Chapter 8: Non-Bony Fish

Big Skate, Image from Wikipedia

The Classification of Non-Bony Fish

All fishes are part of the Kingdom Animalia and Phylum Chordata (having spinal cords.) From there the non-bony fishes can be divided into two classes: Class Agnatha (Jawless Fishes) and Class Chondrichthyes (sharks and rays.)

Class Agnatha (Jawless Fishes)

Image from PBS

Image from PBS.

The members of class Agnatha are startling to look at. Long and eel like, but lacking jaws they can appear faceless and lifeless. Hagfish, for example, look more worm-like than fish-like. They have no mouth, no eyes, no fins and not much of anything else belonging to a typical fish. But they are fish, and some of the deep oceans most able scavengers.

They can be found in both the Atlantic and Pacific oceans, where they live at depths of 100 to 3,150 feet. They will play the role of decomposer, depending on their sense of smell to find food. Hagfish can also invade the bodies of sick fish by entering through their mouths and settling in their stomachs (Gross huh?). Once lodged inside, they can eat their hosts from the inside out. Hagfish are sometimes called “slime hags” because of their ability to produce copious amounts of slime. This slime protects them from the digestive juices of their hosts. The slime also is a great deterrent for potential predators.

Image from: Wikipedia

Lampreys are the closest living ancestors to Hagfish yet they live very different lifestyles. As larval lamprey called are called ammocoetes.  Ammocoetes burrow into the gravel of freshwater streams and spend up to seven years filter feeding.  They are an important part of freshwater ecosystems. When they grow into adults, they grow a ring of teeth, move into salt water and become parasites to other fishes. They attach themselves to fishes, latch on with their teeth, and secrete an anticoagulant into their hosts blood stream. Just like a leech or a mosquito, this keeps the fish’s blood from clotting and allows a lamprey to drink their victims blood. Lamprey rarely kill their hosts but they do leave scars. Much like salmon, adults return to freshwater to spawn in freshwater.   Unlike salmon, they do not have site fidelity, meaning they don’t return to their natal streams. Pacific lamprey have been around since before trees existed on earth, yet their numbers are in steep decline.  They have survived 5 mass extinctions but they may not survive this one. Here is a great read on Pacific Lamprey.

Class Chondrichthyes (Sharks, Skates, and Rays)

A comparison of features from sharks, skates, and rays.

A comparison of features from sharks, skates, and rays. Image from Mississippi State University Marine Fisheries Ecology

The class Chondrichthyes includes all sharks, skates, rays, and sawfish. To most untrained eyes sharks and rays look nothing alike. With the noticeable exception of ray’s kite-like body and mouth underneath instead of in front of their body, sharks and rays are structurally very similar. All members of the class Chondrichthyes have a skeleton made of cartilage rather than bone. This is useful because cartilage is lighter and more flexible than bone. Chondrichthyes lack an air bladder, which means they have a natural tendency to sink. As such, most sharks have pectoral fins that are shaped and function much like airplane wings. Water moving over their fins, gives them lift. Rays’ wings are nothing more than over-sized pectoral fins, and also serve to provide lift while swimming.Most sharks and rays have five to seven gill slits instead of the one that their cousins, the bony fish’ have. Almost all sharks and rays have a movable jaw and well developed teeth. Sharks and rays have placoid, or hook-shaped scales that give them a rough and armor like skin. There are several hundred shark and ray species that inhabit all of the oceans habitats.

Special Adaptations of Sharks and Rays

Ampullae of Lorenzini– The AOL are tiny pores on the bottom side of a sharks snout. Water can penetrate into these pores, allowing the shark to detect tiny electrical pulses that are created by the muscle movements of other fish.

Lateral Line– Similar (in some respects) to the Ampullae of Lorenzini, the lateral line is a row of pressure sensitive pores that run the length of the sharks body. The lateral line allows the shark to orient to particle movement or sound. It consists of structures called neuromasts which are located in canals that lie just below the surface of the skin or the scales.

Basking Shark

A basking shark, the second largest shark alive today, a filter feeder. Image from Wikipedia

Sharp Rough Skin– Sharks and rays also share the same kind of skin: instead of scales, they have small tooth-like spikes called denticles. The spikes can be so sharp, in fact, that ocean going cultures have long used shark skin as sandpaper.

Teeth– Thanks to the movie “Jaws,” sharks are known for their teeth, but no two shark species has exactly the same teeth. As such, teeth or bite marks can be used as an identifying characteristic of individual species. Those huge, triangular teeth familiar from Jaws have serrated edges that equip the white shark for cutting into a fare of seals, sea lions, porpoises, and even sea turtles. Other species will have teeth that are more pointed and spear-like, which are much better adapted for grabbing than for cutting. Not all sharks have teeth. In fact, there are several shark species (like the basking shark pictured to the below) that are strictly filter feeders.

Shark Reproduction

Sharks, for the most part, are sexually dimorphic and reproduce using internal fertilization. Male sharks have claspers. It’s one of the ways you can tell male and female sharks apart. Claspers are used to deliver sperm to the cloaca of a female shark. Shark fetuses develop internally, but there are some sharks that will lay unusually shaped egg cases. If you want to know more about shark reproduction, and how strange it can get, read “50 Shades of Grey Reef Shark” from the marine science blog Southern Fried Science.

Sharks are in Trouble

A thresher shark was killed after becoming stuck in a gillnet. (Brian Skerry) Image from Smithsonian.

Sharks are especially vulnerable to habitat destruction and fishing pressure. Sharks are unusually long lived and give birth to a relatively small number of young when compared to other types of fish. Many species breed only every other year and have gestational periods as long as thirteen months.

Sharks are sometimes caught as by-catch by fishermen using floating long lines or drift nets. However, for the last 30 years sharks, have been increasingly targeted for their fins. Shark meat can be eaten, but most sharks that are intentionally harvested are taken only for their fins. Shark fins may be sold for several hundred dollars a pound. Shark fin soup is a very popular (and very expensive) dish served in many Asian countries. In the last twenty years, many species of sharks have seen their populations decline by as much as 90%.

Questions to Research:

  1. Listen to an An ode to the Pacific lamprey.  Tell me two things you learn.
  2. Explain how hagfish can use slime as defense against potential predators. This YouTube video may also help.
  3. Anatomically speaking sharks and rays are really more alike than they are different. However, they use their bodies very differently. Watch a video of a manta ray swimming and a hammerhead shark swimming. Describe the difference.
  4. Examine the picture at the very bottom of the page. Then go to the Shark-O-Matic to identify what species of shark this is. Click here to read about this shark and to check your answer. How would you describe this sharks feeding strategy?
  5. Whale sharks and basking sharks are the largest of the sharks. Yet they feed on the smallest of prey (plankton.) Watch either video and describe in your own words what this looks like.
  6. We often consider salmon sharks a “local species” in South Central Alaska. Yet, in the winter salmon sharks travel great distances. Where do salmon sharks go after a summer spent in Prince William Sound? This page may also help you.
  7. How is the conservation of sharks connected to the health of Australian coral reefs?
  8. Read the attached article about tracking great white sharks.  Why are they tagging sharks and what are they learning?
  9. After reading the article, use the shark tracking app to track a shark.  Pick a shark, tell me its name and species, and then tell me where the shark has gone the time it was tracked.
  10. In 2020 about 10 people were killed worldwide by sharks. Each year almost 100 million sharks are harvested, most for their fins. The hunter is now the hunted. Select any of the articles provided, then explain why the survival of sharks is increasingly under threat.  Global Shark Conservation 

Image from Wikipedia