Bony Fish Facts

Most of the world's fish species are categorized into two types: bony fish and cartilaginous fish. In simple terms, a bony fish (Osteichthyes) is one whose skeleton is made of bone, while a cartilaginous fish (Chondrichthyes) has a skeleton made of soft, flexible cartilage. A third type of fish, including eels and hagfish, is the group known as Agnatha, or jawless fish. 

The cartilaginous fish include sharks, skates, and rays. Virtually all other fish fall into the class of bony fish which includes over 50,000 species.

Description

All bony fishes have sutures in their neurocranium and segmented fin rays derived from their epidermis. Both bony fish and cartilaginous fish breathe through gills, but bony fish also have a hard, bony plate covering their gills. This feature is called an "operculum." Bony fish may also have distinct rays, or spines, in their fins.

And unlike cartilaginous fish, bony fish have swim or gas bladders to regulate their buoyancy. Cartilaginous fish, on the other hand, must swim constantly to stay afloat. 

Species

Bony fish are considered to members of the class Osteichthyes, which is subdivided into two main types of bony fish:

• Ray-finned fishes, or Actinopterygii
• Lobe-finned fishes, or Sarcopterygii, which includes the coelacanths and lungfishes.

The subclass Sarcopterygii is made up of about 25,000 species, all characterized by the presence of enamel on their teeth. They have a central axis of bone that acts as a unique skeletal support for fins and limbs, and their upper jaws are fused with their skulls. Two major groups of fishes fit under the Sarcopterygii: the Ceratodontiformes (or lungfishes) and the Coelacanthiformes (or coelacanths), once thought to be extinct.

Actinopterygii includes 33,000 species in 453 families. They are found in all aquatic habitats and range in body size from under a half inch to over 26 feet long. The Ocean sunfish weighs up to over 5,000 pounds. The members of this subclass have enlarged pectoral fins and fused pelvic fins. Species include Chondroste, which are primitive ray-finned bony fishes; Holostei or Neopterygii, the intermediate ray-finned fishes like sturgeons, paddlefish, and bichirs; and Teleostei or Neopterygii, the advanced bony fishes such as herring, salmon, and perch. 

Habitat and Distribution

Bony fish can be found in waters all around the world, freshwater and saltwater both, unlike cartilagenous fish who are found only in salt waters. Marine bony fish live in all the oceans, from shallow to deep waters, and in both cold and warm temperatures. Their lifespans range from a few months to over 100 years.

An extreme example of bony fish adaptation is the Antarctic icefish, which lives in waters so cold that antifreeze proteins circulate through its body to keep it from freezing. Bony fish also comprise virtually all freshwater species living in lakes, rivers, and streams. Sunfish, bass, catfish, trout, and pike are examples of bony fish, as are the freshwater tropical fish that you see in aquariums. 

Other species of bony fish include:

• Tuna
• Atlantic cod
• Red lionfish
• Giant frogfish
• Seahorses
• Ocean sunfish

Diet and Behavior

A bony fish's prey depends on the species but may include plankton, crustaceans (e.g., crabs), invertebrates (e.g., green sea urchins), and even other fish. Some species of bony fish are virtual omnivores, eating all manner of animal and plant life. 

Bony fish behavior varies greatly, depending on the species. Smaller bony fish swim in schools for protection. Some like the tuna swim continually while others (stonefish and flatfish) spend most of their time lying on the seafloor. Some such as morays only hunt at night; some like butterfly fishes do so during the day; and others are most active at dawn and dusk. 

Reproduction and Offspring

Some bony fish are born sexually mature or become mature shortly after birth; most mature within the first one to five years. The main reproduction mechanism is external fertilization. During the spawning season, females release hundreds to thousands of eggs in the water, and males release sperm and fertilize the eggs.

Not all bony fish do lay eggs: Some are live-bearing. Some are hermaphrodites (the same fish has both male and female genitalia), and other bony fish switch genders over time. Some, like the seahorse, are oviparous, meaning the eggs are fertilized in the parent who feeds them from a yolk sac. Among seahorses, the male carries the offspring until they are born. 

Evolutionary History

The first fish-like creatures appeared over 500 million years ago. Bony fish and cartilaginous fish diverged into separate classes about 420 million years ago.

Cartilaginous species are sometimes seen as more primitive, and for good reason. The evolutionary appearance of bony fish eventually led to land-dwelling vertebrates with bony skeletons. And the gill structure of bony fish gill was a feature that would eventually evolve into air-breathing lungs. Bony fishes are therefore a more direct ancestor to humans. 

Conservation Status

Most bony fish species are classed as Least Concern by the International Union for Conservation of Nature (IUCN), but there numerous species that are Vulnerable, Near Threatened, or Critically Threatened, such as Metriaclima koningsi of Africa.

Sources

• "Bony and Ray-Finned Fishes." Endangered Species International, 2011. 
• Class Osteichthyes. The Biology Classroom of Mr. Pletsch. University of British Columbia, February 2, 2017.
• Hastings, Philip A., Harold Jack Walker, and Grantly R. Galland. "Fishes: A Guide to Their Diversity." Berkeley, University of California Press, 2014.
• Konings, A. "Metriaclima ." The IUCN Red List of Threatened Species: e.T124556154A124556170, 2018. koningsi
• Martin, R.Adam. Fathoming Geologic Time. ReefQuest Centre for Shark Research.
• Plessner, Stephanie. Fish Groups. Florida Museum of Natural History: Ichthyology.