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A Closer Look Into the Fintastic World of Fish

Sabrina Lane
February 2024
Seahorses are fish. They have gills and a swim bladder.

What do fishes eat? What are the characteristics of fishes and what features do fishes share?

This month, we take the spotlight and shine it on fishes. Before diving deeper into the ways and wiles of our fishy friends, however, let’s take a little detour.

Is the plural of fish ‘fish’ or ‘fishes’? Well, both are correct. That said, the norm is to use the plural ‘fish’ when referring to two or more fish that belong to the same species and to use the plural ‘fishes’ when referring to two or more fishes that belong to different species. Therefore, when referring to two, three, or more yellow-edged morays, use ‘fish’. However, when referring to a group composed of three yellow-edged morays, two northern stargazers, one giant trevally, and one bluefish, use ‘fishes’.

Now that that’s out of the way, let’s start discovering the world of fishes and gain a deeper appreciation of the most numerous of all vertebrates. Learn what fishes eat and their characteristics. Differentiate between bony fish (osteichthyes) and cartilaginous fish (chondrichthyes), and understand the many adaptations of fishes for survival.

A Welcome Note from Sabrina

Sabrina is a Zoological Manager at SeaWorld Yas Island, Abu Dhabi. She has worked with SeaWorld for over 18 years, with a big focus on husbandry of large tropical saltwater systems, including bony fish, sharks, rays, eels, and invertebrates. Throughout her career, she has worked with various teams and animals, successfully launching guest interaction programmes, such as ray feeding and swimming with sharks. Sabrina has played a crucial role in the pre-opening process of the Endless Ocean realm, by ensuring all animals fully adapt to their new environment, and by helping to bring the SeaVenture experience to Abu Dhabi.

Fish habitats: where do fishes live?

Fishes live in either freshwater or saltwater. Their habitats include lakes, ponds, rivers, streams, springs, wetlands, canals, reservoirs, coral reefs, kelp forests, bays, and deep-sea habitats like abyssal plains, hydrothermal vents, and cold seeps.

Factors influencing fish habitat

Fishes choose their habitat based on several factors. These factors include the availability of food, amount of light, oxygen levels, competition for resources, and risk of predation. All things being equal, fishes choose the habitat that would maximize their net energy gains.

Simplistically speaking, the ideal habitat provides more benefits (for instance, due to an abundance of prey) than risks (for instance, the presence of many predators). The former makes it easier to find food, while the latter means a lot of energy spent avoiding and evading predators. It is also important to note that fishes’ preferred habitats may change throughout their life cycle.

A school of sweetlips
A school of colorful ribbon sweetlips

Scientific facts about fish: what is fish?

Fishes are a group of aquatic animals. They are vertebrates like us, so all fishes have skeletons inside their bodies. However, whereas we are warm-blooded animals that breathe air, grow hair, and feed on milk as babies, most fishes are cold-blooded, live underwater, and absorb oxygen from the water through their gills.

All fishes fall under one of three groups: Agnatha (jawless fish), Osteichthyes (bony fish) and Chondrichthyes (cartilaginous fish). The following is a summary of the scientific classification of fishes:


Kingdom: Animalia

Phylum: Chordata

Subphylum: Vertebrata

Divisions: Agnatha (jawless), Gnathostomata (with jaw)

Classes: Cyclostomata (under Agnatha), Chondrichthyes (under Gnathostomata), Osteichthyes (under Gnathostomata)


Class Cyclostomata: jawless fish

One of the primary characteristics of fishes under the Agnatha division is their lack of jaws. Instead, they have a circular mouth with rows of teeth. The parasitic lamprey and hagfish are examples of jawless fishes, but some do not consider them true fishes because they only have a rudimentary skeleton.

Class Chondrichthyes: cartilaginous fish

Sharks, skates, and rays are Chondrichthyes. They have a skeleton made of cartilage, which is lighter, more flexible and less dense than bone.

Class Osteichthyes: bony fish

The skeleton of bony fishes is calcified bone. Most fishes belong to Class Osteichthyes; it accounts for more than 400 families and 29,000 species. There are bony freshwater fishes (e.g., goldfish and trout) and saltwater fishes (e.g., perch and trevally), and their size can vary greatly from less than seven millimeters (e.g., the male photocorynus spiniceps angler fish) to more than four meters (e.g., swordfish). They can also weigh 2,205 kilograms (e.g., ocean sunfish).

Characteristics of Osteichthyes

Aside from the calcified bone skeleton, standard Osteichthyes characteristics include:

  • Jaws
  • Paired fins
  • Paired nostrils
  • A pair of gill openings
  • Operculum (flap or covering that protects the gills)
The jawless lamprey fish
Manta Ray is an example of a cartilaginous fish

The body parts of fish (fins of fish, fish brain, etc.)

All fishes have a mouth, fins, and a brain. Most have jaws and a backbone, except jawless fishes. Fishes may have two jaws: the oral jaws that open and close the mouth, and pharyngeal jaws in the throat. Almost all fishes, including jawless fishes, have teeth. Toothless fishes include the sturgeon, pipefish, sea horse, and stout infantfish.

The following are the body parts of fish. Note that some fishes don’t have all these parts:

  • Eyes: Fish eyes can detect colors and perceive distance, and fishes use their eyes to find food and escape predators.
  • Nares: Most fishes have a pair of nostrils, which they use for smelling, not breathing.
  • Mouth: Mouths are for catching and consuming food.
  • Operculum: The operculum is a bony aperture that covers and protects the gills. Only bony fishes have an operculum.
  • Gills: Found on the side of the head, the gills have blood vessels that capture oxygen from water.
  • Scales: Most but not all fishes have scales. Jawless fishes (e.g., hagfish and lampreys) have none. Catfish, stout infantfish, paddlefish, and most species of blennies also have no scales. Cartilaginous fishes have placoid scales that do not grow with the fish; instead, new scales grow to fill in the gaps between old scales.
  • Vent: Fishes excrete waste, extra water, eggs, and sperm through their vent or anus.
  • Fins: Fishes can have pectoral, pelvic, anal, caudal, dorsal, and adipose fins, and bony fishes have paired fins.
  • Barbels: Some fishes, such as catfish, sturgeon, and long-barbel goatfish, have barbels or “whiskers” on their head that help them detect food.
  • Lateral line system: The lateral line system consists of hundreds of vibration and pressure receptors known as neuromasts. These help fishes detect the direction and the rate at which water is moving, allowing them to sense their own movement and the presence of stationary objects around them. It also alerts them to the movement of nearby prey and predators. The neuromasts may be found on the surface of the skin on the head, trunk and tail fin or embedded on the floor of lateral line canals, located just beneath the skin, which let water in through pore-like openings.
  • Gas bladder: Most bony fishes have a gas bladder in their body cavity. Also known as swim bladders, gas bladders contain gas (typically oxygen) and serve as buoyancy organs that allow fish to maintain hydrostatic equilibrium. They enable fishes to swim at their preferred depth instead of sinking or floating. The gas bladder also serves as a resonating chamber that helps fishes hear and produce sound.
A visible endoskeleton of a Siamese glassfish
The ocean sunfish also known as mola-mola can weigh up to 2,200 kilograms

Some bony fish species

The following are eight bony fishes in our care, with info about their natural environment, and the SeaWorld Yas Island, Abu Dhabi realms where you can find them. Make a list and look for them when you visit.

Maori wrasse

Māori wrasse

Cheilinus undulatus - Indo-Pacific region coral reefs
SeaWorld Realm: Endless Ocean

Orange spotted grouper

Orange-spotted grouper

Epinephelus coioides - Turbid coastal reefs and rocky areas of the Western Pacific Ocean and Indian Ocean
SeaWorld Realm: Endless Ocean

Moray eel

Moray eel

Gymnothorax sps. - Shallow-water reefs and rocks, freshwater and saltwater, tropical and subtropical waters
SeaWorld Realm: Endless Ocean

Powder blue tang

Powder blue tang

Acanthurus leucosternon - Reef environments, Indo-West Pacific region
SeaWorld Realm: Endless Ocean, Tropical Ocean

Golden trevally

Golden Trevally

Gnathanodon speciosus - Rocky and coral reefs, sand flats, Indo-Pacific region
SeaWorld Realm: Abu Dhabi Ocean, Tropical Ocean, Endless Ocean

Long barbel goatfish

Long-barbel goatfish

Parupeneus macronema - Reef habitats, Indo-West Pacific region
SeaWorld Realm: Abu Dhabi Ocean, Endless Ocean

Blue spotted boxfish

Bluespot boxfish

Ostracion meleagris - Pacific Ocean, Indian Ocean
SeaWorld Realm: Endless Ocean

Clownfish

Clownfish

Sebae sps. - Red Sea, Pacific Ocean
SeaWorld Realm: Endless Ocean, One Ocean, Animal Care Center

All fishes have endoskeletons, but jawless fishes don’t have a true backbone, and only bony fishes have a skeleton of calcified bone.

The beautiful but venomous lionfish

What do fishes eat?

Fish can be herbivorous, carnivorous and omnivorous. Herbivorous fishes eat only plants, carnivorous fishes eat other animals, while omnivores can eat both plants and animals.

The plants that fishes eat include algae, phytoplankton, seaweed, duckweed, and other aquatic plants. Carnivorous fishes often eat zooplankton and other fish. For instance, salmon eats Atlantic herring, sand lance, and lanternfish, among others.

Water animals that don’t have endoskeletons, known as invertebrates, are an essential food source for fishes. They include mollusks like squid, clams, snails, octopus and cuttlefish and crustaceans like crabs, prawns, shrimp, and krill. Fishes also eat worms, insects, spiders, and mites.

It may surprise you to find that fishes can eat birds, amphibians, reptiles, and mammals. Bass eat frogs regularly, while trout and pike have been known to eat shrews. If a small duckling, frog, snake, or mouse falls into the water, fishes like the pike, trout, salmon, or largemouth bass may eat it.

A stargazer waiting to ambush its prey
A largemouth bass hovers almost just below the surface searching for food
Characteristics of fishes at a glance
Mudskippers can breathe through their skin

Adaptation of fish: how have fishes adapted to survive?

Fishes develop characteristics and behaviors that improve their ability to survive and thrive. This is known as adaptation, and species pass their adaptations from generation to generation.

All living things adapt to their surroundings. For instance, we humans adapted by developing a large and complex brain, the ability to walk upright on two feet, longer thumbs that can touch the fingers on the same hand (i.e., opposable thumbs), and a bigger lung capacity. Meanwhile, fishes have adapted in the following ways:

Body shape

Fishes adapted body shapes suited to their feeding habits and habitat.

  • The butterflyfish has a disc-like body shape – i.e., laterally compressed or flattened side-to-side with a tall, rounded silhouette (i.e., compressiform). This allows it to navigate the narrow cracks or vertical crevices of coral reefs.
  • Tuna has a torpedo- or bullet-like shape (i.e., fusiform) which is streamlined for fast swimming and open-water predation.
  • Moray eels have an elongated body shape (i.e., anguiliform) suitable for maneuvering into holes and fissures