Class Osteichthyes
Introduction
Osteichthyes is a class of jawed fishes having a bony endoskeleton. It is the largest class of vertebrates and includes a diverse group of marine and freshwater bony fishes.
The other group of jawed fishes are cartilaginous fishes, which are included in the class Chondrichthyes.
Osteichthyes Classification
Osteichthyes is a class of fishes included in the division Gnathostomata, which includes all the vertebrates having jaws. Jawless fishes are included in the division Agnatha and the class Cyclostomata.
Gnathostomata is divided into two superclasses, viz. Pisces (having fins) and Tetrapoda (bear limbs).
Pisces is divided into two classes:
Osteichthyes- Bony fishes
Chondrichthyes- Cartilaginous fishes
Osteichthyes is subdivided into two subclasses:
1.Sarcopterygii- lobe-finned fish
2.Actinopterygii- ray-finned fish
Kingdom : Animalia
Phylum : Chordata
Subphylum : Vertebrata
Division : Gnathostomata
Superclass : Pisces
Class : Osteichthyes
Osteichthyes characteristics
With more than 29,000 species of bony fishes, it is the largest class of vertebrates.
It includes both marine and freshwater fishes, most of the commercially used fishes are included in this class.
The size ranges from less than 8 mm in Paedocypris progenetica, which is also the smallest known vertebrate to 4 m and weigh about 1500 kg in the ocean sunfish (Mola mola).
Their endoskeleton is made up of bones.
Paired and median fins are present, which are supported by long rays of cartilage or bone. Fleshy lobed fins are present in sarcopterygians.
These types of fins are supported by bones having joints.
These are adapted to live at the bottom of the sea.
The tail is mostly homocercal.
They contain a swim bladder or air bladder, which provides buoyancy to them and prevents sinking.
The swim bladder also facilitates gaseous exchange.
The mouth is terminal.
Bony fishes contain four pairs of gills. A protective covering of a bony flap known as the operculum protects the gills.
The sarcopterygians, lungfishes and lobe-finned fishes contain lungs.
The skin is covered by bony dermal scales known as ganoid, cycloid or ctenoid scales.
They are poikilotherms or cold-blooded animals and lack the capacity to regulate their internal body temperature. Some of the larger marine fishes like tuna, swordfish, etc. show some level of endothermy.
The heart is two-chambered, contains one auricle and one ventricle. Lungfishes have a three-chambered heart with two auricles and one ventricle.
The brain has a small olfactory lobe and cerebellum. There are ten pairs of cranial nerves present.
The lateral line organ contains hydrodynamic receptors. The sensory unit is called neuromasts. It helps in sensing vibration, water pressure, navigation and locating their prey.
Cloaca is absent, different genital and anal openings are present.
They are ammonotelic and have mesonephric kidneys.
Sexes are mostly separate but some are hermaphrodites. They are mostly oviparous and lay a huge amount of eggs. Fertilisation is mostly external and direct development.
In the male Hippocampus, the brood pouch is present, where eggs incubate.
Osteichthyes Examples
Some of the prominent examples of bony fishes are:
Ray-finned fishes
Marine bony fishes
.Hippocampus (Sea horse)
.Exocetus (Flying fish)
.Lophius (Angler fish)
Freshwater bony fishes
.Labeo rohita (Rohu)
.Labeo catla (Katla)
.Clarias (Walking catfish or Magur)
.Mystus (Catfish)
Aquarium bony fishes
.Betta splendens (Fighting fish)
.Pterophyllum (Angelfish)
Lobe-finned fishes
Lungfishes
.Lepidosiren
.Neoceratodus
.Protopterus
Coelocanth Latimeria is a living fossil and the oldest known lineage of Sarcopterygii.
Sea Horse
Scientific Classification
Common Name : Seahorse
Kingdom : Animalia
Phylum : Chordata
Class : Osteichthyes
Order : Syngnathiformes
Family : Syngnathidae
Genus Species : Hippocampus spp.
Description
Seahorses are elongate with rigid body armor and swim upright. Pectoral fins on the sides and a small dorsal fin on the back of a seahorse's body wave rapidly to move the seahorse through the water. They feed using small mouths at the end of tubular snouts.Size : The various species range in size from about 5 to 36 cm (2 to 14 in.) in length.
Seahorses are at once weird and wonderful, exotic and underwhelming and unique among bony fishes. Underneath their obvious horsey-head charm, seahorse anatomy is really, really different from other fishes.
The most obvious thing about the seahorses is their body design – an upright torso connecting a horse-shaped head and a monkey-like tail. They’re bony fishes, but pretty much the only bony fish that swims upright. When they swim.
If searching for them carries an air of anticipation, finding them is like watching grass grow. When spotted, they’re likely to just be hanging out attached to a sea rod or something, ignoring us and wishing we would go away. It works. Eventually we do.
Looks aren’t everything. Far more than just fish with horse-shaped heads, they’re complex little guys.
Brief
For fishes, they’re notoriously bad swimmers, among the worst in the ocean.
Yet they’re super-predators of tiny shrimps and other planktonic delectables, and have few predators of their own.
They may not swim well but their horse-shaped heads and square tails power body mechanics that contribute to their success in capturing prey.
Their finless tails are prehensile, enabling them to grasp onto solid objects and anchor themselves.
Unlike most bony fishes, they lack scales, sporting an armor of interlocking bony rings, covered by a thin layer of skin.
Internally, seahorses lack teeth and a stomach and have to eat pretty much continually to live.
They’re largely monogamous, with charming, elaborate daily rituals between mates.
Beyond their general horsey shape, they may be best known for the fact that the male does the childbearing.
Distribution
Found in oceans worldwide, seahorses are represented by some 44 species in the genus Hippocampus (from the Greek for horse and sea monster), ranging in size from the recently defined grain-of-rice-sized “Japan pig” pygmy seahorse (Hippocampus japapigu) found in coastal Japanese waters to the foot-long big bellied seahorse (Hippocampus abdominalis) found in southern Australia and New Zealand seas. Members of the Syngnathiadae Family, they’re related to pipefishes and seadragons.
Most of us probably associate seahorses with tropical seas because that’s where we see them. But a species like the lined seahorse (Hippocampus erectus) is found not just in the Caribbean but in the Atlantic as far north as Cape Cod. Two species are native to British and Irish waters as far north as the Shetland Islands. Three species are found in the Mediterranean. Obviously, the greatest number of species are found in the Indo-Pacific basin.
They typically inhabit shallow waters – reefs, ledges, mangrove forests and eel grass beds – but move into deeper waters during rougher winter months.
Anatomy
Despite their shapes, seahorses are bona fide bony fishes, equipped with bony skeletal structures, swim bladders and gills. Their shapes, of course, makes them unique, and they’re the only fishes who swim in an upright posture.
Bony skeletons they have, but more like exoskeletons. Their bodies are encased in a structure of interlocking plates covered by a fleshy skin. They lack the usual fishy scales. This bony structure, in effect, confers on them a suit of armor that accounts for their lack of predators – they’re hard to eat. They’re most vulnerable to being grabbed by the pincers of larger crabs.
Fortunately, they’re masters of camouflage. Not only do their colors range from red and orange to purple and white, they can change their colors to blend in with their surroundings.
Swimming
On the other hand, seahorse anatomy makes the little guys the slowest-swimming fishes in the ocean. Without tail fins, their propulsion is provided by a single, small dorsal fin that beats furiously – as much as 50 times per second. Their small pectoral fins don’t help much with speed, mainly serving to steer the little fish around. And, swimming upright, these horse-shaped fishes are hardly streamlined for rapid movement through the water.
Put all this together and they typically move at a heart-racing 5 ft./1.5 m. per hour. It is heart-racing. After a strenuous journey they can die of exhaustion. Still, they have one swimming trait nearly all other fishes lack. They can move upwards, downwards and backwards.
And the lack of swimming finesse may not matter. Like a lot of small fishes in the sea, they tend to spend their lives pretty much in one small patch of seabottom. Females forage around territories of about 1,100 sq. ft./100 sq. m. Perhaps because males are busy taking care of the babies, they have a particularly confined territory – 5½ square feet/.5 square meters.
Tail
If their tails aren’t much use for swimming, another weirdness of seahorse anatomy is they’re prehensile – like monkeys’ tails that can be wrapped around tree branches and held tight. In seahorse world, this means they can anchor themselves firmly onto a sea rod or seagrass stalk or mangrove root and hang on in place.
And, despite their lack of swimming agility, if they want to travel they can lock on to loose seaweed or debris with their tails and hitchhike for long distances. Doing so may be more a matter of anchoring onto something firm than intending to shift territories.
Hunting
Here’s where weird seahorse anatomy pays off: They’re really, really good at capturing prey – mostly small mysid shrimps, copepods, other invertebrates and occasionally, small fishes and fish larvae that drift by in the water or move along the seafloor. Anchoring themselves with their tails, their curved bodies and horse-shaped head-and-neck design maximize the speed and distance with which they can capture prey.
Normally, any prey in question can sense vibrations in the water caused by a predator’s movement and bolt away at extremely high speeds. But the horsey heads of seahorses, researchers have determined, are craftily shaped to produce minimal disturbances as they approach their targets, letting them get very close to their target before striking.
Like most other fishes, seahorses feed by drawing the prey in with suction. Depending on the species, the effective striking distance ranges from only about one millimeter to three centimeters. And they’re fast. A strike takes less than one millisecond.
They have a 90 percent success rate, among the highest for any fish (great white sharks achieve about 55 percent).
Toothless and Stomachless
Seahorse hunting techniques depend on their habitat. In areas with little vegetative cover, they’ll sit and wait for prey to come to them – the tiny shrimps, copepods and other stuff that drift by in the planktonic soup of the ocean currents. In habitats with significant vegetative cover – seagrass patches and mangrove forests, for example – they’re likely to move around and hunt while swimming. The slender headshape of seahorse anatomy is useful here, too: It’s helpful for probing in nooks and crannies.
And they need that 90 percent kill rate. Another seahorse anatomy weirdness: they have neither teeth nor stomachs. They’re extremely good at catching prey; they’ve very bad at digesting it, which means they have to eat pretty much continuously to stay alive and healthy. Scientists estimate that a typical seahorse consumes as many as 3,000 prey each day.
Daddy Care
Seahorses are widely famous for the fact that the males do the childbearing – one of the few animal species that does this. During mating, a female seahorse deposits her eggs in a brood pouch on the male’s abdomen, after which he fertilizes and nurtures them with nutrients and oxygen until they hatch, usually a period of several weeks.
The result is fully formed teeny, tiny seahorses. The number varies, from as few as five to as many as a thousand. Volume is good; the rate of survival into adulthood is estimated to be less than one percent. They may be well-nourished and protected during pregnancy but once they’re born they’re on their own. Emerging from their daddy’s pouch headfirst, fully able to swim, they drift with the plankton until they find a territory suitable for settling down.
Male lined seahorses (Hippocampus erectus), a common species found in the Caribbean, develop their brood pouches at about five to seven months. They’re considered mature at just over two inches in length and can grow to about seven inches during lifespans that can last as long as four years.
Monogamous
Seahorses often mate for life. Some are more serially monogamous, staying together only for extended mating seasons. Whichever, it’s strategy that fosters reproductive success during the course of a mating season.
In any event, male and female seahorse couples start each day with a greeting ritual, moving through complex, rhythmic twists and twirls, sometimes changing colors and interlocking their tails. For one thing, the rituals ensure that their reproductive cycles are synched.
These dances last for several minutes before they separate for the day to pursue their predation strategies, the female going off into her significantly larger hunting territory.
Vulnerable
Seahorses’ major goal in life may be to just mind their own business, eat well and produce little miniature seahorses. They have few predators among reef denizens but they do have us humans. The nonprofit Seahorse Trust estimates that more then 1.5 million seahorses are taken from the oceans annually for the traditional Chinese medicine trade, one million for sale as dried curios and one million for the aquarium pet trade.
On its Red List, the International Union for the Conservation of Nature (IUCN) lists two species as endangered, 12 as vulnerable, one as near threatened and 17 as of least concern. But it also indicates that data is insufficient for 17 species.
Life cycle of Seahorses
Seahorses court for several days at a time. The courting ritual involves dancing, changing colour and entwining tails. Sometimes, more than one male seahorse will compete to win the affections of a female, or vice versa. Males can inflate their pouch by pumping water through it to display its emptiness. This is to entice the female to deposit her eggs in it.
Egg Transfer: The female deposits her egg in the water in order to transfer to the males pouch. The male releases his sperm directly into seawater to fertilize the eggs. They are then embedded in the pouch wall and become surrounded by a spongy tissue. The number of eggs varies between species.
Pregnancy: Although the male carries the baby, the initial conditions in the pouch are determined by the nutrients provided by the female, but when the eggs are in the pouch conditions are controlled by the male. He provides oxygen and nutrients through a network of capillaries. The embryos will remain in the pouch for two to six weeks, depending on species and temperature, as they develop into fully formed juveniles.
Birth: When the male seahorse is ready to give birth he has muscular contractions to expel the young, which are known as fry, from the pouch.
Young: Newborns measure between two and twelve millimetres, depending on species. The number of young produced ranges between 100 and 200 for most species, but can be as low as 5 for the smaller species, or more than 1,500 for larger species. The fry will often grasp floating or still objects, and even each other, with their tails.
Juveniles: Newly released young undergo only small changes after emerging from the father. During growth some body proportions may change but they don’t experience major body changes. Some newborns emerge with a small fin which is lost over time.
Adult: Mature male seahorses have the ability to become pregnant any time during the breeding season, which varies with species. This is thought to be influenced by environmental conditions such as water temperature.
Fun Facts
1.A seahorse is a type of fish closely related to pipefishes and belonging to the scientific family Syngnathidae. Roughly 35 species of seahorse occur worldwide.
2.The seahorse's scientific genus name, Hippocampus, is Greek for "bent horse."
3.The seahorse may appear as if it wears armor; its body is covered with bony rings and ridges.
4.Seahorses are well camouflaged among the relatively tall eelgrasses and seaweeds in which they make their homes. A seahorse often moors itself in the water by curling its prehensile tail around seagrass and coral branches.
5.The seahorse's small mouth, located at the end of its tube-like snout, sucks up tiny plankton and fish larvae.
