the seabed may be much more long-lived.

Jellyfish have been in existence for at least 500 million years,

And possibly 700 million years or more,

Making them the oldest multi-organ animal group.

The name jellyfish,

In use since 1796,

Has traditionally been applied to Medusae and all similar animals,

Including the comb jellies.

The term jellies,

Or sea jellies,

Is more recent,

Having been introduced by public aquaria in an effort to avoid use of the word fish.

With its modern connotation of an animal with a backbone,

Though shellfish,

Cuttlefish,

And starfish are not vertebrates either.

In scientific literature,

Jelly and jellyfish have been used interchangeably.

Many sources refer to only siphozoans as true jellyfish.

A group of jellyfish is called a smack,

Or a smuck.

The term jellyfish broadly corresponds to Medusae,

That is,

A life-cycle stage in the Medusazoa.

The American evolutionary biologist Paulin Cartwright gives the following general definition.

Typically,

Medusazoan cnidarians have a pelagic,

Predatory jellyfish stage in their life cycle.

Staurozoans are the exceptions,

As they are stocked.

The Merriam-Webster Dictionary defines jellyfish as follows.

A free-swimming marine silenderate that is the sexually reproducing form of a hydrazoan,

Or siphozoan,

And has a nearly transparent saucer-shaped body and extensible marginal tentacles studded with stinging cells.

Given that jellyfish is a common name,

Its mapping to biological groups is inexact.

Some authorities have called the comb jellies and certain salps jellyfish,

Though other authorities state that neither of these are jellyfish,

Which they consider should be limited to certain groups within the Medusazoa.

The subphylum Medusazoa includes all cnidarians with a Medusa stage in their life cycle.

The basic cycle is egg,

Planula larvae,

Polyp,

Medusa,

With the Medusa being the sexual stage.

The polyp stage is sometimes secondarily lost.

The subphylum include the major taxa,

Siphozoa,

Large jellyfish,

And the subphylum,

Cubizoa,

Box jellyfish,

And hydrazoa,

Small jellyfish,

And excludes anthozoa,

Corals,

And sea anemones.

This suggests that the Medusa form evolved after the polyps.

Medusazoans have tetramera symmetry,

With parts in four or multiples of four.

The four major classes of Medusazoan cnidaria are Siphozoa,

Or sometimes called true jellyfish,

Though there are no more truly jellyfish than the others listed here.

They have tetraradial symmetry.

Most have tentacles around the outer margin of the bowl-shaped bell,

And long oral arms around the mouth in the center of the sub-umbrella.

Cubizoa,

Box jellyfish,

Have a rounded box-shaped bell,

And their valerium assists them to swim more quickly.

Box jellyfish may be related more closely to Siphozoan jellyfish than either are to the hydrazoa.

Hydrazoa medusae also have tetraradial symmetry,

Nearly always have a vellum,

Diaphragm used in swimming,

Attached just inside the bell margin,

Do not have oral arms,

But a much smaller central stalk-like structure,

The manubrium,

With terminal mouth opening,

And are distinguished by the absence of cells in the mesoglia.

Hydrazoa show great diversity of lifestyle,

Some species maintain the polyp form for their entire life,

And do not form medusa at all,

Such as hydra,

Which is hence not considered a jellyfish,

And a few are entirely medusal and have no polyp form.

Staurozoa,

Stalk jellyfish,

Are characterized by a medusa form that is generally sessile,

Oriented upside down,

And with a stalk emerging from the apex of the bell,

Which attaches to the substrate.

At least some Staurozoa also have a polyp form that alternates with the medusoid portion of the life cycle.

Until recently,

Staurozoa were classified with a cyphozoa.

There are over 200 species of cyphozoa,

About 50 species of Staurozoa,

About 50 species of Cubizoa,

And the Hydrazoa includes about 1,

000 to 1,

500 species that produce medusa,

But many more species that do not.

Since jellyfish have no hard parts,

Fossils are rare.

The oldest unambiguous fossil of a free-swimming medusa is Burgesso medusa from the mid-Cambrian Burgess Shale of Canada,

Which is likely either a stem group of box jellyfish,

Cubizoa,

Or a Craspida that clade including Staurozoa,

Cubizoa,

And Cyphozoa.

Other claimed records from the Cambrian of China and Utah in the United States are uncertain and possibly represent ctenophores instead.

The main feature of a true jellyfish is the umbrella-shaped bell.

This is a hollow structure consisting of a mass of transparent jelly-like matter known as mesoglia,

Which forms the hydrostatic skeleton of the animal.

The mesoglia is 95% or more composed of water and also contains collagen and other fibrous proteins,

As well as wandering amebocytes that can engulf debris and bacteria.

The mesoglia is bordered by the epidermis on the outside and the gastrodermis on the inside.

The edge of the bell is often divided into rounded lobes known as lapids,

Which allow the bell to flex.

In the gaps or niches between the lapids are dangling rudimentary sense organs known as rapaellae,

And the margin of the bell often bears tentacles.

On the underside of the bell is the manubrium,

A stalk-like structure hanging down from the center.

There are often four oral arms connected to the manubrium,

Streaming away into the water below.

The mouth opens into the gastrovascular cavity,

Where digestion takes place and nutrients are absorbed.

This is subdivided by the 4-6 septa into a central stomach and four gastric pockets.

Near the free edges of the septa,

Gastric filaments extend into the gastric cavity.

These are armed with nematocytes and enzyme-producing cells and play a role in subduing and digesting the prey.

In some cyphozoans,

The gastric cavity is joined to radiocanals,

Which branch extensively and may join a marginal ring canal.

Cilia in these canals circulate the fluids in a regular direction.

The box jellyfish is largely similar in structure.

It has a squarish,

Box-like bell.

A short pedallium or stalk hangs from each of the four lower clavicles.

One or more long,

Slender tentacles are attached to each pedallium.

The rim of the bell is folded inwards to form a shelf,

Known as a valarium,

Which restricts the bell's aperture and creates a powerful jet when the bell pulsates,

Allowing the box jellyfish to swim faster than true jellyfish.

Hydrozoans are also similar,

Usually with just four tentacles at the edge of the bell,

Although many hydrozoans are colonial and may not have a free-living medusal stage.

Stalk jellyfish are attached to a solid surface by a basal disc and resemble a polyp,

The oral end of which has partially developed into a medusa with tentacle-bearing lobes.

And a central manubrium with four-sided mouth.

Most jellyfish do not have specialized systems for osmoregulation,

Respiration,

And circulation,

And do not have a central nervous system.

Nematicides,

Which deliver the sting,

Are located mostly on the tentacles.

True jellyfish also have them around the mouth and stomach.

Jellyfish do not need a respiratory system because sufficient oxygen diffuses through the epidermis.

They have limited control over their movement,

But can navigate with the pulsations of the bell-like body.

Some species are active swimmers most of the time,

While others largely drift.

The rophalia contain rudimentary sense organs,

Which are able to detect light,

Waterborne vibrations,

Odor,

And orientation.

A loose network of nerves called a nerve net is located in the epidermis.

Although traditionally thought not to have a central nervous system,

Nerve net concentration and ganglion-like structures could be considered to constitute one in most species.

A jellyfish detects stimuli and transmits impulses both throughout the nerve net and around a circular nerve ring to other nerve cells.

The rophalia ganglia contain pacemaker neurons,

Which control swimming rate and direction.

In many species of jellyfish,

The rophalia include ocelli,

Light-sensitive organs able to tell light from dark.

These are generally pigment spot ocelli,

Which have some of their cells pigmented.

The rophalia are suspended on stalks with heavy crystals of calcium carbonate at one end,

Acting like gyroscopes to orient the eyes skyward.

Certain jellyfish look upward at the mangrove canopy while making a daily migration from mangrove swamps into the open lagoon where they feed and back again.

Box jellyfish have more advanced vision than the other groups.

Each individual has 24 eyes,

Two of which are capable of seeing color,

And four parallel information processing areas that act in competition,

Supposedly making them one of the few kinds of animals to have a 360 degree view of its environment.

The study of jellyfish eye evolution is an intermediary to a better understanding of how visual systems evolved on earth.

Jellyfish exhibit immense variation in visual systems,

Ranging from photoreceptive cell patches seen in simple photoreceptive systems to more derived complex eyes seen in box jellyfish.

Major topics of jellyfish visual system research with an emphasis on box jellyfish include the evolution of jellyfish vision from simple to complex visual systems,

The eye morphology and molecular structures of box jellyfish,

Including comparisons to vertebrate eyes,

And various uses of vision including task-guided behaviors and niche specialization.

Experimental evidence for photosensitivity and photoreception in Cnidarians antecedes the mid 1900s,

And a rich body of research has since covered evolution of visual systems in jellyfish.

Jellyfish visual systems range from simple photoreceptive cells to complex image-forming eyes.

More ancestral visual systems incorporate extraocular vision,

Vision without eyes,

That encompass numerous receptors dedicated to single-function behaviors.

More derived visual systems comprise perception that is capable of multiple task-guided behaviors.

Although they lack a true brain,

Cnidarian jellyfish have a ring-nervous system that plays a significant role in motor and sensory activity.

This net of nerves is responsible for muscle contraction and movement,

And culminates the emergence of photosensitive structures.

Across Cnidaria,

There is large variation in the systems that underlie photosensitivity.

Photosensitive structures range from non-specialized groups of cells to more conventional eyes,

Similar to those of vertebrates.

The general evolutionary steps to develop complex vision include from more ancestral to more derived states,

Non-directional photoreception,

Directional photoreception,

Low-resolution vision,

And high-resolution vision.

Increased habitat and task complexity has favored the high-resolution visual systems common in derived Cnidarians,

Such as box jellyfish.

Basal visual systems observed in various Cnidarians exhibit photosensitivity representative of a single task or behavior.

Extraocular photoreception,

A form of non-directional photoreception,

Is the most basic form of light sensitivity and guides a variety of behaviors among Cnidarians.

It can function to regulate circadian rhythm,

As seen in eyeless hydrazoans,

And other light-guided behaviors responsive to the intensity and spectrum of light.

Extraocular photoreception can function additionally in positive phototaxis in planula larvae of hydrazoans,

As well as in avoiding harmful amounts of UV radiation via negative phototaxis.

Directional photoreception,

The ability to perceive direction of incoming light,

Allows for more complex phototactic responses to light,

And likely evolved by means of membrane stacking.

The resulting behavioral responses can range from guided spawning events timed by moonlight to shadow responses for potential predator avoidance.

Light-guided behaviors are observed in numerous cyphozoans,

Including the common moon jelly,

Aurelia Rita,

Which migrates in response to changes in ambient light and solar position,

Even though they lack proper eyes.

The low-resolution visual system of box jellyfish is more derived than directional photoreception,

And thus box jellyfish vision represents the most basic form of true vision,

In which multiple directional photoreceptors combine to create the first imaging and spatial resolution.

This is different from the high-resolution vision that is observed in camera or compound eyes of vertebrates and cephalopods that rely on focusing optics.

Critically,

The visual systems of box jellyfish are responsible for guiding multiple tasks or behaviors in contrast to less derived visual systems in other jellyfish that guide single behavioral functions.

These behaviors include phototaxis,

Based on sunlight,

Positive,

Or shadows,

Negative,

Obstacle avoidance,

And control of swim pulse rate.

Box jellyfish possess proper eyes,

Similar to vertebrates,

That allow them to inhabit environments that lesser-derived Medusa cannot.

In fact,

They are considered the only class in the clade Medusazoa that have behaviors necessitating spatial resolution and genuine vision.

However,

The lens in their eyes are more functionally similar to cup eyes exhibited in low-resolution organisms and have very little to no focusing capability.

The lack of the ability to focus is due to the focal length exceeding the distance to the retina,

Thus generating unfocused images and limiting spatial resolution.

The visual system is still sufficient for box jellyfish to produce an image to help with tasks such as object avoidance.

Box jellyfish eyes are a visual system that is sophisticated in numerous ways.

These intricacies include the considerable variation within the morphology of box jellyfish's eyes,

Including their task behavior specification,

And the molecular makeup of their eyes,

Including photoreceptors,

Opsins,

Lenses,

And synapses.

The comparison of these attributes to more derived visual systems can allow for a further understanding of how the evolution of more derived visual systems may have occurred,

And puts into perspective how box jellyfish can play the role as an evolutionary developmental model for all visual systems.

Box jellyfish visual systems are both diverse and complex,

Comprising multiple photosystems.

There's likely considerable variation in visual properties between species of box jellyfish given the significant interspecies morphological and physiological variation.

Eyes tend to differ in size and shape,

Along with number of receptors,

Including opsins,

And physiology across species of box jellyfish.

Box jellyfish have a series of intricate lensed eyes that are similar to those of more derived multicellular organisms,

Such as vertebrates.

Their 24 eyes fit into four different morphological categories.

These categories consist of two large,

Morphologically different medial eyes,

A lower and upper lensed eye,

Containing spherical lenses,

A lateral pair of pigment slit eyes,

And a lateral pair of pigment pit eyes.

The eyes are situated on rophalia,

Small sensory structures,

Which serve sensory functions of the box jellyfish and arise from the cavities of the x-umbrella,

The surface of the body on the side of the bells of the jellyfish.

The two large eyes are located on the midline of the club and are considered complex because they contain lenses.

The four remaining eyes lie laterally on either side of each rophalia and are considered simple.

The simple eyes are observed as small invaginated cups of epithelium that have developed pigmentation.

The larger of the complex eyes contains a cellular cornea created by a mono-ciliated epithelium,

Cellular lens,

Homogenous capsule to the lens,

Vitreous body with prismatic elements,

And a retina of pigmented cells.

The smaller of the complex eyes is said to be slightly less complex,

Given that it lacks a capsule,

But otherwise contains the same structure as the larger eye.

Box jellyfish have multiple photosystems that comprise different sets of eyes.

Evidence includes immuno-cytochemical and molecular data that show photopigment differences among the different morphological eye types,

And physiological experiments done on box jellyfish to suggest behavioral differences among photosystems.

Each individual eye type constitutes photosystems that work collectively to control visually guided behaviors.

Box jellyfish eyes primarily use cPRCs,

Ciliary photoreceptor cells,

Similar to that of vertebrate eyes.

These cells undergo phototransduction cascades,

Process of light absorption by photoreceptors that are triggered by c-opsins.

Available opsin sequences suggest that there are two types of opsins possessed by all cnidarians,

Including an ancient phylogenetic opsin,

And a sister ciliary opsin to the c-opsins group.

Box jellyfish could have both ciliary and cnidops,

Cnidarian opsins,

Which is something not previously believed to appear in the same retina.

Nevertheless,

It is not entirely evident whether cnidarians possess multiple opsins that are capable of having distinctive spectral sensitivities.

Comparative research on genetic and molecular makeup of box jellyfish's eyes versus more derived eyes seen in vertebrates and cephalopods focuses on lenses and crystalline composition,

Synapses,

And PAX genes and their implied evidence for shared primordial ancestral genes in eye evolution.

Box jellyfish eyes are said to be an evolutionary development model of all eyes based on their evolutionary recruitment of crystallines and PAX genes.

Research done on box jellyfish,

Including Trypidalia cystifora,

Has suggested that they possess a single PAX gene,

PAXB.

PAXB functions by binding to crystalline promoters and activating them.

PAXB in situ hybridization resulted in PAXB expression in the lens,

Retina,

And stethocysts.

These results and the rejection of the prior hypothesis that PAX6 was an ancestral PAX gene in eyes has led to the conclusion that PAXB was a primordial gene in eye evolution and that the eyes of all organisms likely share a common ancestor.

The lens structure of box jellyfish appears very similar to those of other organisms,

But the crystallines are distinct in both function and appearance.

Weak reactions were seen within the sera and there were very weak sequence similarities within the crystallines among vertebrate and invertebrate lenses.

This is likely due to differences in lower molecular weight proteins and the subsequent lack of immunological reactions with antisera that other organisms' lenses exhibit.

Jellyfish range from about 1 mm in bell height and diameter to nearly 2 m in bell height and diameter.

The tentacles and mouthparts usually extend beyond this bell dimension.

The smallest jellyfish are the peculiar creeping jellyfish in the genera Staurocladia and Eleutheria,

Which have bell disks from 0.

5 mm to a few mm in diameter.

With short tentacles that extend out beyond this,

Which these jellyfish use to move across the surface of seaweed or the bottoms of rocky pools.

Many of these tiny creeping jellyfish cannot be seen in the field without a hand lens or microscope.

They can reproduce asexually by fission,

Splitting in half.

Other very small jellyfish have bells about 1 mm,

Are the hydromedusae of many species that have just been released from their parent polyps.

Some of these live only a few minutes before shedding their gametes in the plankton and then dying,

While others will grow in the plankton for weeks or months.

The lion's mane jellyfish,

Cyanea capillata,

Was long cited as the largest jellyfish and arguably the longest animal in the world,

With fine thread-like tentacles that may extend up to 36.

5 meters long,

Though most are nowhere near that large.

They have a moderately painful but rarely fatal sting.

The increasingly common giant,

Nomura's jellyfish,

Found in some but not all years in the waters of Japan,

Korea and China in summer and autumn,

Is another candidate for largest jellyfish.

In terms of diameter and weight,

Since the largest Nomura's jellyfish in late autumn can reach 2 meters in bell diameter and about 200 kilograms in weight,

With average specimens frequently reaching 0.

9 meters in bell diameter and about 150 kilograms in weight.

The large bell mass of the giant Nomura's jellyfish can dwarf a diver and is nearly always much greater than the lion's mane,

Whose bell diameter can reach 1 meter.

Reviews

Enjoyed what I heard. Jellies are kinda cute in a way

🥱😴😴😴 thank you Ben! I am really loving the sea creatures! I end up dreaming about the ocean 🌊

I never thought that jellyfish aren't really fish!?