Stingrays | Gentle Reading For Sleep
Drift off with this calm bedtime reading that helps with sleep and eases insomnia. In this soothing episode, you’ll learn all about stingrays while letting your mind relax into gentle rest. Benjamin’s steady, peaceful narration shares fascinating facts about these graceful creatures of the sea, blending education with relaxation. There’s no whispering, no hypnosis—just calm storytelling designed to ease stress, anxiety, and sleeplessness. Press play, settle in, and let your thoughts drift away as you discover the world of stingrays. Happy sleeping!
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's episode is about stingrays.
A special thanks to Carissa Jones for sponsoring today's episode.
Stingrays are a group of sea rays,
A type of cartilaginous fish.
They are classified in the suborder Myelobatidae of the order Myelobatiformes and consist of eight families.
Hexatriginidae,
Sixgill Stingray,
Plesiobatidae,
Deepwater Stingray,
Urolophidae,
Stingrays,
Uritriginidae,
Round Rays,
Daisyadidae,
Whiptail Stingrays,
Potamotriginidae,
River Stingrays,
Chimneridae,
Butterfly Rays,
And Myelobatidae,
Eagle Rays.
There are about 220 known stingray species organized into 29 genera.
Stingrays are common in coastal,
Tropical,
And subtropical marine waters throughout the world.
Some species,
Such as the Thorntail Stingray,
Are found in warmer temperate oceans,
And others,
Such as the Deepwater Stingray,
Are found in the deep ocean.
The River Stingrays and a number of Whiptail Stingrays are restricted to freshwater.
Most Myelobatoids are demersal,
Inhabiting the next to lowest zone in the water column,
But some,
Such as the Pelagic Stingray and the Eagle Rays,
Are pelagic.
Stingrays diverge from their closest relatives,
The Pan Rays,
During the late Jurassic period,
And diversified over the course of the Cretaceous into the different extant families today.
The earliest stingrays appear to have been benthic,
With the ancestors of the Eagle Rays becoming pelagic during the early-late Cretaceous.
Permineralized stingray teeth have been found in sedimentary deposits around the world,
As far back as the early Cretaceous.
The oldest known stingray taxon is Deciatus speedonensis.
From the Hot Rivian of England,
Whose teeth most closely resemble that of the extant Sixgill Stingray.
Although stingray teeth are rare on sea bottoms compared to the similar shark teeth,
Scuba divers searching for the latter do encounter the teeth of stingrays.
Full-body stingray fossils are very rare,
But are known from certain lagerstätten that preserve soft-bodied animals.
The extinct Cyclobatus of the Cretaceous of Lebanon is thought to be a skate that had convergently evolved a highly stingray-like body plan,
Although its extant taxonomic placement is still uncertain.
True stingray fossils become more common in the Eocene.
With the extinct freshwater stingrays Heliobatus and Astrotrigon,
Known from the Green River Formation.
A diversity of stingray fossils is known from the Eocene-Montebalca Formation from Italy,
Including the early Stingray Aurechia,
As well as Desiomyliobatus,
Which is thought to represent a transitional form between stingrays and eagle rays,
And the highly unusual Liciniobatus,
Which had an extremely short and slender tail with no sting.
The mouth of the stingray is located on the ventral side of the vertebrate.
Stingrays exhibit hyostolic jaw suspension,
Which means that the mandibular arch is only suspended by an articulation with the hyomandibula.
This type of suspension allows for the upper jaw to have high mobility and protrude outward.
The teeth are modified placoid scales that are regularly shed and replaced.
In general,
The teeth have a root implanted within the connective tissue,
And a visible portion of the tooth is large and flat,
Allowing them to crush the bodies of hard-shelled prey.
Male stingrays display sexual dimorphism by developing cusps,
Or pointed ends,
To some of their teeth.
During mating season,
Some stingray species fully change their tooth morphology,
Which then returns to baseline during non-mating seasons.
Spiracles are small openings that allow some fish and amphibians to breathe.
Stingray spiracles are openings just behind its eyes.
The respiratory system of stingrays is complicated by having two separate ways to take in water to use the oxygen.
Most of the time stingrays take in water using their mouth,
And then send the water through the gills for gas exchange.
This is efficient,
But the mouth cannot be used when hunting because the stingrays bury themselves in the ocean sediment and wait for prey to swim by.
So the stingray switches to using its spiracles.
With the spiracles,
They can draw water free from sediment directly into their gills for gas exchange.
These alternate ventilation organs are less efficient than the mouth,
Since spiracles are unable to pull the same volume of water.
However,
It is enough when the stingray is quietly waiting to ambush its prey.
The flattened bodies of stingrays allow them to effectively conceal themselves in their environments.
Stingrays do this by agitating the sand and hiding beneath it.
Because their eyes are on top of their bodies and their mouths on the undersides,
Stingrays cannot see their prey after capture.
Instead,
They use smell and electroreceptors,
Similar to those of sharks.
Stingrays settle on the bottom while feeding,
Often leaving only their eyes and tails visible.
Coral reefs are favorite feeding grounds and are usually shared with sharks during high tide.
The stingray uses its paired pectoral fins for moving around.
This is in contrast to sharks and most other fish,
Which get most of their swimming power from a single caudal tail fin.
Stingray pectoral fin locomotion can be divided into two categories,
Undulatory and oscillatory.
Stingrays that use undulatory locomotion have shorter,
Thicker fins for slower motile movements and benthic areas.
Longer,
Thinner pectoral fins make for faster speeds and oscillation mobility in pelagic zones.
Visually distinguishable oscillation has less than one wave going,
Opposed to undulation having more than one wave at all times.
Stingrays use a wide range of feeding strategies.
Some have specialized jaws that allow them to crush hard mollusk shells,
Whereas others use external mouth structures called cephalic lobes to guide plankton into their oral cavity.
Benthic stingrays,
Those that reside on the sea floor,
Are ambush hunters.
They wait until prey comes near,
Then use a strategy called tenting.
With pectoral fins pressed against the substrate,
The ray will raise its head,
Generating a suction force that pulls the prey underneath the body.
This form of whole body suction is analogous to the buccal suction feeding performed by ray finned fish.
Stingrays exhibit a wide range of colors and patterns on their dorsal surface to help them camouflage with a sandy bottom.
Some stingrays can even change color over the course of several days to adjust to new habitats.
Since their mouths are on the underside of their bellies,
They catch their prey,
Then crush and eat with their powerful jaws.
Like its shark relatives,
The stingray is outfitted with electrical sensors called ampullae of Lorenzini.
Located around the stingray's mouth,
These organs sense the natural electrical charges of potential prey.
Many rays have jaw teeth to enable them to crush mollusks,
Such as clams,
Oysters,
And mussels.
Most stingrays feed primarily on mollusks,
Cretaceans,
And occasionally on small fish.
Freshwater stingrays in the Amazon feed on insects and break down their tough exoskeletons and mammal-like chewing motions.
Large pelagic rays like the manta use ram feeding to consume vast quantities of plankton and have been seen swimming in acrobatic patterns through plankton patches.
The venom of the stingray has been relatively unstudied due to the mixture of venomous tissue secretion cells and mucous membrane cell products that occurs upon secretion from the spinal blade.
The spine is covered with the epidermal skin layer.
During secretion,
The venom penetrates the epidermis and mixes with the mucus to release the venom on its victim.
Typically,
Other venomous organisms create and store their venom in a gland.
The stingray is notable in that it stores its venom within tissue cells.
The toxins that have been confirmed to be within the venom are cystitines,
Peroxyrhodoxin,
And galactin.
Galactin induces cell death in its victims,
And cystitines inhibit defense enzymes.
In humans,
These toxins lead to increased blood flow in the superficial capillaries and cell death.
Despite the number of cells and toxins that are within the stingray,
There is little relative energy required to produce and store the venom.
The venom is produced and stored in the secretory cells of the vertebral column at the mid-distal region.
These secretory cells are housed within the ventrolateral grooves of the spine.
The cells of both marine and freshwater stingrays are round and contain a great amount of granule-filled cytoplasm.
The stinging cells of marine stingrays are located only within these lateral grooves of the stinger.
The stinging cells of freshwater stingray branch out beyond the lateral grooves to cover a larger surface area along the entire blade.
Due to this large area and an increased number of proteins within the cells,
The venom of freshwater stingrays has a greater toxicity than that of marine stingrays.
Stingrays are usually very docile and curious,
Their usual reaction being to flee any disturbance,
But they sometimes brush their fins past any new object they encounter.
Nevertheless,
Certain larger species may be more aggressive and should be approached with caution.
The giant oceanic manta ray,
Giant manta ray or oceanic manta ray,
Is a species of ray in the family Mobility and the largest type of ray in the world.
It is circumglobal and is typically found in tropical and subtropical waters.
But can also be found in temperate waters.
Until 2017,
The species was classified in the genus Manta along with the smaller reef manta ray.
DNA testing revealed that both species are more closely related to rays of the genus Mobula than previously thought.
As a result,
The giant manta was renamed Mobula by Rostris to reflect the new classification.
The giant oceanic manta ray can grow up to a maximum of 9 meters in length and to a disc size of 7 meters across,
With a weight of about 3,
000 kilograms.
But the average size commonly observed is 4.
5 meters.
It is dorsoventrally flattened and has large triangular pectoral fins on either side of the disc.
At the front,
It has a pair of cephalic fins which are forward extensions of the pectoral fins.
These can be rolled up in a spiral for swimming or can be flared out to channel water into the large forward pointing rectangular mouth when the animal is feeding.
The teeth are in a band of 18 rows and are restricted to the central part of the lower jaw.
The eyes and the spiracles are on the side of the head behind the cephalic fins and the gill slits are on the ventral undersurface.
It has a small dorsal fin and the tail is long and whip-like.
The manta ray does not have a spiny tail as do the closely related devil rays but has a knob-like bulge at the base of its tail.
The skin is smooth with a scattering of conical and ridge-shaped tubercles.
The coloring of the dorsal upper surface is black,
Dark brown,
Or steely blue,
Sometimes with a few pale spots and usually with a pale edge.
The ventral surface is white,
Sometimes with dark spots and blotches.
The markings can often be used to recognize individual fish.
Mobula birestrus is similar in appearance to Mobula alfredi and the two species may be confused as their distribution overlaps.
However,
There are distinguishing features.
The oceanic manta ray is larger than the reef manta ray,
4 to 5 meters on average,
Compared to 3 to 3.
5 meters.
However,
If the observed rays are young,
Their size can easily bring confusion.
Only the color pattern remains an effective way to distinguish them.
The reef manta ray has a dark dorsal side with usually two lighter areas on top of the head,
Looking like a nuanced gradient of its dark-dominating black coloration and whitish to grayish.
The longitudinal separation between these two lighter areas forms a kind of Y.
While for the oceanic manta ray,
The dorsal surface is deep dark and the two white areas are well marked without grating effect.
The line of the separation between these two white areas forms a T.
The two species can also be differentiated by their ventral coloration.
The reef manta ray has a white belly,
Often with spots between the bronchial gill slits and other spots spread across trailing edge of pectoral fins and abdominal region.
The oceanic manta ray has also a white ventral coloration with spots clustered around lower region of its abdomen.
Its cephalic fins inside of its mouth and its gill slits are often black.
The giant oceanic manta ray has a widespread distribution in tropical and temperate waters worldwide.
In the northern hemisphere,
It has been recorded as far north as Southern California and New Jersey in the United States.
Aomori Prefecture in Japan,
The Sinai Peninsula in Egypt,
And the Azores in the northern Atlantic.
In the southern hemisphere,
It occurs as far south as Peru,
Uruguay,
South Africa,
And New Zealand.
It is an ocean-going species and spends most of its life far from land,
Traveling with the currents and migrating to areas where upwellings of nutrient-rich water increases the availability of zooplankton.
The oceanic manta ray is often found in association with offshore oceanic islands.
When traveling in deep water,
The giant oceanic manta ray swims steadily in a straight line,
While further inshore it usually basks or swims idly.
Mantas may travel alone or in groups of up to 50.
They sometimes associate with other fish species as well as seabirds and marine mammals.
About 27% of their diet is based on filter feeding and they will migrate to coastlines to hunt varying types of zooplankton.
Such as copepods,
Mycids,
Shrimp,
Ephasiids,
Decapod larvae,
And on occasion varying sizes of fish.
When foraging,
It usually swims slowly around its prey,
Herding the plankton creatures into a tight group before speeding through the bunched up organisms with its mouth open wide.
While feeding,
The cephalic fins are spread to channel the prey into its mouth and the small particles are sifted from the water by the tissue between the gill arches.
As many as 50 individual rays may gather at a single plankton-rich feeding site.
Research published in 2016 proved about 73% of their diet is mesopelagic deep water sources including fish.
Earlier assumptions about exclusively filter feeding were based on surface observations.
The giant oceanic manta ray sometimes visits a cleaning station on a coral reef where it adopts a near stationary position for several minutes while cleaner fish consume bits of loose skin and external parasites.
Such visits occur most frequently at high tide.
It does not rest on the seabed as do many flatfish as it needs to swim continuously to channel water over its gills for respiration.
The oceanic manta has one of the largest brains weighing up to 200 grams,
Five to ten times larger than a whale shark brain.
It heats the blood going to its brain and is one of the few animals,
Land or sea,
That might pass the mirror test,
Seemingly exhibiting self-awareness.
The giant freshwater stingray is a species of stingray in the family Daciatidae.
It is found in large rivers and estuaries in Southeast Asia and Borneo.
Though historically it may have been more widely distributed in South and Southeast Asia.
The widest freshwater fish and the largest stingray in the world,
This species grows up to 2.
2 meters across and can exceed 300 kilograms in weight.
It has a relatively thin oval pectoral fin disc that is widest anteriorly and a sharply pointed snout with a protruding tip.
Its tail is thin and whip-like and lacks fin folds.
This species is uniformly grayish brown above and white below.
The underside of the pectoral and pelvic fins bear distinctive wide,
Dark bands on their posterior margins.
Bottom-dwelling in nature,
The giant freshwater stingray inhabits sandy or muddy areas and preys on small fishes and invertebrates.
Females give live birth to litters of one to four pups,
Which are sustained to term by a maternally produced histotroph.
The first scientific description of the giant freshwater stingray was authored by Dutch ichthyologist Pieter Bleker in an 1852 volume of the journal Verhandelingen van uit Batavius Genootschap van Kunsten en Wetenschappen.
His account was based on a juvenile specimen 30 centimeters across,
Collected from Jakarta,
Indonesia.
Bleker named the new species Polylepis,
From the Greek poly,
Many,
And lepis,
Scales.
And assigned it to the genus Trigon.
However,
In subsequent years,
Bleker's description was largely overlooked.
And in 1990,
The giant freshwater stingray was described again by Supab Mangaprasad and Tyson Roberts in an issue of the Japanese Journal of Ichthyology.
They gave it the name Hemantura chapraya,
Which came into widespread usage.
In 2008,
Peter Last and B.
Mabel Monjaji Matsumoto confirmed that T.
Polypsis and H.
Chapraya referred to the same species.
And since Bleker's name was published earlier,
The scientific name of the giant freshwater stingray became Hemantura polypsis.
This species may also be called the giant freshwater whipray,
Giant stingray,
Or freshwater whipray.
There's a complex of similar freshwater and estuarine stingrays in South Asia,
Southeast Asia,
And Australasia,
That are or were tentatively identified with E.
Polypsis.
The Australian freshwater urogymnus were described as a separate species,
Urogymnus deliensis,
In 2008.
The freshwater urogymnus in New Guinea are probably U.
Dullanasis,
Rather than U.
Polypsis,
Though confirmation awaits further study.
Trigon fluviatilis from India,
As described by Nelson Anandale in 1909,
Closely resembles and may be conspecific with U.
Polypsis.
On the other hand,
Comparison of freshwater whipray DNA and amino acid sequences between India and Thailand has revealed significant differences.
Finally,
Additional research is needed to assess the degree of divergence amongst populations of U.
Polypsis,
Inhabiting various drainage basins across its distribution,
So as to determine whether further taxonomic differentiation is warranted.
In terms of the broader evolutionary relationships between the giant freshwater whipray and the rest of the family Daciatidae,
A 2012 phylogenetic analysis based on mitochondrial DNA reported that it was most closely related to the porcupine ray,
And that they,
In turn,
Formed a clade,
With the mangrove whipray and the tube-mouthed whipray.
This finding adds to a growing consensus that the genus Hemantura sensu lato is paraphyletic.
The giant freshwater stingray has a thin,
Oval,
Pectoral-finned disc,
Slightly longer than wide and broadest towards the front.
The elongated snout has a wide base and a sharply pointed tip that projects beyond the disc.
The eyes are minute and widely spaced.
Behind them are large spiracles.
Between the nostrils is a short curtain of skin with a finely fringed posterior margin.
The small mouth forms a gentle arch and contains four to seven paupilae,
Two to four large at the center and one to four small to the sides on the floor.
The small and rounded teeth are arranged into pavement-like bands.
There are five pairs of gill slits on the ventral side of the disc.
The pelvic fins are small and thin.
Mature males have relatively large claspers.
The thin cylindrical tail measures 1.
8 to 2.
5 times as long as the disc and lacks fin folds.
A single serrated stinging spine is positioned on the upper surface of a tail near the base.
At up to 38 centimeters long,
The spine is the largest of any stingray species.
There is a band of heart-shaped tubercles on the upper surface of the disc,
Extending from before the eyes to the base of the sting.
There is also a midline row of four to six enlarged tubercles at the center of the disc.
The remainder of the disc upper surface is covered by tiny granular denticles,
And the tail is covered with sharp prickles past the sting.
This species is plain grayish-brown above,
Often with a yellowish or pinkish tint towards the thin margins.
In life,
The skin is coated with a layer of dark brown mucus.
The underside is white with broad dark bands,
Edged with small spots on the trailing margins of the pectoral and pelvic fins.
The tail is black behind the spine.
The giant freshwater stingray reaches at least 1.
9 meters in width and 5 meters in length.
And can likely grow larger.
With reports from the Mekong and Chao Phraya rivers of individuals weighing 500 to 600 kilograms,
But it is not impossible that it is 1500 kilograms,
Or even 2000 kilograms.
It ranks among the largest freshwater fishes in the world.
In June 2022,
It was reported that a specimen caught in the Mekong River had broken the record for the largest strict freshwater fish ever documented.
The largest sturgeon species can far exceed this size,
But they are anadromous.
The individual weighed 661 pounds or 300 kilograms,
And was measured at 3.
98 meters long and 2.
2 meters wide.
