The World of Sharks
Sharks belong to a distinct class (Chondrichthyes) of sea creatures, separate from fish.
All sharks share the general physical traits of having rough, scaleless skin that covers a cartilage skeleton.
They range in size from the less than one foot long pygmy shark to the forty foot long whale shark.
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From that starting point, the potentially five hundred different types of sharks found in the world's oceans generally divide into eight orders based on their physical differences.
- Order Hexanchiformes: Cow and Frilled Sharks have six or seven sets of gills, as opposed to most shark species with five sets of gills.
- Order Squaliformes: The Dogfish Sharks have two dorsal fins.
- Order Squatiniformes: Angelsharks are flat bodied bottom feeders.
- Order Pristiophoriformes: Known generally as sawsharks because of the shape of their long, edged noses.
- Order Heterodontiformes: Bullhead Sharks get their name from the shape of their heads, which are above average in size, ridged above the eyes.
- Order Orectolobiformes: They go by the common name carpet sharks and are known for their sometimes colorful, camouflage bodies. The largest shark in the world, the whale shark (Rhincodon typus) fits in this group.
- Order Lamniformes: The common name of the group is Mackerel Sharks and it includes the Great White Shark.
- Order Carcharhiniformes: Ground Sharks are the most common types of sharks and include species such as hammerheads, tigersharks and catsharks.
Over one-half (55.8% of global) of all the world's shark species, some 278 species, are categorized as deep water sharks, or sharks that live 200 meters below the ocean's surface.
With 119 documented species, catsharks (Family Scyliorhinidae) comprise the largest group of of deep water sharks.
Most people consider sharks as ocean villains. Perhaps for that reason, declining shark populations receive less than adequate attention.
The Food and Agricultural Organization (FAO) promoted one of the earliest shark conservation efforts, developing an International Plan of Action for the Conservation of Sharks, built on the principle of states voluntarily creating shark management plans for their territorial waters.
Despite this early conservation effort, shark populations continue to decline. The latest Atlantic Shark Study, for example, notes:
"The release of the first ever IUCN Red List of Threatened Species assessment of northeast Atlantic sharks, rays and chimaeras reveals that 26 percent are threatened with extinction and another 20 percent are in the Near Threatened category."
Because many shark species swim in oceans that cross state boundaries, devising a workable shark management plan faces many roadblocks. The issue of declining shark populations extends beyond the loss of a couple of species, it possibly means large scale changes in ocean ecosystems.
Their status as predators at the top of the ocean food chain means declining shark populations often correlate with increases in the populations of their preferred prey. A chain reaction sets in with the increased population of lesser predators causing decreases in populations of their (the lesser predators) prey.
Reports on declines in the bay scallop fishing industry, for example, are linked to decreases in the Atlantic shark population. As shark populations declined, the populations of one of their preferred preys, Cownose rays, increased. The cownose rays then began feeding unimpeded on the bay scallop populations.
Often considered physical oddities, the coastal lifestyle and appearance of hammerhead sharks (Sphyrnidae) continues to intrigue their human observers.
Why did they evolve to have long, flat heads?
Without any data to back it up, the casual observer might easily hypothesize that the long, flat heads provide them with better eyesight.
Recent research published in the Journal of Experimental Biology 212, 4010-4018, confirms what many casual observers suspected.
Not only do the wide set eyes of hammerhead sharks provide them with better forward looking depth perception of ocean objects, but also the eyes set on the sides of their heads provide them with better backward looking ability.
One hammerhead species, the scalloped hammerhead, was measured as having a full 360o visual field.
The research results appear to mesh with another line of inquiry into the shape of the hammerhead shark's head. According to research presented in the Journal Copeia Vol. 1995, No. 2, the head of the hammerhead shark also evolved to help it with improved mobility.
The author states, "The present results, together with observation of their swimming behavior, strongly suggest that the hammerhead sharks utilize their head for hydrodynamic purposes in various ways."
Taken together the research suggests that the mystery of the hammerhead shark can be explained as an evolutionary process that provided them with the dual ability to more effectively see potential food sources and more effectively maneuver their bodies to catch the prey once sighted.
© 2013. Patricia A. Michaels