🐠 Fish Taxonomy

• Fish are members of the Animalia Kingdom (animals) and are also classified into the Phylum Chordata.
• In order to be a chordate an animal must:
  • Have a notochord (a slim and flexible rod that supports the body) at some point in their lives.
  • Have a tubular nerve chord along their back (dorsal surface) with the brain developing from a swelling found at the anterior end (front) of this tube.
  • Paired gill slits at some stage of their life history
  • Segmentation of at least part of their body
  • A post-anal tail at some stage in their life history
  • A ventral heart
  • An endoskeleton
• Fish are further classified into the Vertebrata Subphylum.
• In order to be a vertebrate, an animal must have a vertebral column, or backbone.
• This backbone encloses, supports and protects the spinal cord.
• Fish are vertebrates that live in water and breathe with gills.
• Fish are ectotherms, or cold-blooded.
• Fish have either backbone of cartilage or bone.
• Most fish are adapted to live in salt or fresh water. Most fish have fins and scales, which cover and protect the body.
• The body systems of the fish, such as the digestive, circulatory are well developed.

👉🏻 The sub-phylum Vertebrata is further divided into the following:

Class — Cyclostomata

• All living members of the class Cyclostomata are ectoparasites on some fishes.
• They have an elongated body bearing 6-15 pairs of gill slits for respiration.
• Cyclostomes have a sucking and circular mouth without jaws.
• Their body is devoid of scales and paired fins. Cranium and vertebral column are cartilaginous.
• Cyclostomes are marine but migrate for spawning to fresh water.
• After spawning, within a few days, they die. Their larvae, after metamorphosis, return to the ocean.
• Examples: Petromyzon (Lamprey) and Myxine (Hagfish).

Class — Chondrichthyes

• They are marine animals with streamlined body and have cartilaginous endoskeleton.
• Mouth is located ventrally.
• Notochord is persistent throughout life.
• Gill slits are separate and without operculum (gill cover).
• The skin is tough, containing minute placoid scales.
• Teeth are modified placoid scales which are backwardly directed.
• Their jaws are very powerful. These animals are predaceous.
• Due to the absence of air bladder, they have to swim constantly to avoid sinking.
• Heart is two-chambered (one auricle and one ventricle).
• Some of them have electric organs (e.g., Torpedo) and some possess poison sting (e.g., Trygon).
• They are cold-blooded (poikilothermous) animals, i.e., they lack the capacity to regulate their body temperature.
• Sexes are separate. In males pelvic fins bear claspers. They have internal fertilization and many of them are viviparous.
• Examples: Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), Trygon (Sting ray).

Class — Osteichthyes

• It includes both marine and fresh water fishes with bony endoskeleton.
• Their body is streamlined.
• Mouth is mostly terminal.
• They have four pairs of gills which are covered by an operculum on each side.
• Skin is covered with cycloid/ctenoid scales.
• Air bladder is present which regulates buoyancy.
• Heart is two chambered (one auricle and one ventricle).
• They are cold-blooded animals.
• Sexes are separate. Fertilization is usually external. They are mostly oviparous, and development is direct.
• Bony fish are again classified into ray finned group (perch, and catfish) and lobe finned group (lungfish). Ray finned fish have thin, flexible skeleton rays. Lobe finned fish have muscular fins supported by bones.
• Examples:
  • Marine — Exocoetus (Flying fish), Hippocampus (Sea horse),

• Freshwater — Catla (Katla), Labeo (Rohu), Clarias (Magur)

• Aquarium — Betta (Fighting fish), Pterophyllum (Angel fish)

👉🏻 Difference between the two classes is summarized as follows:

• Phytoplankton-eaters
  • Chinese silver carp
  • Indian ‘catla’ carp
  • Indian ‘rohu’ carp
  • Milk fish
• Water plant eaters
  • Chinese grass carp
  • Chinese ‘Wuchang’ bream
  • Big gourami
  • Tilapia
  • Zill’s tilapia
• Predatory fish species (fish eaters)
  • Snakehead species
• Omnivores (eat everything available)
  • Barb species
  • Crucian carp
  • Chinese mud carp
  • Common carp
  • Catfish species
  • Indian ‘mrigala’ carp
  • Tilapia species
• Larvivorous fishes
  • Fishes which prefer mosquito larvae and feed on them throughout their life are known as larvicidal fishes.
  • The example are Carassius auratus, Lebistes, Gambusia affinis, Gambusia holbrooki, Tilapia, Notopterus notpterus etc.

Transgenic fish

• A transgenic fish is one which carries one or more foreign genes.
• The foreign genes are selectively incorporated by microinjection into the eggs with a view to produce transgenic fish lines carrying such foreign genes.

Predatory fishes

• Fishes which prey on smaller fishes or hatchlings i.e. Clupisoma garua, Silonia silondia, Rita rita, Wallago attu, Nandus nandus, Channa striatus, Ailiaberg etc.

Weed fishes

• These fishes have good fecundity and attain sexual maturity in summer and breed even without rain prior to the monsoon and therefore their young ones are abundant in number during monsoon.
• The examples are Ambassis, Barilius barila, Chela cochins, Gad usia chapra, Punius ticto, Oxygaster bacaila etc.
• Fecundity: The capacity of an individual fish to produce female reproductive cells is known as fecundity. The fecundity is a quality of the species which ensures high rate of intraspecific competition and more rapid rate of evolution i.e. the species with high fecundity have a greater biotic potentiality.

Ocean Zones

• Oceanic zones were created by oceanographers to make it easier to study individual areas of the oceans.
• They were also designed to study animals living in specific areas of the oceans.
• The Ocean can be divided into zones based on depth (Vertically) and distance from the land (Horizontally).
• Major vertical zones of the ocean begin at sea level and end at the deepest points in the ocean.
• Another set of zones in the ocean are horizontal.
• Starting at the seashore they cover the ocean floor. These zones include the seashore, continental shelf, continental slope, deep ocean floor, and ocean trenches.

• Continental margin is the submerged area of the continents. It includes:
  • Continental Shelf: (Flat gradually sloping seafloor) from shoreline to 200m. End of Shelf is called Shelf Break.
  • Continental Slope (Steeply sloping seafloor) seaward of shelf break.
  • Continental Rise (Moderately sloping seafloor) seaward of slope.

Various classification of Ocean Zones

Pelagic and Benthic Zones

• One simple ocean zone classification is between the Water and the Ocean Floor.
• The water is referred to as the Pelagic Zone (The organisms that swim through the water column are known as Nekton).
• The Ocean floor is referred to as the Benthic Zone (The organisms that live here are benthic organisms or Benthos).
• The classification of the Pelagic Zones:
  • The pelagic zone is the region of a body of water (lake, river, or ocean) that is not associated with the bottom (see benthic zone) or shore (see littoral zone).
  • This habitat zone is truly a three-dimensional habitat space. Some fish that occupy the pelagic zone never encounter the bottom or shore throughout their lives.
• The pelagic zone is divided into zones based on light penetration from the surface (depth from surface)
  • Epipelagic (sunlight: 0 - 200m)
  • Mesopelagic (twilight: 200 - 1,000m)
  • Bathypelagic (no light: 1,000 - 4,000m)
  • Abyssopelagic (4,000 m — 6,000m)
  • Hadopelagic (6,000 m — 10,000m)
• The bathypelagic zone is also called the midnight zone because no light penetrates the area and the water is near freezing.

Epipelagic Zone

• The epipelagic zone is the water from the surface of the sea down to 200 metres.
• It is also referred to as the surface waters or the sunlit zone and includes the photic zone.
• The photic zone is defined as the surface waters down to the point where the sunlight has attenuated to 1 percent of the surface value.
• This depth depends on how turbid the water is, but in clear water can extend to 200 metres, coinciding with the epipelagic zone.
• The photic zone has sufficient light for phytoplankton to photosynthesize.

Horizontal division of the Pelagic Zone

Neritic Zone

• The Neritic zone lies above the continental shelf.
• It extends from the low-tide mark outward from the seashore to where the depth of the water reaches 200 meters (656 feet).
• This water is lit by sunlight and relatively shallow.
• The majority of sea life lives in this zone because it has well-oxygenated water, low pressure, and a fairly stable temperature.

Oceanic Zone

• The Oceanic zone extends from the edge of the continental shelf over the continental slope to the ocean floor beyond.
• It begins where the water is over 200 meter (656 feet) deep.
• This zone contains deep trenches, volcanoes, and ocean basins.

Light Zones in the Ocean

Euphotic Zone

• The uppermost layer of the world’s oceans is bathed in sunlight during the daytime.
• This bright ocean layer is called the sunlit zone or the euphotic zone (euphotic means well lit" in Greek) or the epipelagic zone (epipelagic means “upon the sea”).
• The depth of this zone depends on the clarity or murkiness of the water.
• In clear water, the euphotic zone can be quite deep; in murky water, it can be only 50 feet deep.
• On average, it extends to about 660 feet (200 meters); the depth of the ocean averages about 13,000 feet or 4 000 m
• In this zone, there is enough light for photosynthesis to take place, so many plants and other photosynthetic organisms live in this zone and food is abundant.
• Photosynthesis is a process in which sunlight and carbon dioxide gas are converted into food (chemical energy contained in carbohydrates) and oxygen gas.
• Photosynthesis in the oceans creates approximately 90% of the Earth’s gaseous oxygen.
• Most of the oxygen is produced by phytoplankton.
• These primary producers (also called autotrophs) are the first link in the food chain in the oceans.
• Because of this food source, many animals also live in this zone.
• In fact, most of the life in the ocean is found in this zone, although it is the smallest ocean zone in terms of volume of water.

Disphotic Zone

• The middle layer of the world’s oceans receives only faint, filtered sunlight during the daytime. This is because the seawater absorbs the sunlight. This barely-lit ocean layer is called the twilight zone or the disphotic zone (disphotic means “poorly lit” in Greek) or the mesopelagic zone (mesopelagic means “middle sea”).
• This zone appears deep blue to black in color.
• The depth of this zone depends on the clarity or murkiness of the water.
• On average, this zone extends from 660 to 3,300 feet (200 to 1,000 m).
• In the disphotic zone, there is enough light to see during the day, but not enough light for photosynthesis to take place, so no plants live in this zone.
• The amount of light decreases with depth. Because of this, food is not abundant.
• The water in the disphotic zone is cold (the temperature ranges from 41 to 39 degrees F) and decreases with depth. The pressure is high — it can be up to 1,470 psi (pounds per square inch) and increases with depth. The amount of dissolved oxygen in the water is less than in the sunlit zone.
• Thus, the Ocean can be divided broadly into Photic and Aphotic Zones based on the penetration of light.
• The Photic Zone is further divided into Euphotic Zone and Disphotic Zone.

Benthic Zone

• The benthic zone is ocean floor all the way from the low-tide mark outward to the deepest trenches on the ocean floor.
• Animals have been found living in all areas of the ocean.
• Most plant and animal life lives on the continental shelf that is relatively shallow, and sunlight penetrates the water to support plant life.
• Many different types of plants and animals live in the zone and are often attached to the rocks.

Classification of the Benthic Zones

• Littoral
• Bathyl
• Abyssal
• Hadal

  

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