Shark

Sharks (superorder Selachimorpha) are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago.^[1]
Since that time, sharks have diversified into 440 species, ranging in size from the small dwarf lanternshark, Etmopterus perryi, a deep sea species of only 17 centimetres (6.7 in) in length, to the whale shark, Rhincodon typus, the largest fish, which reaches approximately 12 metres (39 ft 4 in) and which feeds only on plankton, squid, and small fish by filter feeding. Sharks are found in all seas and are common down to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater, with a few exceptions such as the bull shark and the river shark which can live both in seawater and freshwater.^[2] They breathe through five to seven gill slits. Sharks have a covering of dermal denticles that protects their skin from damage and parasites, and improves their fluid dynamics so the shark can move faster. They have several sets of replaceable teeth.^[3]
Well-known species such as the great white shark, tiger shark, blue shark, mako shark, and the hammerhead are apex predators, at the top of the underwater food chain. Their extraordinary skills as predators fascinate and frighten humans, even as their survival is under serious threat from fishing and other human activities.

Anatomy

Teeth

The teeth of the tiger shark are oblique and serrated for sawing through flesh.

Main article: Shark teeth

Shark teeth are embedded in the gums rather than directly affixed to the jaw, and are constantly replaced throughout life. Multiple rows of replacement teeth grow in a groove on the inside of the jaw and steadily move forward as in a "conveyor belt"; some sharks lose 30,000 or more teeth in their lifetime. The rate of tooth replacement varies from once every 8–10 days to several months. In most species teeth are replaced one at a time, except in cookiecutter sharks the entire row of teeth is replaced simultaneously.^[7]
Tooth shape depends on diet: sharks that feed on mollusks and crustaceans have dense flattened teeth for crushing, those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting. The teeth of plankton-feeders such as the basking shark are smaller and non-functional.^[8]

Skeleton

Shark skeletons are very different from those of bony fish and terrestrial vertebrates. Sharks and other cartilaginous fish (skates and rays) have skeletons made of cartilage and connective tissue. Cartilage is flexible and durable, yet has about half the density of bone. This reduces the skeleton’s weight, saving energy.^[9] Sharks have no rib cage and therefore on land a shark's own weight can crush it.^[10]

 

Guppy Fish

The guppy (Poecilia reticulata), also known as the millionfish,^[1] is one of the most popular freshwater aquarium fish species in the world. It is a small member of the Poeciliidae family (females 4–6 centimetres (1.6–2.4 in) long, males 2.5–3.5 centimetres (1.0–1.4 in) long) and like all other members of the family, is live-bearing.

Taxonomy

Robert John Lechmere Guppy discovered this tiny fish in Trinidad in 1866, and the fish was named Girardinus guppii in his honour by Albert Günther later that year. However, the fish had previously been described in America. Although Girardinus guppii is now considered a junior synonym of Poecilia reticulata, the common name "guppy" still remains.
Over time guppies have been given a variety of taxonomic names, although Poecilia reticulata is the name currently considered to be valid.^[2]

Distribution

Guppies are native to Antigua and Barbuda, Barbados, Brazil, Guyana, Netherlands Antilles, Trinidad and Tobago, the U.S. Virgin Islands and Venezuela.^[3][4]
However, guppies have been introduced to many different countries on all continents, except Antarctica. Sometimes this has occurred accidentally, but most often as a means of mosquito control, the hope being that the guppies would eat the mosquito larvae slowing down the spread of malaria. In many cases, these guppies have had a negative impact on native fish faunas.^[5]