The Selachii: selachos (cartilaginous fish) roughly 490 species of sharks
roughly 660 species of skates and rays: dorsoventrally flattened, pectoral fins enlarged
and fused to body
Big skate Spotted eagle ray
The hyostylic jaw (found in most living sharks)
upper jaw attached to skull by only one ligament (in yellow)
the hyomandibula (= epihyoid [red]) articulates with the skull and the upper jaw
the ceratohyoid [green] articulates with the lower jaw.
NOTE: the hyoid arch is now involved primarily with feeding!
Many Selachii are top end (apex) predators. As a predatory shark approaches its
the lower jaw gapes wide open and the upper jaw is pushed forward by the
hyomandibula. The amount it can be pushed forward depends on the length of the
ligament that attaches the upper jaw to the skull (this varies among different species of
As the upper jaw moves forward, the lower jaw begins to close (pushed up by the
ceratohyoid), lower jaw teeth stab into the food item (spiking the prey to hold it) and the
very sharp upper teeth reach the surface of the food, slashing it. As all this is happening
the shark’s eyes roll tailward in their sockets to protect them from injury.
If the prey is large, the shark will begin thrashing its head violently from side-to- side,
maximizing the cutting efficiency of its serrated teeth.
Currently, there are many ideas about the benefits of pushing the upper jaw forward out
of the mouth, but no real data.
Three-dimensional computer analysis of the great white shark’s jaw mechanics indicated
that the bite force of a 240 kg white shark (1602 N) is about half that of a similar sized
lion (3300 N). The lion is thus more mechanically efficient, possibly (in part) because its
skull/jaws are made of bone. In fact the creature that has the highest bite force relative to
its size is the piranha: BUT when you calculate the bite force of the largest known great white shark, it works
out to 18,216 N, which is the highest absolute force calculated for any living species.
The force is so large because the shark is so big!
Many other selachiian species (generally benthic) have teeth modified into grinding
plates for crushing hard prey like molluscs
The four largest Selachii, however, are filter feeders:
basking shark: largest specimen was 12.2 m and 41,887 lbs, which makes it the second
largest fish in the ocean
whale shark: at up to 12.6 meters long and 47,000 lbs (and possibly more but not
confirmed) this is the largest fish in the ocean
megamouth shark manta ray (largest ray) (extremely rare shark)
The last fabulous jaw modification: Euhyostyly (skates and rays)
the jaws are suspended from the skull by the hyomandibula (no ligaments). Skates and
rays can protrude their jaws more than sharks. The amount of protrusion depends
upon the size and position of the hyomandibula
the ceratohyoid is reduced or absent red = upper jaw; blue = lower jaw, green =
hyomandibula, pale yellow = skull. Top: Resting position, jaw retracted; Middle:
depression of the lower jaw, hyomandibula opens the mouth; Bottom: upper jaw
grab, suck and chew feeders along the bottom (except for the big filter feeding manta
prey on invertebrates like clams, mussels, starfish so their teeth are organized in
Sensory systems: Just like the ratfish, lamprey and all ray-finned fish (we will discuss
them in the next lecture), all Selachii can to detect water movements via the lateral line
system along the head and side of the body (shown below as a series of dots):
Just like the ratfishes (and a few ray finned fishes), the Selachii can also detect the weak electric fields around living organisms using the ampullae of Lorenzini. This ability is
more developed in the Selachii than in any other vertebrate group:
Distribution of ampullae of Lorenzini in a skate and cat shark
more ampullae of Lorenzini (black dots on the snout) in sharks
The function of the ampullae of Lorenzini is to detect weak electrical fields from
prey items. For example, spotted cat sharks can find a small flounder hiding under sand,
even if a layer of agar is placed between the sand and the flounder (the agar stops
chemicals associated with smell and taste being transmitted).
Incredibly sensitive (<0.000000010V/cm), operates during the day or night but the fields
being detected are very weak, so only good when the shark/ray/ratfish is within a few cm
from the prey item (not a long distance sense).
Olfaction is a long distance sense and all chondrichthyans have an incredibly good sense
of smell. However, although sharks are considered to be the ultimate odour- sensing
aquatic predators, a recent study demonstrated that they are in fact not more sensitive to a
range of amino acids usually associated with locating prey items than are various ray-
finned fishes: both groups have the ability to detect amino acids in concentrations from ~
10 to 10 mol/liter.
Reproduction in the Selachii
As you would expect, fertilization is internal via claspers in all Selachii (remember it is
internal in all Chondrichthyes!)
Not much is known