EEB267 - Lecture 5

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Ecology & Evolutionary Biology
Deborah Mc Lennan

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 prey: 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 shark). 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 maximally protruded.   grab, suck and chew feeders along the bottom (except for the big filter feeding manta ray)   prey on invertebrates like clams, mussels, starfish so their teeth are organized in grinding plates  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 ~ -7 -9 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
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