angel sharks

The angel sharks are an unusual group of sharks, with their flattened bodies and broad pectoral fins that give them a strong resemblance to skates and rays. The 16-odd known species are all classified in a single genus Squatina belonging to its own family Squatinidae and order Squatiniformes. They occur worldwide in temperate and tropical seas.

While the forward part of the body is broad and flattened, the rear part retains a muscular appearance more typical of other sharks. The eyes and spiracles are on top, and the five gill slits are on bottom. Both the pectorals and the pelvic fins are large and held horizontally. There are two dorsal fins, no anal fin, and unusually for sharks, the lower lobe of the caudal fin is longer than the upper lobe. Most types grow to a length of 1.5 meters (5 ft), with the Japanese angel shark Squatina japonica known to reach 2 meters.

Angel sharks are bottom-dwellers, burying themselves in sand or mud, then lunging to snap up prey, which includes fish, crustaceans, and various types of mollusks. The Pacific angel shark Squatina californica is also known to leave the bottom at night to forage. Although they are not normally aggressive, they do bite when stepped on or handled.

Angel sharks are ovoviviparous, with litters known up to 13 pups.

The sharks were long considered of no commercial interest, but in 1978, Michael Wagner, a fish processor in Santa Barbara, California began to promote angel sharks, and 310 metric tons were taken off California in 1984. The fishery devastated the population, and is now regulated

The molluscs or mollusks are the large and diverse phylum Mollusca, which includes a variety of familiar creatures well-known for their decorative shells or as seafood. These range from tiny snails, clams, and abalone to the octopus and squid (which are considered the most intelligent invertebrates). There are some 70,000 described species within this phylum [1].

The giant squid, which until recently had not been observed alive in its adult form is the largest invertebrate although it is likely that the Colossal Squid is even larger. The scientific study of molluscs is called malacology.



Anatomy
Molluscs are triploblastic protostomes. The principal body cavity is a blood-filled hemocoel. It is unknown whether they have a true coelom (eucoelom); any coelomic cavities have been reduced to vestiges around the hearts, gonads, and metanephridia ( kidney-like organs). The body is often divided into a head, with eyes or tentacles, a muscular foot and a visceral mass housing the organs.

Molluscs have a mantle, which is a fold of the outer skin lining the shell, and a muscular foot that is used for motion. Many molluscs have their mantle produce a calcium carbonate external shell and their gill extracts oxygen from the water and disposes waste. All species of the phylum Mollusca have a complete digestive tract that starts from the mouth to the anus. Many have a feeding structure, the radula, mostly composed of chitin. Radulae are diverse within the Mollusca, ranging from structures used to scrape algae off rocks, to the harpoon-like structures of cone snails. Cephalopods (squid, octopuses, cuttlefish) also possess a chitinous beak. Unlike the closely related annelids, molluscs lack body segmentation.

Development passes through one or two trochophore stages, one of which (the veliger) is unique to the group. These suggest a close relationship between the molluscs and various other protostomes, notably the Annelids.

Mollusc fossils are some of the best known and are found from the Cambrian onwards.

The octopus is a cephalopod of the order Octopoda that inhabits many diverse regions of the ocean, especially coral reefs. The term may also refer to only those creatures in the genus Octopus. In the larger sense, there are 289 different octopus species, which is over one-third the total number of cephalopod species.

Octopuses are characterized by their eight arms (not tentacles), usually bearing suction cups. These arms are a type of muscular hydrostat. Unlike most other cephalopods, the majority of octopuses — those in the suborder most commonly known, Incirrata — have almost entirely soft bodies with no internal skeleton. They have neither a protective outer shell like the nautilus, nor any vestige of an internal shell or bones, like cuttlefish or squids. A beak, similar in shape to a parrot's beak, is their only hard part. This enables them to squeeze through very narrow slits between underwater rocks, which is very helpful when they are fleeing from morays or other predatory fish. The octopuses in the less familiar Cirrata suborder have two fins and an internal shell, generally lessening their ability to squeeze into small spaces.

Three defensive mechanisms are typical of octopuses: ink sacs, camouflage, and autotomising limbs. Most octopuses can eject a thick blackish ink in a large cloud to aid in escaping from predators. They also have specialized skin cells both for colour changing ( chromatophores) and light reflection and refraction ( iridophores and leucophores). They use this ability to blend into the environment to hide, communicate with other octopuses, or warn. The very poisonous Blue-ringed Octopus becomes bright yellow with blue rings when it is provoked. When under attack, some octopuses can autotomise their limbs, in a similar manner to skinks and other lizards. The crawling arm serves as a distraction to would-be predators; this ability is also used in mating. A few species, such as the Mimic Octopus have a fourth defense mechanism. They can combine their highly flexible bodies with their colour changing ability to accurately mimic other, more dangerous animals such as lionfish and eels.

When octopuses reproduce, they use a specialized arm called a hectocotylus to insert spermatophores (packets of sperm) into the female's mantle cavity. The hectocotylus is usually the third right arm. In some species, the female octopus can keep the sperm alive inside her for weeks until her eggs are mature. After they have been fertilized, the female lays roughly 200,000 eggs (this figure dramatically varies between species). The female hangs these eggs in strings from the ceiling of her lair. After the eggs hatch, the young larval octopuses must spend a period of time drifting in clouds of plankton, where they feed on copepods, larval crabs and larval seastars until they are ready to sink down to the bottom of the ocean, where the cycle repeats itself. In some deeper dwelling species, the young don't go through this period. This is a dangerous time for the larval octopuses; as they become part of the plankton cloud they are vulnerable to many plankton eaters.

Octopuses have a relatively short life span, and some species live for as little as six months. Larger species, such as the North Pacific Giant Octopus, may live for up to five years under suitable circumstances. However, reproduction is a cause of death: males can only live for a few months after mating, and females die shortly after their eggs hatch, for they neglect to eat during the (roughly) one month period spent taking care of their unhatched eggs.

Octopuses have three hearts. Two pump blood through each of the two gills, while the third pumps blood through the body. Octopus blood contains the copper-rich protein hemocyanin for transporting oxygen. Less efficient than the iron-rich hemoglobin of vertebrates, the hemocyanin is dissolved in the plasma instead of being bound in red blood cells and gives the blood a blue colour.



Intelligence
Octopuses are highly intelligent, probably the most intelligent of any of the invertebrates, with their intelligence supposedly comparable to that of the average housecat. Maze and problem-solving experiments show that they have both short- and long-term memory, although their short lifespans limit the amount they can ultimately learn.

An octopus has a highly complex nervous system, only part of which is localized in its brain. Two-thirds of an octopus's neurons are found in the nerve cords of its arms, which have a remarkable amount of autonomy. Octopus arms show a wide variety of complex reflex actions arising on at least three different levels of the nervous system. Some octopuses, such as the mimic octopus, will move their arms in ways that emulate the movements of other sea creatures.

In laboratory experiments, octopuses can be readily trained to distinguish between different shapes and patterns. They are able to open jars after learning from observation. Octopuses have also been observed in what may be described as play; repeatedly releasing bottles or toys into a circular current in their aquariums and then catching them. Octopuses often break out of their aquariums and sometimes into others in search of food. They have even boarded fishing ships and opened holds to eat crabs.

In many countries, including the United States and the United Kingdom, octopuses are on the list of experimental animals on which surgery may not be performed without anesthesia.


Sensation
Octopuses have keen eyesight. Although their slit-shaped pupils might be expected to afflict them with astigmatism, it appears that this is not a problem in the light levels in which an octopus typically hunts. Surprisingly, they do not appear to have colour vision, although they can distinguish the polarization of light. Attached to the brain are two special organs, called statocysts, that allow the octopus to sense the orientation of its body relative to horizontal. An autonomic response keeps the octopus's eyes oriented so that the pupil slit is always horizontal.

Octopuses also have an excellent sense of touch. The octopus's suckers are equipped with chemoreceptors so that the octopus can taste what it is touching. The arms contain tension sensors so that the octopus knows whether its arms are stretched out. However, the octopus has a very poor proprioceptive sense. The tension receptors are not sufficient for the octopus brain to determine the position of the octopus's body or arms. (It is not clear that the octopus brain would be capable of processing the large amount of information that this would require; the flexibility of an octopus's arms is much greater than that of the limbs of vertebrates, which devote large areas of cerebral cortex to the processing of proprioceptive inputs.) As a result, the octopus does not possess stereognosis; that is, it does not form a mental image of the overall shape of the object it is handling. It can detect local texture variations, but cannot integrate the information into a larger picture.

The neurological autonomy of the arms means that the octopus has great difficulty learning about the detailed effects of its motions. The brain may issue a high-level command to the arms, but the nerve cords in the arms execute the details. There is no neurological path for the brain to receive feedback about just how its command was executed by the arms; the only way it knows just what motions were made is by observing the arms visually.


Locomotion
Octopuses move about by crawling or swimming. Their main means of slow travel is crawling, with some swimming. Their only means of fast travel is swimming.

They crawl by walking on their arms, usually on many at once, on solid surfaces, while supported in water. In 2005 it was reported that some octopuses can walk on two arms on a solid surface, while at the same time imitating a coconut or a clump of seaweed.

They swim by expelling a jet of water from a contractile mantle, and aiming it via a muscular siphon.