sharks

Cartilaginous

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A Blacktip Reef Shark (Carcharhinus melanopterus) has a fully cartilaginous skeleton (photo credit: David Clode).

If you’ve ever eaten a fish fillet, you’ve probably had the ‘pleasure’ of picking out fish bones from your meal.  While most fish are, indeed, bony (superclass: Osteichthyes), certain lineages of fish have cartilaginous skeletons, meaning that their skeletal structure is composed of cartilage, like a human ear or nose, rather than bone.  Elasmobranchs (sharks, skates, and rays) and chimera (class: Chondrichthyes) have always had cartilaginous skeletons.

 

Atlantic Sturgeon | NOAA Fisheries

Atlantic Sturgeon (Acipenser oxyrinchus oxyrinchus) have secondarily derived cartilaginous skeletons (photo credit: NOAA).

Chondrosteans (sturgeon and paddlefish; subclass: Chondrostei) are another group of cartilaginous fish – but with a more complex evolutionary history.  The ancestors of sturgeon and paddlefish had fully ossified, bony skeletons (they are members of Osteichthyes) but they have evolved to have secondarily derived cartilagenous skeletons (i.e., cartilage ➡ bone ➡ cartilage).  They do still retain some bony elements – scutes which are modified bony scales.

 

 

phylogeny

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Phylogeny of fishes (diagram credit: evolution.berkeley.edu).

Phylogeny is the representation of evolutionary relationships, often displayed as a “tree” (i.e., branching diagram) where the most closely related organisms are connected by a node and more distantly related organisms have multiple steps back in the tree before they are connected.  Nodes are often characterized by biogeographic events and evolutionary innovations.

In fishes, jawless fishes (such as hagfish and lampreys) are the most primitive, cartilaginous, and without paired fins; sharks and rays are cartilaginous with jaws and placoid denticles; and bony fish are the most diverse and derived fishes, with scales (see cycloid and ctenoid), swim bladders, and bony skeletons.

 

 

Heterocercal

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The heterocercal tail of a shark is a caudal fin with unequal lobes.

The heterocercal tail of a shark is a caudal fin with unequal lobes.

A heterocercal tail is a caudal fin composed of two asymmetrical lobes.  Often, such as the case in many sharks, the vertebral column passes through the upper lobe, making it the larger of the two lobes.  A heterocercal tail is contrasted with a homocercal tail which has equal lobes.

Ampullae of Lorenzini

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The ampullae of Lorenzini on this chimera are the two rows of dots on its snout (NOAA)

The ampullae of Lorenzini on this chimaera are the two rows of dots on its snout (NOAA)

Ampullae of Lorenzini are a network of electroreceptors, sensory organs that detect electric fields in water, found in chondrichthyes (sharks, rays, and chimaeras).  The ampullae are a series of symmetrical pores, concentrated around the snout and nose, connected by gel-filled canals.  They can conduct electrical impulses so small, that chondrichthyes are likely to be more sensitive to electric fields than any other group of animals.  Because all muscle contractions produce a weak electrical field, these electroreceptors make sharks, rays, and chimaeras highly capable of detecting other organisms, such as prey, nearby in water.

Ampullae of Lorenzini, shown in red, are a network of electroreceptors which can detect electrical impulses from other organisms in water.

Ampullae of Lorenzini, shown in red, are a electroreceptors that can detect electrical impulses from other organisms in water.

 

Elasmobranch

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Spotted Eagle Rays are a member of the catilaginous subclass Elasmobranchii along with other rays, sharks, and sawfishes (NOAA).

Spotted Eagle Rays are a member of the cartilaginous subclass Elasmobranchii along with other rays, sharks, and sawfishes (NOAA).

Elasmobranchs, including sharks, rays, and sawfishes, belong to the taxonomic subclass of cartilaginous fish Elasmobranchii.  Like most chondrichthyes, they have exposed gills, no swim bladder, internal fertilization, and placoid denticles.  They differ from the other subclass, chimaera (subclass: Holocephali), in that they have rigid dorsal fins, placoid denticles cover most of their bodies, and they usually have spiracles (modified gill slits directly behind the eye).

Chondrichthyes

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Chimaeras are a predominately deep sea branch of elasmobranchs with most found deeper than 200m (NOAA).

Chimaeras are a predominately deep sea branch of chondrichthyes with most found deeper than 200m (NOAA).

Chondrichthyes are a taxonomic class of cartilaginous fishes that encompass sharks and rays (elasmobranchs) and chimaera.  Though there are exceptions, in general, Chondrichthyes have exposed gills, no swim bladder, internal fertilization, and placoid denticles.  These characteristics differentiate them from the more evolutionarily derived branch of fishes, bony fish (Osteichthyes).

Placoid denticles

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While these Great White placoid denticles may look like scales, they are actually modified teeth (Trevor Sewell/Electron Microscope Unit, University of Cape Town).

While these Great White placoid denticles may look like scales, they are actually modified teeth (Trevor Sewell/Electron Microscope Unit, University of Cape Town).

Placoid denticles are found on sharks, rays, and chimaeras.  Not really a true “scale,” like ctenoid or cycloid scales, placoid denticles are actually modified teeth.  They have an inner tissue component, which contains both blood vessels and nerves, that is covered by a layer of dentin and an outer enamel.  They form a tough protective skin layer for sharks, rays, and chimaeras and also have shown to reduce friction and drag so that these fish can swim more efficiently through water.