MARINE VERTEBRATES Fish
Lecture Notes
-
These
share 3 fundamental characteristics, backbone, which enclose
the nerve cord, and bilaterally symmetry and a presence of an
endoskeleton.
THE FISH
There
are over 22,000 species of fish which make up more than half
of all vertebrate species and most (58%) are marine.
Jawless
fish, Agnatha, first appeared about 550 million years ago.
They occupied the worlds rivers and seas and lived unchanged
for about 100 million years until 2 revolutionary
developments occurred, namely biting jaws which developed
from the front gill arches and fins became paired. These
enabled fish to be able to eat a variety of foods and swim
better. The jawless fish population declined after this.
Chondrichthyes evolved before bony fish. These cartilaginous
fish had a ventral mouth and a skeleton made of cartilage.
The bony fish had a true bone skeleton, specialized mouth,
and a swim bladder.
In 1938, off the coast of South Africa,
near the Chalumna River, a fisherman brought up a strange
fish. It weighed a hundred pounds and was 4 feet long. The
body was covered with circular scales and when it was
identified by Prof. JBL Smith, it was found to be a living
member of an ancient class of fish that was thought to have
gone extinct 70 million years ago. It was the Coelacanth, the
oldest true fish.
The body
forms of the fish are adaptations to the environment or
special behavior patterns. The streamlined or fusiform shaped
fish allow rapid movement through the water ad are found in
predatory fish. Fish compressed from side to side can easily
move through plants and in narrow spaces. Flattened
dorso-ventrally, depressed, are usually bottom dwellers.
Attenuated...elongated eels live in sand, mud or under rocks.
Fins aid
in locomotion. The dorsal and anal fin are used as rudders to
prevent rolling, paired pectoral and pelvic fins are used in
turning, balancing, and braking. The caudal or tail fin is
used mostly for pushing against the water.
In
sharks, and other cartilaginous fish, the fins play a role to
stop the fish from sinking because of the lack of an air
bladder. (dissection)
Three
types of fins are found on the tail, heterocercal, the top is
taller than the bottom of the fin, homocercal, both top and
bottom of tail are the same...symmetrical, and diphycercal,
the tail ends in a point.
The
mouth of a fish and its teeth are adapted for the type of
feeding the fish carries out. There are 5 types of feeding
methods:
1.
predators, specialized teeth for grasping and chewing,
2.
nibblers, take small bites,
3. food
strainers, use of gill rakers to strain the food floating in
the currents,
4. food
suckers, bottom feeders who draw food through the mouth like
a vacuum cleaner and
5.
parasites, attach to another fish and live off its juices. (Candiru or
Vampire Fish)
Fish
have a one way digestive system. Food enters the mouth and
passed to the pharynx where it is funneled to the esophagus.
In parrot fish, sucker fish, etc. the pharynx is equipped
with teeth that grind, grasp and tear the food before it
enters the esophagus. Often the esophagus can expand to
accommodate almost anything a fish can get in its mouth. The
walls of the digestive tube are coated with mucus to help
food move easily through the gut. The food then enters the
stomach where chemical digestion begins. An elongated stomach
is characteristic of meat eaters. A sac-shaped stomach is
that of fish that consume both plant and animal materials,
some stomachs function as grinding organs and some can expand
to accommodate large pieces of food. Puffers, globefish,
porcupine fish can pump water into their stomachs to inflate
themselves. Parrot fish, pipefish, and seahorses don't have
stomachs and digestion and absorption take place in the
intestine.
Food
passes from the stomach to the intestine where chemical
digestion continues and the end products are absorbed. The
intestine is folded, coiled and spiraled to increase surface
area. Meat eaters have shorter intestines than plant eaters.
Nutrients absorbed enter the blood and are transported to
various parts of the fish by the circulatory system.
Circulation
Blood is
pumped from the heart to gills where gas exchange takes
place. Blood carries oxygen via red blood cells to all parts
of the body, transports CO2, digested food, wastes etc. and
returns to the two chambered heart. The heart has one atria
and one ventricle. White blood cells are also present and
produce antibodies, aid in blood clotting and destroy germs
and microorganisms that enter the blood.
Respiration
Most
fish obtain oxygen directly from seawater using a gill. Water
is taken in through the mouth and pumped over the gills. The
gill is located behind the mouth in the gill chamber and
consists of several gill arches. Each gill arch supports many
gill rakers and gill filaments. Dissolved O2 diffuses across
the thin membranes of the gill and enters the blood. The gill
rakers are positioned to stop particles suspended in water
from damaging the filaments. Water pumped past the gill
leaves the gill chamber through the gill slit. The gill slits
in bony fish are covered with an operculum. Jawless and
cartilaginous fish have open gill slits. Sharks and rays have
a modified gill slit, the spiracle which works with the mouth
to bring water into the gill chambers. It is found on the
dorsal surface of rays.
Gas
Exchange
Thin
membranes of gill filaments make gas exchange easy. Diffusion
occurs and O2 is picked up by the blood. Diffusion is
increased by two features,
1.-surface
of gill filaments increased by branching increasing area of
membranes which come in contact with water and
2.-water
flowing over gills move in opposite direction of blood
movement in the gills. This counter current system enables
blood to pick up a maximum amount of O2 from the surrounding
water. Adaptations include: carp gulping air which can
diffuse into the gill filaments, lungfish with air sacs,
enlargements of the gill chamber etc.
Buoyancy
Fish
with well-developed swim bladders can remain poised at a
desired level with minimum effort by increasing or decreasing
the amount of gas in the bladder. The bladder can be filled
by gulping air or release of gas from the blood through gas
glands. When the fish goes deeper, the increased pressure
squeezes gas in the bladder, decreasing gas volume of the
bladder. The fish becomes heavier/denser. The fish restores
gas volume by secreting gas from the blood. As the fish moves
up, there is less pressure on the bladder, and the gas
expands. The fish becomes lighter and more buoyant so in
order to prevent the bladder from overfilling, the fish must
reabsorb the gas. Fish with an air-tube connecting the
bladder to the digestive tract can let gas escape through the
mouth, (open swim bladder) but a closed swim bladder system,
reabsorption is necessary. If a fish was pulled up from deep
water quickly, its swim bladder could explode! Vertical
movement is limited by the swim bladder...predators like
sharks, don't have to worry about that because have no swim
bladder.
Temperature
Temperature
has a profound effect on metabolism of fish because most
cold-blooded (ectothermic). Generally a rise in temperature
speeds up the metabolism and a drop slows down metabolism and
reduces swimming speed etc. Most fish generate heat but lose
it rapidly to the surroundings because they lack insulation.
Predatory fish have evolved a countercurrent system for
conserving heat and muscles can stay warm with energy
producing reactions.
Excretion.....Disposal
of wastes produced during metabolism
Bony
fish have salt secreting cells in the gills called chloride
cells that remove excess salt.
Sight
Like the
human eye in some ways but the density difference between the
water and cornea of the eye is not so great so the lens of
the fish is hard, dense and round to maximize refraction.
Light is focused by moving away or toward the retina. The
eyes bulge out with the lens protruding through the pupil.
Iris is not adjustable. Color vision is developed in fish
mainly in shallow clear water helping locate food, breeding
partners and avoiding predators. The eyes are on either side
of the head increasing the visual field which helps for
animals with no neck!
Hearing
and Balance
Ears,
lateral line organs and swim bladders aid in detecting
underwater sounds, maintaining balance and enabling some fish
to produce sounds. The inner ear, labyrinth, functions for
hearing and balance. Fluid filled canals and receptor cells,
neuromast sense cells are sensitive to movement in fish. It
bends and pushes ciliary hairs of the neuromast sense cells
and the impulse is sent to the auditory nerve to the brain.
Ear stones, otoliths, move in the tubes in conjunction with
fish movements, shifting positions,and this helps with the
balance and hearing. The neuromast cells are also found in
the lateral line organ, detecting low frequency vibrations.
The swim bladder vibrates as sounds in water pass through the
fish. These vibrations are passed to the inner ear. The toad
fish generates sounds in this way.
Smell...chemoreception
Detection
of dissolved substances in water is how the fish can smell
its food. Taste buds in the mouth taste the food, barbels,
whisker-like appendages, contain taste buds which enable the
fish to probe muddy waters.
Electroperception
The
ability of fish to detect weak electrical currents in
water/or generate them. This is used for communication and
examining the environment. Some can discharge several hundred
volts, stunning its prey and easily capturing them to eat.
Cartilaginous fish have sense organs in the head called the
ampullae of Lorenzini that can detect weak electrical fields.
It is used to help locate their prey, assist in navigation
and maybe even detect currents.
Outer
Covering
The
entire skin of the fish is alive, even the scales are covered
by a thin layer of living cells, the epidermis the only
protective material covering the epidermis is slime secreted
by mucus glands scattered over the body. The mucus reduces
friction and protects against bacteria etc. Fish odor or body
odor is found in the slimy covering. This allows fish to
recognize their species.
Scales
form a protective outer covering. There are 4 types of fish
scales. Sharks and their cartilaginous relatives possess
tooth-like Placoid scales. The placoid scale is
composed of a core of pulp and covering of dentine. Shark
teeth, spines of sting rays, tooth-like projections of saw
fish are modified placoid scales.
3 types
of non-placoid scales are common to bony fish. Platelike Ganoid
scales made of bone are found on primitive fish like the gar
pike and sturgeon. Cycloid and Ctenoid
scales
form an overlapping covering like roof shingles on a house.
The scales are thin and flexible allowing great mobility.
Cycloid scales are mainly found on soft-ray fish and ctenoid
scales are mostly found on spiny ray fish.
Coloration
This
serves in functions like camouflage, looking for a mate,
advertising the fact its poisonous or showing willingness to
remove parasites. Two types of cells control color namely
Chromatophores and Iridocytes. Chromatophores are starshaped
pigment cells located under transparent scales or in the thin
cell layer overlying the scales. The nervous system and
endocrine system seem to control the redistribution of
pigments within the chromatophore. Iridocytes are pigment
cells containing reflecting granules (work like mirrors). The
silvery stripes and iridescence of some fish result from
light being reflected by iridocytes. There is warning
coloration, cryptic coloration (blend), disruptive
coloration, and countershading.
Defense
and Migration
Fish
that don't swim well have evolved protective devices. Some
include the sharp spines of the surgeon fish and trigger fish
or the protective armor of the trunk fish, seahorse and
pipefish. Seahorses have a prehensile tail which they use to
hold on to underwater branches and remain motionless.
Globefish and puffers expand their bodies by pumping water
into their stomachs. Coloration...countershading is where the
dorsal side is darker than the ventral side making the fish
difficult to see. The clownfish has contrasting colors to
deceive the predator (disruptive contrast).
Secretions...unicorn
fish expels a cloud of ink, and stonefish and scorpion fish
possess poison glands.
Schooling
fish apparently have a decided advantage over non-schooling
fish.
Migration
There
are two reasons for seasonal migration. 1. food and
2.breeding. Migratory fish travel thousands of miles to
return to the same place each year. If they breed in fresh
water they are anadromous and if they breed in salt
water they are catadromous. A third type remain in the
ocean and move on definite pathways between feeding and
breeding areas. The ocean wanderings of migratory fish
correspond to ocean currents.
Reproduction
and life history
Many
ways have evolved for marine fish to reproduce.The sexes are
usually separate and both sexes have paired gonads located in
the body cavity. In most marine fish, the gonads produce
gametes only during certain periods of time. This is crucial
because both sexes need to be ready at the same
time...especially in those that migrate to breed. The timing
is controlled by sex hormones which are released into the
blood to stimulate the production of gametes. They are
released by being triggered by environmental factors like
temperature, light, and food availability.
Some
fish are hermaphrodites and though able to fertilize
themselves they usually breed with other individuals to
ensure fertilization between species. This is also common in
deep water fish as an adaptation to the dark depths and the
chance of NOT finding another fish of the opposite sex.
Sex
reversal also occurs in some fish. Individuals begin life as
males but eventually change into female or vice versa.
Fertilization
in fish is usually external but in some cases, internal
fertilization happens. Different types of development of the
egg occurs. Usually external fertilization involves the
production of many, I mean many, eggs. They end up floating
in the plankton and most don't survive. This is OVIPAROUS, or
the egg develops outside the body and feeds off the yolk.
Some
fish, mainly cartilaginous ones, have eggs that develop
inside the body and the young are born alive. This is
OVOVIVIPAROUS. Some rock fish have this method of
reproduction.
In some
sharks and rays, the embryos actually feed off nutrients of
the mother. This is like mammals and is known as VIVIPAROUS.
Other
Fish
Lampreys..cooler
waters a larval stage during which they little resemble the
adult in structure or lifestyle. Adult eel-like and has one
or two dorsal fins a caudal fin and no paired fins. The
mouth, jawless, is a disk adapted for sucking, with a complex
arrangement of teeth, their arrangement specific to the
species and therefore used in classification (Agnatha-2
classes, 1 order each, Lampreys have 3 families with a total
of 72 species (12/17 genera/2/4 families). They have 7
external gill openings, water goes through the mouth to the
chambers. They are covered in mucus, its toxic. They spawn
upstream though some are freshwater and some salt. They may
migrate for a few months to longer. They migrate back after
spawning and here the difference between non and parasitic
lampreys comes into play. The non, don't feed and just breed
and die. The parasitic feed on blood and fluids of fish for
up to 2 years. They detect their prey by sight and attach on
the lower surface. They move in with the sucker closed to
reduce water resistance but open it before the attack. The
teeth come into play now. The only way fish free themselves
is by coming to the surface and turning so the lampreys head
is in the air.. 4lbs of fish blood can feed a lamprey from
metamorphosis to spawning. Moved into great lakes with the
opening of the Welland canal.decimating the trout fisheries.
Hag fish
or slime eels are eel like have a fleshy fin, and flattened
caudal region. They have 4 to 6 tentacles around the mouth.
No jaws or stomach but are parasites of larger fish and
defend themselves by releasing slime. They can tie their
bodies into knots and posses several sets of hearts. They may
reach 28" and live in cold oceanic waters preying on
dying fish using the tongue to burrow a hole in the side of
the fish, loops its body around the fish and thrusts its head
into the body of the fish. They feed quickly and may soon be
completely inside the fish eating the flesh, worms etc. There
are about 20 species and their skin is used in the making of
leather goods.
Sturgeons
are the largest and longest lived freshwater fish and provide
food! Caviar! The sturgeon and paddle fish are the only
survivors of an ancient group of fish. They migrate to and
from the sea.Sizes are impressive...1800lbs,
Bowfin
and Garfish...bowfin can survive out of water using their air
bladder as a lung. Garfish, long bodied predators long jaws
and many teeth and armor like scales.
Tarpons,
Eels, Notacanths...weird that these unalike fish are grouped
together but all have larva unlike the adult.
Bristlemouths...luminous
organs, bristle like teeth eyes on stalks
Lizard
fish and Lantern fish...sit on pelvic fin and lower tail lobe
Spiny
finned fish latest flowering of bony fish evolution.
Billfish
The term
billfish encompasses two closely related families, the
Istiophoridae: sailfish, spearfish, and marlins and the
Xiphiidae, the swordfish. The term billfish is usually
reserved for the istiophorids and call the only member of the
family Xiphiidae, a swordfish. Both families of fish have
members whose upper jaw is extremely elongated and narrow.
These are all fast-swimming, aggressive fish in the open
ocean with a tall dorsal fin and lunate (quarter moon shaped)
tail. They have a unique circulatory system that keeps the
warmth generated by their active swimming retained in the
muscles. This makes their muscles slightly warmer than the
surrounding water and gives them an advantage over the
slower-moving, completely cold-blooded fishes. Most are
tropical and subtropical in distribution but are often caught
in temperate waters, especially in summer months.
They are
among the oceans fastest swimmers. Sailfish have been clocked
at a minimum of 70 MPH for short bursts and probably cruise
at 20 to 30 MPH. The long bill is thought to be a cutwater
which aids these fish in very fast swimming. It is also used
to decimate schools of fish by thrashing it back and forth
horizontally through the water and is thought to be used in
"battles" with other bill fish. The lunate tail and
narrow, keeled peduncle (area just before the tail) are also
adaptations for fast swimming. Even the dorsal and pelvic
fins of the sailfish fit into neat grooves on the body to
prevent any unwanted drag. Although marlin may be somewhat
slower than sailfish, and swordfish slower yet,
"slower" in this case is still close to 40-50 MPH.
There
are eleven species of billfishes and swordfish, five of which
are found in the Gulf of Mexico. The term includes two fish
families, which inhabit tropical and sub-tropical oceans
worldwide. The family Istiophoridae contains ten members: the
marlins (genus Makaira), the spear fishes and white marlins
(genus Tetrapturus), and the sailfishes (genus Istiophorus).
The swordfish, Xiphias gladius, is the only member of the
family Xiphiidae.
Swordfish
can easily be distinguished from the istiophorids. The sword
or bill of the swordfish is a broad flat blade making up
about one-third of the body length. In addition, the fish
lacks pelvic fins, and in the adult, teeth and scales.
Although
all billfish are large, (at least 5 feet), the two edible
species outdo themselves. Both marlins and swordfish are
taken at lengths of 6 to 15 feet and 300 lbs to 1000 lbs.
They
undergo long distance migrations and range far afield for
their food. One fish, tagged off the Virgin Islands, was
caught four months later 4,500 miles away off the coast of
West Africa.
Young
fish less than an inch in length, may take refuge from
predators under floating Sargassum or sea grass. Here they
prey on smaller, less active fish who are also seeking safe
harbor.
Adult
billfish eat a wide variety of fish: flying fish, scad,
mullet, round herring, ballyhoo, mackerel, tuna, and jacks.
They also eat squids and other billfish. Only humans and
mackerel sharks, killer whales and bigger billfish eat
billfish.
The bill
can be used in aggressive encounters with other billfish as
evidence of billfish caught with pieces of bill embedded in
their bodies. Whales have also been attacked by billfish.
Catch
them on hook and line, baited with live mullet, mackerel or
squid though the numbers are dropping rapidly.
Large
marlin are always females. Whether males change sex as they
attain greater size and become females or just stop growing
as fast as the females is presently being studied. Small
specimens of males and females are found its just only
females are the record catches!
Sharks.
Over 350
million yrs ago, something new happened in the primeval
seas...an entirely new class of vertebrates evolved which
were quite different from anything before. They had tiny
tooth-like body armor called placoid scales, exposed
strap-like gill openings, unique paired copulatory organs and
flexible skeletons of cartilage. These groups settled on a
lifestyle that has persisted to the present. They are slow
growing, late maturing, produce small numbers of well formed
young of whom a mother invests a lot of resources during
development but virtually nothing after the young are born.
Most
people think sharks are large, fast swimming elegant savage
predators. This is true of some species but only a minority.
The group should be of general interest because of the
intriguing aspects of biology found in sharks, the
exceptional sense of smell, electropreception, and giving
birth to live young.
One
notable feature is its teeth. In the highly predaceous
sharks, these are large and razor sharp used for cutting and
shredding their prey into bite size pieces. Some however are
bottom feeding species and eat mollusks and crustaceans and
their teeth are flattened for crushing the shells of their
prey. The fish eaters have long thin teeth to help catch and
hold their prey.. A shark may have up to 3000 teeth arranged
in 6 to 20 rows according to the species. In most sharks only
the first row or two are actively used for feeding. The
remaining rows are used for holding prey. They are in various
stages of formation with the newest at the back. As a tooth
in the functioning row breaks or is worn down, it falls out
and a replacement tooth moves forward in a sort of conveyor
belt system. They can be replaced every few days. This keeps
the functional row sharp. The shark may use over 20,000 teeth
in a lifetime and the strength of the jaws can exert a biting
strength of 3000kg per sq. cm. (44000 lb/sq. in. on the
teeth. (humans are 150lbs).
Sharks
find their prey through a number of sensory systems. Many
have poor eyesight but some are real good. Some have barbels
around their mouth to taste the sea bed for prey. All sharks
have a very keen sense of smell. Their nostrils are used only
for smelling, not for breathing. The part of the sharks brain
that deals with smelling is twice as large as the rest of the
brain. Sharks can detect 1 part of blood per million parts of
seawater (1 drop in 25 gal). They have a lateral line system
which is a series of canals on the entire body and head which
are filled with a jelly-like substance, which are sensory
receptors which pick up pressure waves caused by movements of
other animals or even by the shark itself. On the snout are
ampullae of Lorenzini, a series of electro-receptive pits
which are the most sensitive electro-perceptive devices found
in any animal. They are capable of picking up one-millionth
of a volt, which is less than the electric charge produced by
nerves in an animals body. The sharks can find their prey
from the prey's natural electric output.
Sharks..The
six and seven gilled sharks have extra sets of gills
preferring cold water some being filmed at 1800m.
The
orectoloboids are five closely related families of sharks
ranging from 3'long to the whale shark 50' long. All but the
whale shark are bottom dwellers and spend most of their time
just sitting on the ocean floor and actually skeletons
modified so they can actually use their fins for walking on
the ocean floor. They have sensory barbels around their
mouth. The whale shark has gill arches specially modified to
act like a sieve to filter out the planktonic organisms upon
which it feeds. Because it of its bulk it needs constant fuel
and feeds as it swims.
The
thresher, mackerel are among the largest group of sharks in
the world. They have a long upper lobe on their caudal fin
and use it by swimming through a school of small fish,
thrashing the tail and killing or stunning the fish which are
then eaten.
The
family of mackerel sharks include the great white, mako and
basking sharks. Their caudal fins have lobes being nearly
equal in length. Most if not all are homothermic which means
they can keep their body temperature above that of their
surroundings. The Mako is probably the fastest, measured at
60mph and is known to have outswum and eaten swordfish. The
great white/white death/white pointer etc. mainly feeds on
marine mammals, the only shark to do so, and has broad
serrated teeth designed for biting large chunk of flesh from
whales and seals. It is viviparous, meaning the embryos
develop in the uterus. The basking shark is another filter
feeder and the second largest in size...45'.
The
requiem sharks, are the typical sharks, the bull shark, which
can enter freshwater and has been found more than a thousand
miles from the mouth of large rivers. The largest is the
Tiger shark..18' and the most dangerous. It swallows almost
anything, including roles of tar paper, shoes, gas cans,
license plates, cans of paint and human parts The young tiger
has dark bands on a silvery background fading with age. (see
article)
Hammerhead
sharks, also in this group, have large lateral expansions of
their head on which the eyes are set. They grow to 15' and
the expanse of the head gives a better field of vision, and a
more expansive electro-detecting system.
Shark
Reproduction
Success
is due to the reproductive adaptations. Males have claspers
which release sperm into the cloaca. Young are born as
miniatures of the adults and few in number. Their large size
reduces predators and allow for more nutrients for the young.
Eggs are produced with large yolks.
3 types
of egg laying:
1. ovipariy- egg laying...bull head, some nurse sharks
2. ovovivipary- thin shelled eggs hatching in the uterus before
full development. No placental development, use yolk. Tiger,
dogfish.
3. Vivipary-no shells-some yolk but mostly nourished by mother
through the placenta. Bull, lemon, hammerhead, great white.
Comparison
of cartilaginous and bony fish
CHONDRICHTHYES
OSTEICHTHYES
skeleton
cartilage bone
skull no
sutures sutures
teeth
not fused to jaws fused
nostrils
single..each side double
swim
bladder absent present
intestine
spiral valve none
fert.
internal mostly external
scales
placoid ganoid, cycloid ctenoid
jaws not
protrusible protrusible
gills no
operculum operculum
Skates Rays and Chimaeras.
Examples
are the eagle rays, electric rays, guitar fish, mantas, sawfish, skates, stingrays, and
the chimaeras. There are about 300 species of rays and skates
which have flattened bodies and love usually on the bottom
(Demersal). There are some rays that look like sharks and
some sharks with flattened bodies so the way to tell is that
the skates and rays all have heir gill slits on the ventral
surface. The enlarged pectoral fins are fused to the head and
the eyes are on top of the head. The stingrays have a
whiplike tail usually equipped with spines for defense.
Poison glands produce venom. Skates are like rays but lack a
whiplike tail and stinging spines. Some can have electric
organs .
Rat Fish
..about 25 species of this deep water chimaera, is separated
because the gill slits are covered by a flap of skin. The
long tail gives it the name "ratfish".
Chapter Questions
Fish
Reading and Questions
1.
How often do some reef fish reverse gender?
2.
What are sex-changing fish known as?
3.
What is the most common type of sex reversal?
4
Name 3 species of
reef fish that change sex several times?
5.
Do fish change appearance when they go back to the one sex each time?
6.
How is this sex reversing a benefit?
Readings
Return to
Selections
Here
is some extra information collected from web sites on
The
Rays and Manta Ray
Thanks
to all for this educational resource!
Manta Ray: The
Good Devil
For centuries it
was thought that the manta ray, or "devilfish," was an
evil ocean beast capable of destroying fishing boats and
devouring people. In fact, the manta ray is one of the gentlest
creatures in the sea whose only wish, it seems, is to float
peacefully in the water sucking in mouthfuls of water in search
of its favorite food -- plankton.
It's
understandable why people have been frightened by this tremendous
fish. The manta ray is the largest member of the and can measure
up to 19 feet long and weigh as much as 2,300 pounds. Some manta
rays have been caught that measure an unbelievable 23 feet
across. This giant sea creature takes its name from the Spanish
word "manta" meaning blanket. With its enormous black
fins extended, the manta ray really does look like a huge
floating blanket.
Manta rays live
mainly in tropical waters and feed on zooplankton, small fish and
crustaceans. Like other rays, the Manta has a skeleton made out
of cartilage instead of bone, and its nostrils, mouth and gill
slits are ventral, or located on its underside. Manta rays can be
found browsing through sandflats in search of food or, sometimes,
leaping clear out of the water. Manta rays give birth to live
young. When their babies are born they are wrapped up in their
little wing flaps like pigs in a blanket. Baby manta rays are
called "pups."
Manta rays usually
swim in pairs, and although they seem frightening, manta rays are
so gentle that many will even let humans pet them. Some
experienced divers have actually taken rides on the backs of
these beautiful, harmless fish.
Despite their
size, manta rays are usually graceful and tend to be tolerant
with divers.
Manta rays were
once called devil fish by sailors who saw large "horns"
extending forward from their heads. These horns turned out to be
ingenuous scoopers which, when unfurled, guide plankton into the
manta’s mouth. Manta wingspans can reach up to 20 feet, and
they glide though the water like birds. Both gentle and graceful,
they have become a favorite attraction for divers. Their range is
circumtropical, with sightings in the Atlantic from as far north
as New England and south to Brazil. Mantas prefer plankton-rich
waters and are regular visitors to a few select locations. In the
winter, mantas visit San Benedicto Island, south of Cabo San
Lucas, Mexico. Here, they glide past a pinnacle called The Boiler
hoping to take advantage of the resident cleaner fish. Mantas
also hang out in Yap, Micronesia at Manta Ridge, another cleaning
station. From March through July, mantas visit the island of
Tobago, feeding in the rich waters fed by the Orinoco estuary in
South America. On Hawaii’s Kona Coast, mantas are drawn each
evening to the lights of a large hotel. Night divers watch as the
mantas feed on the plankton that is attracted by the
lights. [from: http://www.letsfindout.com/subjects/undersea/rfimanta.html ]
devil ray
also called MANTA
RAY, any of several genera of marine rays
comprising the family Mobulidae (class Selachii). Flattened, and
wider than they are long, devil rays have fleshy, enlarged
pectoral fins that look like wings; extensions of these fins,
looking like devils' horns, project as the cephalic fins from the
front of the head. Devil rays have long, whiplike tails
provided, in some species, with one or more stinging spines.
Devil rays,
related to sharks and skates, are found in warm waters along
continents and islands. They swim at or near the surface,
propelling themselves by flapping their pectoral fins and, at
times, leaping or somersaulting out of the water. They feed on
plankton and small fishes that they sweep into their mouths with
their cephalic fins.
Atlantic manta (Manta birostris)
The smallest of
the devil rays, species Mobula diabolis of
Australia, grows to no more than 60 cm (2 feet) across, but the ,
or giant devil ray (Manta birostris; see ),
largest of the family, may grow to more than 7 m (23 feet) wide.
The Atlantic manta is a well-known species, brown or black
in colour and very powerful, but inoffensive. It does not, old
tales to the contrary, envelop pearl divers and devour them
[from "devil
ray" Encyclopędia Britannica Online
http://www.eb.com:180/bol/topic?eu=30645&sctn=1 [Accessed April 19,
1999]. ]
Habitat: Manta
rays typically are found in the open ocean, but will occasionally
pass over reefs.
Average adult
size: Adults can grow to be more than 20 feet across, from
wing-tip to wing-tip.
Natural history:
Manta rays swim near the surface of the open sea. Their bad
reputation as a "devilfish" is undeserved. Hardly
man-eaters, they cruise slowly through the water, feeding on
microscopic plankton. The manta uses the fins on either side of
its head to herd plankton into its mouth. It will sometimes swim
somersaults in the water to take advantage of a particularly rich
area of plankton. Mantas are known to jump out of the water or
"breach." They will have one or two babies per
"litter."
Range: Found in
Brazil north to Bermuda.
Pictures of
Mantas
http://www.big.or.jp/~ishigaki/manta/E_manta.html
http://emoo.imaginary.com:4243/objbrowse/~manta
Rays
any of the
cartilaginous fishes of the order Batoidei, related to and placed
with them in the class Chondrichthyes (or Selachii). The order
includes 300 to 350 species.
Rays are
distinguished from sharks by a flattened, disklike body, with the
five gill openings and the mouth generally located on the
underside. Rays are further distinguished from sharks by
their greatly enlarged, winglike pectoral fins, which extend
forward along the sides of the head above the gill openings. Many
rays swim and breathe differently from sharks, propelling
themselves with their pectoral fins and taking in water for
respiration through large openings (spiracles) on the upper
surface of the head, rather than through the mouth. The ray's
tail is generally long and slender and in many species bears one
or more sharp, saw-edged, venomous spines that can be used to
inflict painful wounds.
Rays are
predominantly marine and are found in all oceans. Many are
slow-moving bottom dwellers. feed on plankton and small animals;
others take various fishes and invertebrates, sometimes damaging
commercially valuable shellfish beds. Other than skates, most or
possibly all rays bear living young. Fertilization is
internal, the male introducing sperm into the female by means of
special copulatory organs (claspers) that are the modified edges
of the pelvic fins.
Rays can be
classified into the following groups: electric rays,
sawfishes, skates, and various families of rays that have
slender, whiplike tails equipped with spines and that are
all-inclusively called , or whip-tailed rays.
The (suborder
Torpedinoidei) are distinguished by large paired electric organs
between the pectoral fins and the head, with which they can give
powerful shocks either for defensive purposes or to kill prey.
The electric rays have a smooth and naked skin; the head
and trunk with the pectoral fins form a circular disk, and the
tail is short and stout. About 20 species are known to inhabit
warm seas, with some reaching a weight of 200 pounds (90 kg).
All other types of
rays, which lack electric organs, generally have a rough
skin, often bearing strong spines. The (family Pristidae) have a
snout that is modified into a long blade possessing a series of
strong teeth on each side. About six species are known from warm
seas, frequenting sandy shores and estuaries.
In the (suborder
Rajoidei), the large pectoral fins extend to the snout and
backward, stopping abruptly at the base of a slender tail. In
contrast to other rays, skates produce eggs; these are
large and oblong in shape with dark, leathery shells having a
tendril at each corner by which they become fastened to seaweed
or other objects. Skates lack the long, slender barbed spine that
distinguishes stingrays. The most widespread skates belong to the
genus Raja of the family Rajidae.
The remaining rays
comprise the suborder Myliobatoidei and consist of whip-tailed rays
(family Dasyatidae), butterfly rays (Gymnuridae),
stingrays (Urolophidae), eagle rays (Myliobatidae), devil rays
(or mantas; Mobulidae), and cow-nosed rays
(Rhinopteridae). Common to the rays of all these families
is a long, slender, whiplike tail that usually has a barbed spine
connected with a poison gland; this spine is capable of
inflicting serious wounds and is a dangerous weapon when the tail
is lashed. Almost all of these rays are inhabitants of
warm seas, except for a few species of stingray that live in the
rivers of South America. (See .)
The are a group of
fishes that are closely related to the rays and are either
classified as a separate order (Rhinobatiformes) or as a suborder
(Rhinobatoidei) of the ray order (Batoidei).
The majority of
batoid fishes (members of the order Batoidei; i.e., and
allies) are bottom dwellers, preying on other animals on or near
the sea floor. (Rhynchobatidae and Rhinobatidae), butterfly rays
(Gymnuridae), eagle rays (Mylobatidae), and cow-nosed rays
(Rhinopteridae) feed on invertebrates, principally mollusks and
crustaceans. Whip-tailed rays (Dasyatidae) use their broad
pectoral fins to dig shellfish from sand or mud. Skates lie on
the bottom, often partially buried, and rise in pursuit of such
active prey as herring, trapping the victims by swimming over and
then settling upon them, a practice facilitated by the skates'
habit of hunting at night.
(Torpedinidae) are
characteristically bottom fishes of sluggish habits. They feed on
invertebrates and fish, which may be stunned by shocks produced
from the formidable electric organs. With their electricity and
widely extensible jaws, these rays are capable of taking
very active fishes, such as flounder, eel, salmon, and dogfish.
Shallow-water electric rays have been observed to trap
fishes by suddenly raising the front of the body disk, while
keeping the margins down, thereby forming a cavity into which the
prey is drawn by the powerful inrush of water.
Most of the (seven
recognized families of the suborder Myliobatoidea, which includes
all of the typical rays) swim gracefully, with undulations
of the broad, winglike pectoral fins. Some species, especially
the eagle rays, frequently swim near the surface and even
jump clear of the water, skimming a short distance through the
air.
(From Seaworld
info sheets)
The wingspan, or
disc-size, of a southern stingray (Dasyatis americana) such as
the one shown here can reach up to 1.5 m (5 ft.).
All rays belong to
the superorder Batoidea, which includes stingrays, electric rays,
skates, guitarfish, and sawfish. Like sharks—their close
relatives—batoids have skeletons made of tough connective
tissue called cartilage. About 480 species of batoids are
distributed worldwide, particularly in warm and temperate
climates, and are found in oceans, estuaries, freshwater streams,
lagoons, lakes, shallow offshore waters, and coastlines.
Rays primarily
feed on molluscs, crustaceans, worms, and occasionally smaller
fishes. Some rays crush their prey between their blunt teeth,
sometimes referred to as bony plates. Often completely burying
themselves in the sand or soft sediment, rays are camouflaged by
a grayish-brown, often mottled coloration.
Reminiscent of
birds in flight, some rays gently flap their enlarged pectoral
fins, or "wings," to "fly" through and
sometimes even leap out of the water. A ray’s wingspan, or
disc-size, can range from about 30 cm (12 in.) in yellow
stingrays to over 6.1 m (20 ft.) in manta rays.Among the best
know rays are stingrays, which have long, slim, whiplike tails
armed with serrated, venomous spines. A stingray lashes its tail
only as a defensive measure when it is caught, stepped on, or
otherwise disturbed. When wading in shallow waters, people should
shuffle their feet to avoid stepping on a buried stingray. In many
parts of the world, some rays are commercially important food
sources, yet currently, rays are not considered threatened or
endangered. Due to humankind’s impact on the marine
environment, however, concern is mounting for the future of rays
throughout their range.
Food habits
All sharks are
carnivorous and, with a few exceptions, have broad feeding
preferences, governed largely by the size and availability of the
prey. The recorded food of the (Galeocerdo cuvieri), for
example, includes a wide variety of fishes (including other
sharks, skates, and stingrays), sea turtles, birds, sea lions,
crustaceans, squid, and even carrion such as dead dogs and
garbage thrown from ships. Sleeper sharks (Somniosus),
which occur mainly in polar and subpolar regions, are known to
feed on fishes, small whales, squid, crabs, seals, and carrion
from whaling stations. Many bottom-dwelling sharks, such as the
smooth dogfishes (Triakis and Mustelus), take
crabs, lobsters, and other crustaceans, as well as small fishes.
The two giant
sharks, the whale shark (Rhincodon typus) and basking
shark (Cetorhinus maximus), resemble the baleen whales in
feeding mode as well as in size. They feed exclusively or chiefly
on minute passively drifting organisms (plankton). To remove
these from the water and concentrate them, each of these species
is equipped with a special straining apparatus analogous to
baleen in whales. The basking shark has modified gill rakers, the
whale shark elaborate spongy tissue supported by the gill arches.
The whale shark also eats small, schooling fishes.
The (Pristiophoridae) and (Pristidae) share a specialized mode of
feeding that depends on the use of the long, bladelike snout, or
"saw." Equipped with sharp teeth on its sides, the saw
is slashed from side to side, impaling, stunning, or cutting the
prey fish. Saw sharks live in midwaters; sawfishes, like most
other rays, are bottom inhabitants.
(Alopias)
feed on open-water schooling fishes, such as mackerel, herring,
and bonito, and on squid. The long upper lobe of the tail, which
may be half the total length of the shark, is used to frighten
the fish (sometimes by flailing the water surface) into a
concentrated mass convenient for slaughter.
Most sharks and
probably most rays segregate according to size, a habit
that protects smaller individuals from predation by larger ones.
Even among sharks of a size category, dominance between species
is apparent in feeding competition, suggesting a definite nipping
order. Other sharks keep clear of (Sphyrna), whose
manoeuvrability, enhanced by the rudder effect of the head, gives
them an advantage. When potential prey is discovered, sharks
circle it, appearing seemingly out of nowhere and frequently
approaching from below. Feeding behaviour is stimulated by
numbers and rapid swimming, when three or more sharks appear in
the presence of food. Activity soon progresses from tight
circling to rapid crisscross passes. Biting habits vary with
feeding methods and dentition. Sharks with teeth adapted for
shearing and sawing are aided in biting by body motions that
include rotation of the whole body, twisting movements of the
head, and rapid vibrations of the head. As the shark comes into
position, the jaws are protruded, erecting and locking the teeth
into position. The bite is extremely powerful; a (Isurus),
when attacking a swordfish too large to be swallowed whole, may
remove the prey's tail with one bite. Under strong feeding
stimuli, the sharks' excitement may intensify into what is termed
a feeding frenzy, in which not only the prey but also injured
members of the feeding pack are devoured, regardless of size.
In most cases the
initial attraction to the food is by smell. Laboratory studies
have shown that sharks do not experience hunger in the normal
sense of the word, and they are much more prone to be stimulated
to feeding by the olfactory or visual cues announcing the
appearance of prey.
Manta, or
devil, rays swim mostly at or near the surface,
progressing by flapping motions of the pectoral fins. Even the
largest often leap clear of the water. In feeding, a manta
moves through masses of macroplankton or schools of small fish,
turning slowly from side to side and using the prominent cephalic
fins, which project forward on each side of the mouth, to fan the
prey into the broad mouth.
Chimaeras and
ghost sharks dwell near the bottom in coastal and deep waters, to
depths of at least 2,500 metres (about 8,000 feet). They are
active at night, feeding almost exclusively on small
invertebrates and fishe
Reproductive
behavior
Mature individuals
of some species of sharks segregate by sex, coming together only
during the mating season, when the males, at least those of the
larger, more aggressive species, stop feeding. Segregation is a
behavioral adaptation to protect the females, one principal
courting activity used by the male to induce cooperation of the
female in mating being that of slashing her with teeth especially
developed for that purpose. After mating, the sexes again
separate. The pregnant females also tend to keep apart from the
other females of like size. As the time of parturition
approaches, the pregnant females move to particular areas, which
presumably have properties of environment especially suitable as
nursery grounds. When giving birth to their young, they stop
feeding, and, soon after parturition is completed, they depart.
Nursery areas vary
with species. Some sharks--e.g., the bull and sandbar
sharks--use shallow waters of bays and estuaries; the silky shark
uses the bottom far out on oceanic banks such as the Serrana Bank
in the western Caribbean. The Atlantic (Squalus acanthias)
bears its young mostly during the winter far out on the
continental shelf of northeastern America almost two years after
mating.
A few skates that
have been observed mating may be characteristic of other rays.
The male seizes the female by biting the pectoral fin and presses
his ventral surface against hers while inserting one, or in some
species, both claspers into her cloaca. Male skates have one to
five rows of clawlike spines on the dorsal side of each pectoral
fin. These are retractile in grooves of the skin and are used to
hold the female during mating.
The eggs of skates
in aquaria have been observed to be extruded in series, usually
of two but sometimes one, with rests of one to five days between
extrusions. A female of a European skate, Raja brachyura,
laid 25 eggs over a 49-day period in the aquarium located at
Plymouth, England.
Although the
mating of chimaeroids has not been observed, it is generally
presumed that the mode of copulation is similar to that of sharks
and that the male's frontal spine and anterior appendage of the
pelvic fins are probably used in securing the female. Two eggs
are laid simultaneously, one from each oviduct. They are often
carried for a relatively long period before being laid, several
hours or even days, each protruding for the greater part of its
length.
All species of
sharks, rays, and chimaeras produce large, yolk-rich .
These are fertilized internally, for which the males are equipped
with two copulatory organs called claspers along the inner edges
of the pelvic fins. Each clasper has a groove for guidance of
sperm. The few published descriptions of mating sharks and rays
are probably characteristic of the entire group. The male grasps
one of the female's pectoral fins with his teeth to hold her in
position as he inserts a clasper through a cavity (cloaca) and
into a tube (oviduct). Males of most species probably use only
one clasper at a time. The sperm travel to the anterior end of
the oviduct, where they fertilize the eggs. The eggs then move
down the oviduct past the shell gland, where they are covered by
a shell or capsule.
In oviparous
(egg-laying) species, which include some of the sharks, probably
all the skates, possibly some of the guitarfishes, and all of the
chimaeras, the eggs are enveloped in a horny shell, usually
equipped with tendrils for coiling around solid objects or with
spikelike projections for anchoring in mud or sand. The egg cases
of most species are more or less pillow-shaped; those of the
(Heterodontidae) are screw-shaped with a spiral flange. The eggs
of chimaeras are elliptic, spindle-shaped, or tadpole-shaped and
open to the exterior through pores and slits that permit entrance
of water during incubation. An egg of the found in the Gulf of
Mexico measured 30 centimetres (12 inches) long by about 14
centimetres (51/2 inches) wide and was
eight centimetres (three inches) thick. Protected by the shell
and nourished by the abundant yolk, the embryo of an oviparous
species develops for 41/2 to 143/4
months before hatching.
The majority of
sharks and most, possibly all, rays other than the skates
are ovoviviparous (i.e., the egg hatches within the
mother). In this case, the egg is first coated in the shell gland
with a temporary membranous capsule that lasts only during early
development. After emerging from its capsule, the embryo remains
in the oviduct of the mother, nourished by the yolk sac to which
it remains attached. Embryos of some ovoviviparous sharks,
notably the porbeagle (Lamna nasus), mako (Isurus
oxyrinchus), and (Odontaspis taurus), ingest yolks of
other eggs and even other embryos within the oviduct of the
mother after the contents of their own yolk sacs are exhausted.
In the majority of ovoviviparous sharks and rays,
organically rich uterine secretions provide supplemental
nourishment, which is absorbed by the yolk sac and in many cases
by appendages borne on its stalk. In some genera of rays,
vascular filaments producing these secretions extend through the
spiracles and into the digestive tract of the embryos.
Several shark
species are viviparous--i.e., the yolk sac develops folds
and projections that interdigitate with corresponding folds of
the uterine wall, thus forming a yolk placenta through which
nutrient material is passed from the mother.
Back to
referring page
<
Back
to Home Page
