Chapter 9: Bony Fish

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Diversity

The bony fish comprise the largest group of the vertebrates in Phylum Chordata, with over 20,000 species worldwide. They are called bony fish because their bony skeletons are calcified, making them much harder than the cartilage bones of the Chondrichthyes. The bony fishes have evolved a great diversity of sizes and shapes all based on a basic body form with protrusible (extendable) jaws, spiny rayed fins, and a swim bladder to control buoyancy.  This form has been modified in different fish allowing some to become highly maneuverable, others exceptionally fast, and some highly specialized in their feeding behavior.

A Malabar grouper, image from Wikipedia

Many fishes have complex physiologies, sophisticated organ systems, and unique behaviors that have made them highly successful in filling aquatic and marine niches.  To get a sense of the diversity of fishes, click on the link for Arkive.org.

Jaws

One of the most defining features of fish is their jaws.  In the history of life on Earth, the evolution of jaws gave fish tremendous advantages and that advantage has allowed them to dominate the oceans.  The size and shape of a fishes jaws, along with the size and shape of its teeth dictate what a fish eats. Some fish have large jaws with sharp teeth like this grouper.  Other fish, like parrot fish have highly specialized teeth and jaws for feeding on very specific prey (coral, in the case of the parrot fish.)  To see an extreme example of highly specialized jaws click on the link for a video of fish feeding on algae growing on a manatees tail.

A red mullet, image from Wikimedia Commons

Reproduction

In fishes, ovoparity; laying under-developed eggs with external fertilization, is the most common form of reproduction. Salt water is an ideal environment for keeping eggs uniform in temperature and moist.  Some species, like the brown surgeonfish randomly disperse their eggs to the currents, others like the three-spined stickleback will build nests, and a few like several species of African ciclids may brood their eggs in their mouth. Each strategy has its advantages and disadvantages.  For fish that release their eggs into the current, no parental care is involved and when eggs settle out of the plankton, they may be in totally new environments, which prevent them from competing for precious resources with their parents.  The greatest disadvantage to giving birth to immature and vulnerable eggs is that a lot of them get eaten.  Even in immature larval stages, as they first set out to feed on their own, fish are highly vulnerable to predation.  The survival of individual eggs from broadcast spawners is very low, so millions of eggs must be produced in order for the parent to successfully produce offspring. There are other strategies.  Some fish invest more energy in fewer eggs.  A few fish, like the male leafy sea dragon to the left, actually carry their eggs . Male gobies guard the eggs in a nest until they are born. Clink on the link to watch the eggs of developing Atlantic Halibut.

Some fish (particularly deep sea fish) are capable of producing their own light called bioluminescence.  However, researchers have recently learned that lots of fish are capable of absorbing blue light (highly available in the upper ocean) and reflecting back green or red light in a process called Fluorescence.  They do this possibly to attract mates.  Click on the link for a video all about it.

A spotted handfish, image from Arkive.org

Movement

The spotted hand fish, seen to the left, is very unusual.  Its rays create hand-like fins that allow it to walk along the bottom.  For most fish, their fins are adapted for swimming not walking.   Most fish use their back fin called a caudal fin for propulsion. Stiffer, vertically elongated caudal fins like those of a bluefin tuna are designed for speed.  Rounder softer caudal fins like those of a butterfly fish are better for close-cornering and maneuvering.  Not all fish use their caudal fins for propulsion. The ocean sun fish (Mola mola) has no real caudal fin but instead will use exaggerated dorsal and anal fins like oars for swimming. Pectoral fins, pelvic fins, and the dorsal fin help a fish maintain balance and give fish the maneuverability they need.

Body shape also had a great impact on fish movement. Just like in boats and submarines, a longer more fusiform shape like that of a barracuda makes a fish faster.   A flatter, rounder shape like that of a trigger fish, may make give a fish an advantage when maneuvering in close quarters.  The attenuated shape of an eel allows it to move effortlessly in the cracks and crevices of its reef home.

Fish fins, Image from PBS

Avoiding Predators

The main way that fish avoid potential predators is to place themselves in a school.  Schooling has a lot of advantages besides predator avoidance, but in the oceanic world of eat or be eaten schooling reduces the chance that any one individual fish will be eaten by a predator.  Predators have to pick out individual fish to catch, and the large amounts of movement in a school will often confuse a predator and increase the odds of survival for individuals.  Also, predators that are intensely feeding may become satiated (full) while feeding on fish in a school, leaving many fish to escape. Fish use their highly sensitive lateral lines (and to some extent visual cues) to respond to each other’s movement, allowing schools to move in a highly coordinated fashion. Click to watch a school of sardines trying to avoid predators.

Questions for Research

  1. Body Form –  Body form in fishes can be highly varied with the form of a fishes body matching its functional needs.  Go to  A quick course on Ichthyology.  Read and complete the chart below (the whole chart should be copied down into your notes.)
    Body Form Example Function
    torpedo (fusiform) acceleration
    one sided peacock flounder
    pan shaped maneuverability
    attenuated American eel
    unusual
  2. How do billfish like the sailfish use their bills for feeding.  Describe at least two methods.
  3. TED talk by Tierney Thys on the Giant Sun Fish or try Tierney’s website. Describe how they are using new technology to learn about large oceanic fish and then describe one thing they have learned.
  4. Jaws – Jaws are an amazing evolutionary adaptation that appeared first in fish.  It allows fish to potentially voracious predators.  Read about the Two Bite Moray and Gulper Eels.  Make a sketch of these two extreme examples.
  5. Fin Shape – All fish have fins, with most fish having variations of the same basic types.  Use Wikipedia’s Fish Anatomy Page to complete the chart below.
    Caudal fin shape Example Function
    blue shark lift
    homocercal lunate yellowfin tuna
    homocercal truncate brown trout
    triggerfish maneuverability
    stunted, small, or unusual spiny seahorse
  6. Schooling – Explain three advantages (avoiding predators can only be one) of schooling.
  7. Coloration- Dark on top, light on bottom, its called counter coloration and its very common in fish.  In most marine environments it makes a fish hard to see from above (looking into the dark) and hard to see from below (looking into the light above.)  Look at a Blue Fin Tuna to see an example of this.  Make a sketch of the coloration pattern for the tuna.  Next click on the link for coral reef fishes.  Many coral reef fishes are highly colorful with distinctive eye spots or bands of contrasting color.  Explain the possible advantages of these outlandish color patterns.
  8. Reproduction – It will help you to read above before you begin this question.  Contrast the reproductive styles of the peacock grouper with the blue fin tuna  and the leafy sea dragon (click on biology for each.)
  9. Together in class we will listen to a description by Richard Nelson of the connections between spawning salmon and forests.  Describe how salmon (in a very real sense) build forests.
  10. Light weight and highly protective, fish scales were the inspiration for the armor of the middle ages. Obtain a prepared slide of a sheefish fish scale and a microscope, make a sketch of it and describe your impressions.