Electric Eel
The Electric eel (Electrophorus electricus), is an electric fish, and the only species of the genus Electrophorus. It is capable of generating powerful electric shocks, which it uses for both hunting and self-defense. It is an apex predator in its South American range. Despite its name it is not an eel but rather a knifefish.
The electric eel has three abdominal pairs of organs that produce electricity: the Main organ, the Hunter's organ, and the Sachs organ. These organs make up four-fifths of its body, and are what give the electric eel the ability to generate two types of electric organ discharges (EODs), low voltage and high voltage. These organs are made of electrocytes, lined up so that the current flows through them and produces an electrical charge. When the eel locates its prey, the brain sends a signal through the nervous system to the electric cells. This opens the ion channel, allowing positively-charged sodium to flow through, reversing the charges momentarily. By causing a sudden difference in voltage, it generates a current.
The electric eel generates its characteristic electrical pulse in a manner similar to a battery, in which stacked plates produce an electrical charge. In the electric eel, some 5,000 to 6,000 stacked electroplaques are capable of producing a shock at up to 500 volts and 1 ampere of current (500 watts). Such a shock could be deadly for an adult human. (Electrocution death is due to current flow; the level of current that is fatal in humans is roughly 0.75A.)
The Sachs organ is associated with electrolocation. Inside the organ are many muscle-like cells, called electrocytes. Each cell can only produce 0.15 V, though working together the organ transmits a signal of about 10 V in amplitude at around 25 Hz. These signals are what is emitted by the main organ and Hunter's organ that can be emitted at rates of several hundred Hz.
The electric eel is unique among the gymnotiforms in having large electric organs capable of producing lethal discharges that allows them to stun prey. There are reports of this fish producing larger voltages, but the typical output is sufficient to stun or deter virtually any other animal. Juveniles produce smaller voltages (about 100 volts). Electric eels are capable of varying the intensity of the electrical discharge, using lower discharges for "hunting" and higher intensities for stunning prey, or defending themselves. When agitated, it is capable of producing these intermittent electrical shocks over a period of at least an hour without signs of tiring.
Freshwater Mussels
Generally unable to move of their own accord, the parasite larva or glochidium can do little more than clamp down on the first thing that brushes by its cute little jaws, sometimes equipped with vicious fangs and an adhesive tongue. Once attached, the host's skin will grow to encase the tiny hitchhiker, which remains for days or weeks until ready to drop off and settle down as a young mussel. Other species even allow themselves to be swallowed, and settle in the digestive tract where they may absorb nutrients until they are excreted. Usually only compatible with a single fish species, they rely on their parents to infect the right host with all manner of devilry.
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Stonefish
Synanceia is a genus of fish of the family Synanceiidae, the Stonefishes, whose members are venomous, dangerous, and even fatal to humans. It is the most venomous fish in the world. They are found in the coastal regions of Indo-Pacific oceans. They are primarily marine, though some species are known to live in rivers. Its species have potent neurotoxins secreted from glands at the base of their needle-like dorsal fin spines which stick up when disturbed or threatened. The vernacular name of the species, the stonefish, derives from being able to camouflage and transform itself to a gray and mottled color similar to the color of a stone. Divers have sometimes stepped on them, thinking they are stones.
Hairy Angler
The Hairy angler (Caulophryne polynema) is an ocean fish, and was first discovered by a BBC film crew while creating the documentary series The Blue Planet.
The female is about the size of a football and its body is covered in long antennae, which it uses to detect the movements of any prey nearby. The male is a tenth the size; about the size of a ping pong ball. The male also forms a parasitic relation with the female, attaching himself to her flesh and ultimately fusing with her blood stream.
Grenadiers
Grenadiers or rattails (less commonly whiptails) are generally large, brown to black gadiform marine fish of the family Macrouridae. Found at great depths from the Arctic to Antarctic, members of this family are among the most abundant of the deep-sea fishes.
The Macrouridae are a large and diverse family with some 34 genera and 383 species recognized (well over half of which are contained in just three genera, Caelorinchus, Coryphaenoides and Nezumia). They range in length from approximately 10 centimetres (3.9 in) in the graceful grenadier (Hymenocephalus gracilis) to 1.5 metres (4.9 ft) in the giant grenadier, Albatrossia pectoralis. An important commercial fishery exists for the larger species, such as the giant grenadier and roundnose grenadier, Coryphaenoides rupestris. The family as a whole may represent up to 15 percent of the deep-sea fish population.
Portuguese Man-o-War
The Portuguese Man o' War (Physalia physalis), also known as man of war, is a venomous marine invertebrate. The common name comes from a Portuguese war ship type of the 15th and 16th century, the caravel (in Portuguese, Caravela), which had triangular sails similar in outline to the bladder of the Portuguese Man o' War.
While the Portuguese Man o' War resembles a jellyfish, it is in fact a siphonophore – a colony of four kinds of minute, highly modified individuals, which are specialized polyps and medusoids. Each such zooid in these pelagic colonial hydroids or hydrozoans has a high degree of specialization and, although structurally similar to other solitary animals, are all attached to each other and physiologically integrated rather than living independently. Such zooids are specialized to such an extent that they lack the structures associated with other functions and are therefore dependent for survival on the others to do what the particular zooid cannot do by itself.
