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Nekton

Nekton are those animals capable of swimming to the degree that they can overcome many ocean currents and move against them. Nekton include cephalopods, fishes, marine mammals, birds, and reptiles. Neutral buoyancy, the ability to remain in the water without rising or sinking. Neutral buoyancy is of great importance to aquatic animals. It allows animals to spend energy in feeding, escaping predators or mating that would otherwise be needed to maintain its position in the water. Many pelagic marine animals have therefore evolved some form of buoyancy that allows them to hover يحوم or stay motionless without going neither up nor down.

Cephalopods (head footed) Cephalopods belong to Phylum Mollusca, class Cephalopoda and include squids, cut-tlefish, pearly nautilus, and octopus. Cephalopods are the largest of the invertebrates. The squid Architeuthus exceeds lengths of 15 m, including the two long tentacles.

The squid Rossia paci fica The Pacific octopus

Octopus dofleini The western Pacific cuttlefish

Sepiella japonica Buoyancy الطفوية regulating mechanisms by cephalopods

Cuttlefish (Sepia)

The cuttlefish, which are not fish at all, have a calcified internal (i.e. made of calcium carbonate) shell that aids in buoyancy called the cuttlebone and lies on the dorsal side within the body. This shell is the “cuttlebone” sold as a source of calcium for cage birds. The cuttlebone is made up of series of little chambers. The animal can control its own buoyancy by controlling the amount of gas and/or liquid within the chambers. Nautilus (from Greek nautilos, 'sailor')

Nautiloids have a complete external shell made of calcareous chambers. The impressive shell is smooth, coiled, and up to 25 cm in diameter. The shell contains a series of gas-filled chambers that serves as a buoyancy organ. The body occupies the outer, largest chamber has 60 to 90 short, suckerless tentacles used to capture crabs and fish. Squid

The shell is reduced to a chitinous pen that is embedded in the upper surface of the mantle. The pen also holds varying proportions of gas and water. Octopus

Buoyancy is not a problem for octopus because they are benthic.

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Fig sectioned Nautilus shell showing its chambers separated by septa.

Fig Calcified internal shell of the cuttlebone

Fish There are about 20,000 species, which can be found in the different marine habitats including; coral reefs, estuaries, submarine canyons, midwater bathyal, and on the deep-sea bed. All fishes are members of the phylum Chordata, subphylum Vertebrata, and are characterized by a spinal cord, an internal skeleton, a complete digestive system, and various arrangements of fins to control movement. Fish are classified according to their internal skeleton into Cartilaginous fishes (Chon-drichthyes) and Bony fishes (Osteichthyes).

Classification of Fish in Terms of Form

Rover طواف predators (e.g., tuna, marlin) are usually long and torpedo shaped, with the fins spaced along the body for maneuverability.

Lie-in-wait fish predators are also torpedo shaped, but the fins are often

concentrated toward the rear, to help provide the sudden thrust (قوة دافعة) necessary to capture prey.

Surface-oriented fishes (e.g. flying fishes) often have the mouth oriented upward,

to capture surface-bound prey.

Bottom fishes are quite variable in shape: the flounders, plaice, and soles are notable for their flattened shape, and some bottom-roving sharks have strongly flat-tened heads. Sculpins have modified pelvic fins that allow them to adhere to the bottom.

Deep-bodied fishes are flattened laterally and are excellent at maneuvering,

although they are relatively slow swimmers. They are often small and the fins of many are armed with spines, which deter predators.

Eel-like fishes are well adapted for moving in crevices.

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Swimming Swimming purposes Fishes swim to obtain food, escape from predators, and find mates. Many cartilaginous fishes must also swim to flush their gills with water to obtain oxygen for respiration. Mode of swimming Most fishes swim by a rhythmic side-to-side motion of the body or tail. S-shaped waves of contractions moving from head to tail push against the water and force the body forward. Variations on this theme are illustrated in the figure below.

a. Eels and other elongate fishes swim by undulating their body in lateral waves that travel from head to tail.

b. Fast fishes with shorter bodies –like tunas- swim by flexing mainly the caudal portion of the body.

Fig. Cartilaginous fishes: the shark squalus acanthias and the ray Dasyiatis akajei

Fig. Variation in the form of bony fishes: (a) rover predators, (b) lie-in-wait predator, (c) surface-oriented fish, (d) Bottom-feeding flatfish, (e) deep-bodied fish, and (f) eel-like fish.

Fig. Cartilaginous fishes: the shark squalus acanthias and the ray Dasyiatis akajei

Fig. Variation in the form of bony fishes: (a) rover predators, (b) lie-in-wait predator, (c) surface-oriented fish, (d) Bottom-feeding flatfish, (e) deep-bodied fish, and (f) eel-like fish.

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c. Parrotfishes سمكال الببغاء and others swim mainly by moving only fins, that is, the caudal, pectoral, anal, and/or dorsal fins. The ocean sunfish Mola mola, rely upon the flapping of fins for propulsion through the water.

d. Trunkfishes (a kind of boxfish) and porcupine fishes (similar to puffers except that the body is covered with spines) swim slowly by moving the base of the tail while the rest of the body remains immobile.

e. Rays use flapping winglike projections for swimming. Buoyancy Strategies to achieve neutral buoyancy in fish include: 1. Reducing the concentrations of ions. The low salt content of fish cellular fluids reduces bulk density. 2. Storing large amount of lipid. This is especially true in sharks, tunas and their relatives, and other species in which the swim bladder is poorly developed or missed altogether. Epipelagic sharks for example have high lipid (squalene) content in the liver, an organ that may account for up to 25% of the body volume.

(Squalene is a highly unsaturated aliphatic hydrocarbon (C30H50) with important biological properties. It is a metabolic precursor of cholesterol. Squalene is also present in human tissue in small amounts and in some vegetable oils). 3. The presence of nearly horizontal pectoral fins of sharks provide some lift. 4. The use Swim bladder (also called the gas bladder or air bladder) The swim bladder is a flexible-walled, gas-filled sac located middorsally, below the ver-tebral column, and occupies about 5% of the volume of bony fishes (teleosts). The swim bladder absorbs or secretes gas to adjust the depth at which the fish is neutrally buoyant. The pressure increases by approximately 1 atmosphere with each increase of 10 m depth. Thus, if a fish starts to descend, the increased pressure results in a compression of the gas inside the swim bladder. The fish becomes negatively buoyant and will tend to sink. Conversely, if a fish swims into shallower water, there is a decrease in water pressure and so the gas in the swim bladder expands, and the fish tends to float upwards. The swim bladder helps to solve the problems associated with variations of pressure, and thus buoyancy as following:

(a) (b) (c) (d)(a) (b) (c) (d)

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(a) If the fish becomes negatively buoyant, and starts to sink, air enters the swim bladder. In order to do this, a gas gland is present inside the swim bladder wall secretes the gases into the swim bladder. The gas gland is connected to the rete mirabile (means wonderful net) which is a bundle of blood vessels carrying blood to and from the gas gland. These capillaries run parallel to each other carrying the blood in opposite directions in a counter current exchange system. When gas is needed, the gas gland reacts by secreting lactic acid into the blood. This of course has the effect of lowering the pH of the blood, i.e. increasing its acidity. The increase in acidity causes the hemoglobin to release its oxygen as well as reduces the solubility of gases such as oxygen (other gases include N2 and CO2) in the blood. As a result, the partial pressure of oxygen in the venous capillaries is higher than the partial pressure of oxygen arriving in the adjacent arterial capillaries. Oxygen diffuses into the arterial capillaries [out of the venous] raising its partial pressure before the blood flows toward the gas gland. As the process is repeated, the partial pressure of oxygen in the arterial capillaries of the rete builds until it exceeds the partial pressure of oxygen in the swim bladder; therefore oxygen is released into the bladder. If the fish wants more buoyancy its secretory cells must release more lactate. The bladder is lined with the amino acid guanine to prevent resorption of gases.

Fig The structure of swim bladder of a teleost fish. (b) Function the rete mirabile, showing the fate of incoming arterial blood. Countercurrent flow retards loss through venous capillary return. (b) If the fish becomes positively buoyant, and starts to float upwards, gas diffuses out of the swim bladder into the blood. This gas removal process occurs at a site known as the oval body. The gas in the blood is then removed from the body into the surrounding water at the gills. Unlike the rest of the bladder, the oval body does not lined with

Dorsal aorta

Posterior cardinal vein

Oval body

Celiacomesentericartery

To hepaticPortal vein Gas gland

Rete mirabile

Gas bladder

Dorsal aorta

Posterior cardinal vein

Oval body

Celiacomesentericartery

To hepaticPortal vein Gas gland

Rete mirabile

Gas bladder

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guanine and thus, it is highly permeable. The loss of gas from the bladder through the oval is controlled by sphincter عضـلة عاصـرة, that can either obstruct the oval, preventing gas release, or relaxes giving oxygen the opportunity to diffuse out into the blood.

Important Notes: Many bottom-living fish lack swim bladder. Some fishes have a pneumatic duct that runs from the alimentary canal to the swim

bladder. Sharks, rays, and mackerel also lack them. This may be advantageous to them, because

rapid vertical locomotion would be difficult for an animal obliged to contend يكافح continually with the changing gas pressures of a swim bladder.

Feeding

Method of feeding 1. Suction

It is the dominant mode of feeding. The rapid opening of the mouth cavity sucks both water and prey into the mouth. This versatile feeding mechanism can be used to capture reasonably large prey and also can be used to draw much smaller prey, which are captured on extensions of the gill known as gill rakers.

2. Ram feeding

In ram feeding, the predator overtakes the prey with an open mouth. • When the prey is elusive, the predator must pursue at a high speed. • In the case of suspension-feeding ram feeders. The predator swims through the water with the mouth open for minutes at a time (continuous) or for a very short period (intermittent). Continuous ram feeders include basking sharks تشمس المالقرش , Herrings, manta rays, and some mackerels. Fig. Suspension feeding of a basking shark. Water is directed across the gill rakers as the fish is swimming, and plankton is caught on barbs projecting from the gill rakers.

3. Biting: During bite, the mouth cavity are not expanded as in suction feeding.

Gill arch

Gill raker Water flow

Gill arch

Gill raker Water flow

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Schooling

Schooling is a common phenomenon in fish and squid. Schooling is aggregation by mutual attraction, as opposed to aggregation by currents, or by a common attraction of individuals to an attractive microhabitat. In nearly all cases of active movement, schooling is confined to a single species. But feeding schools may consist of one or many species, especially in coral reef environments. Why do fish swim in schools? Providing protection against predation.

Schools can confuse predators, and fishes may either form a ring around the predator or swim in an undulating line. Most tightly schooling fishes have silvery sides, which serve to reflect sunlight and confuse predators. In some cases fish aggregate into approximately spherical schools known as fish balls, which minimize the peripheral area exposed to predators. Schooling increase the swimming efficiency of the fish

This is because the fish in front form an eddy that reduces water resistance for those behind.

Fig. Schooling in fishes: (a) traveling school and (b) feeding school. (c) encirclement of a predator, and (d) undulating to avoid a predator

Body Temperature and the mechanisms to reduce heat loss Ectotherms Nearly all bony fishes are ectotherms, their body temperature is usually within a degree or so of the water temperature. The blood is delivered through main vessels that located centrally, with branches that deliver blood to the outer part of the body. Fish produce metabolic heat, but much of this heat is lost to the water through the gills. Endotherms Occurs in the group Scombroidei, an assemblage of large oceanic fishes that includes the tunas. It also occurs in the shark families Lamnidae and Alopidae. Tunas have their blood vessels just beneath the skin, with smaller vessels delivering blood to the interior. This is different from the typical condition, and it allows heat generated in the body to be conserved. In tunas, heat is generated by red muscle, which is

a b c da b c d

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located near the axial region, instead of closer to the periphery of the body (an arrangement that characterizes most other fishes). Heat loss in tunas is reduced through the whole body with the aid of a countercurrent heat exchanger. The heat exchange involves a series of arteries whose flow is in the direction opposite to that of the adjacent veins. In this way the warmer blood in the veins returning to the gills for a fresh supply of oxygen it comes into close contact with cold, newly oxygenated blood in the arteries, and the heat is kept within the fish's body. The exchanger allows tunas and sharks to maintain body temperatures of greater than 10°C over ambient temperature. Billfishes, such as the swordfish, heat only the brain in a process that is accomplished by passing blood through a specially adapted eye muscle. The butterfly mackerel uses a different eye muscle to accomplish heat production. In both cases, however, a countercurrent mechanism of heat exchange is used to retain heat. These "hot-headed" fishes control and sustain metabolic activity in the brain, abilities that afford them maximum sensory control while hunting over a wide range of seawater temperatures. For example, warming the brain allows swordfish to hunt squids that migrate vertically across a great range of depth and temperature.

Fig. Temperature distribution in a bluefin tuna. Long thermistor needles were used to measure temperature shortly after the tuna was caught. Isotherms, or lines of equal temperature, show the differential (>10°C) between the core body temperature and the environment.

Fig. This photograph shows a cross section through a skipjack tuna. The dark muscle on either side of the vertebral column is maintained at a higher temperature than the rest of the fish thanks to its countercurrent heat exchanger.

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Characteristics of Epipelagic Fishes

Most fish occupy surface to 150m depth.

Lack of habitat diversity; contains less than 2% of known fish species.

Most of habitat is nutrient-poor water.

Most fishes concentrated over continental shelves or in upwelling zones.

Most epipelagics are highly streamlined for continuous travel.

Most are visual predators on zooplanktons or fish

Some filter feeders with long, fine gill rakers, large and small epipelagic fish feed this way.

Silvery in color (scatters light) with countershading.

Countershading: Conditions of the organisms in the water column that are dark colored on top but light colored on the bottom.

Distribution Their distribution can be divided into two basic categories, Oceanic and Neritic Forms:

Oceanic Forms: spend all or part of life in oceanic waters not above continental shelves and subdivided into three categories

• True residents: spend entire life cycle in open ocean. • Partial residents: spend part of life cycle in Open Ocean. These are usually juveniles

of neritic or benthic species and deepsea fish. • Accidental residents: fishes belonging to groups associated with other zones that are

found in the oceanic zone due to currents, storms, etc.

Neritic Forms: • Spend all or most of their lives above continental shelves • Many of these species have the highest abundances among fishes (anchovies, herrings,

menhaden) along with their predators (sharks, jacks, mackerel, etc.) • These fishes are utilizing high productivity waters caused by upwellings and

shoreline (oceanic littoral zone) productivity. Economic Value of Epipelagic Zone

• Fishes occur in huge numbers (anchovy, sardines) • Fishes less numerous but valued food fishes (tunas, salmonids)

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General characteristics of Mesopelagic Fishes Live in depths between 150 and 2,000 m. Many of characteristics of midwater animals are directly related to the lack of food in the mesopelagic.

• Small in size, which is thought to be an adaptation to limited food supply. • Have large mouths, and many have extendible jaws allow them to eat wide range of

prey. • Jaws are equipped with fearsome, long, sharp teeth help keep prey from escaping. • Due to food scarcity they usually have very broad diet and eat just anything they

can fit into their mouths.

• Except vertically migrating fishes, most have lost the swim bladder, to achieve

neutral buoyancy they have developed soft and weak bones. • They don’t swim much; accordingly they have no need for the streamlining. • Mesopelagic fishes often have black backs and silvery sides, reminiscent of the

countershading of epipelagic fishes. • Many species have large tubular eyes that point upward to spot zooplankton prey.

Fishes of these species have relatively short and small mouths, which they use to ingest small prey rapidly.

• Many species possess bioluminescent photophores , probably for several functions, including attracting mates and luring and illuminating prey. The photophores of many bioluminescent mesopelagic fish are concentrated on the ventral surface, produce light that helps the animal blend in with the background light filtering down from the surface. This adaptation which functions in a similar manner to countershading is called counterillumination, confuses predators looking upward into the dim light.

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Bathypelagic and abyssopelagic fishes Below the mesopelagic lies the little-known world of deep sea where sunlight never penetrates, Bathypelagic fishes (depths of 2,000-4,000 m) and abyssopelagic fishes (depths of 4,000-6,000 m).

• Fishes are mostly black. • Like mesopelagic fishes, bioluminescence is ubiquitous in the deep sea. They have

however fewer photophores, and the photophores tend to be on the head and sides for attracting prey and for communication and courtship.

• Many deep-sea animals are blind, and if they have eyes, they are almost always small. • Food is critically scarce, accordingly, deep-sea animals are few, far between, very

small in size (bristlemouth and anglerfishes are usually 10 cm or less), sluggish and sedentary.

• They have flabby, watery, muscles (Indeed, extensive musculature consumes oxygen and is therefore a disadvantage when food is so scarce), weak skeletons, no scales, and poorly developed body systems.

• Nearly all lack functional swimm bladders. • Have huge mouths which may be attributed to the relative rarity of prey in this zone.

They can consume prey much larger than themselves. To do so, they have stomachs that can expand to accommodate the prey once it has been engulfed.

• In addition to the big mouth, other species such as female anglerfishes, have a lure (modified fin ray) protruding upward from the dorsal surface, between the eyes to attract and catch prey.

• Since finding a mate is difficult in this world, many deep-sea fishes are hermaphrodites. Other fishes have evolved away to attract mates such as by sending signals or by having a unique pattern of light organs, by developing powerful sense of smell, releasing chemicals such as pheromones by females, or by having dwarf males (anglerfish), which attach to females.

• The fish fauna of deep-sea bottoms is dominated by eel-like forms. The eel shape may be related to the development of a good lateral line system, which is important in a dark environment.

• Many angler fishes, often have opaque, black digestive tracts, which may serve to help block off the light produced by bioluminescent prey as they are swallowed by the angler fish. This would prevent an obvious cue for yet another predator, who might otherwise obtain a double meal!.

Fig. The deep-water fish Chauliodus uses its specialized backbone to enable the opening of its enormous mouth in order to consume large fish.

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Mammals Whales and Porpoises

Taxonomy Class Mammalia Phylum Chordata Order Cetacea Suborder Odontoceti include the toothed whales (e.g., the sperm whale) and porpoises Suborder Mysticeti Include the baleen whales (e.g., blue whale). General Characteristics of cetacean The body is nearly hairless, elongate, and streamlined, properties that reduce drag as

the animal swims. Forelimbs have been reduced to stabilizing paddles. The hind limbs have been completely reduced and can be identified only by vestigial

bones, which do not protrude from the main trunk. The posterior is strongly muscularized and ends in a pair of flukes, which resemble

the horizontal tail of an airplane. This body design enables very efficient swimming. Some large whales can travel continuously at speeds of over 10 miles per hour, and baleen whales can sustain burst speeds of over 20 miles per hour.

Homeothermic, and all have a thick subdermal layer of fat that retards heat loss. In extreme cold, blood circulation can be restricted to the trunk and brain. Heat is also conserved in the limbs by a countercurrent exchange system.

Reproduce much the same as terrestrial mammals. After a gestation period, the young are born live and underwater, which is unique for marine mammals. The young suckle, and the "milk" has a far higher fat content than that of terrestrial mammals. It has a fishy flavor and is cheesy in consistency. Birth size is also quite large; a blue whale, for example, is about 12 m long at birth. By the time of weaning, after 7 months or so, it has already doubled in length.

Cetacean mothers care for their young. Cetaceans are air breathers and must return to the surface for oxygen. The nasal

opening is the blowhole, located on the back of the head. When a whale reaches the surface, there is often a characteristic loud sneezing sound as carbon dioxide is expelled through special flaps that seal off the nasal opening when the animal dives.

General characteristics of suborder Odontoceti

The suborder Odontoceti includes species such as the sperm whale حوت العنبر, the killer whale, the beluga whale (الدلفين األبيض), and porpoises.

All actively hunt large prey, such as fishes and smaller marine mammals. They have typical mammalian teeth. There is a single blowhole. Most species are excellent divers, a characteristic that is probably related to their

hunting behavior for mobile prey. Sperm whales can dive routinely to depths of 1,000 m.

Range in size from just a few meters in length (the dolphins), to lengths of 15-20 m (the sperm whale).

Most are capable of sophisticated oral communication and can generate a series of sonic and ultrasonic clicking signals. The clicks are also used as a means of

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echo-location; the animals can accurately estimate distance on the basis of the travel and return time of the clicks.

Many species have a bulbous structure, the melon, on the anterior part of the upper skull. The melon is filled with a fine oil. Some suggest that the melon serves as a device to focus returning sound waves, though others think that the melon is a storage site for gaseous nitrogen, so that the animal will not develop the "bends" as it surfaces تتجه للسطح (will be discussed later).

The auditory canal is reduced, so sound is likely transmitted through another route, such as the bones of the lower jaw, which are thin and seem to be good transmitters.

In most of the species, traveling in small groups called pods (also called herds or schools) is the rule. Killer whales (Orcinus orca) travel in pods of usually less than 10, with a dominant male, several females, and a few subordinate males.

General characteristics of suborder Mysticeti (or baleen whales)

These whales are generally larger than toothed whales, range in size from rorquals and gray whales, which are 10-15 m long, to the blue whale, which can (الهرآول)exceed 30 m.

They have a double blow hole. The teeth in the adult are replaced by horny baleen plates, derived from dermal

tissue. Such plates are attached to the upper jaw, from which project sheets composed of individual long straining bristles to strain out larger zooplankton, such as krill. Periodically, the blue whale closes its mouth and the tongue is raised, thus forcing water out through the baleen plates. The zooplankton trapped on the baleen are then swallowed. In the blue and fin whales, the lower mouth is a furrowed متجعد ventral pouch. When closed, the lower mouth appears externally as a series of parallel ridges and furrows below the mouth opening. As the mouth opens, the pouch expands, and may enclose as much as 70 tons of water.

Baleen whales often migrate great distances, often traveling thousands of miles from feeding to breeding areas.

Fig. Feeding mechanism of baleen whales (Right whale –above- and blue whale -below-).

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Pinnipeds Seals ( (الفظ) Sea Lions, and Walruses ,( أو عجل البحرفقمةال

Taxonomy Class Mammalia Phylum Chordata Order Carnivora Suborder Pinnipedia Include the seals, sea lions, walruses, and elephant seals.

General characteristics of Pinnipeds Have hair and lack the large amount of fat. They are streamlined and expert swimmers. The rear legs are modified as flippers and can propel the animal through the water. They range in size from the relatively small (1 m long) sea lions to the enormous

southern elephant seal, which can reach 4000 kg (males) and 6 m in length. In most pinniped species, the teeth are simpler and less specialized than those of

terrestrial mammals. Pinnipeds are carnivores and are usually excellent divers. Pinnipeds are well known for their mating and reproductive habits. Although most

species spend the majority of time at sea, they come to large beach and rocky-shore areas to mate and rear young. Usually, large males arrive first and establish territories. The females and subordinate males then arrive, and the dominant males maintain harems of one to several females.

Seals Seals are divided between the true seals (Family Phocidae) and the eared seals (Family Otariidae).

True seals True seals include the Weddell seal and the elephant seal. They have a small ear hole They have short backward-pointing hind flippers that are used for propulsion in water. These seals do not get about on land very well, in comparison to their elegant propulsion in the water.

Eared seals (Sea lions) They have an external ear and relatively longer necks. They use the anterior flippers for swimming. They can fold the rear flippers forward, sit on them, and use them for propulsion on land.

True seals Eared seals

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Walrus (الفظ) Large pinniped with the upper canine teeth have become modified into large tusks. Sea otters (ثعلب الماء) Sea otters are belong to the mammalian family Mustelidae. Sea otters have evolved a number of traits for life at sea, including streamlined body, modified appendages, and hair that act as a thermal barrier to retard heat loss to seawater. They are marine carnivores, preying mainly on benthic species such as sea urchins and mollusks, and on fishes.

Sirenia (الخيالني) Taxonomy Class Mammalia Phylum Chordata Order Sirenia They include manatees )خروف البحر( , dugongs )األطوم ( , and sea cows. They are generally hairless and streamlined, and they superficially resemble whales. They have blunt and broad muzzles خطم. They are generally sluggish and live in shallow water, They are usually herbivorous eat seaweeds or other aquatic vegetation. Marine Mammals in Gaza Strip Dolphins are regularly reported by Fishermen of Gaza Strip. They have observed that, there are two Dolphin species these are Bottlenose dolphin (Tursiops truncatus) and Common dolphin (Delphinus delphis).

Diving by Marine Mammals Marine mammals do not have gills, hence must get their air supply at the surface. Moreover, these animals have relatively high oxygen consumption rates. One might therefore suppose that they risk running out of oxygen during extended dives. They occasionally submerged for more than an hour, perhaps to escape predators or explore new routes beneath the ice. Special mechanisms, however, have evolved to accommodate these circumstances. Seals, for example, can carry much more oxygen than humans by making use of the following mechanisms:

1. Increased volume of arteries and veins. 2. Have relatively large volume of blood per kilogram of body weight. 3. Have huge spleen, which can store 24 L of blood. The spleen probably contracts

after a dive begin, fortifying the blood with additional erythrocytes loaded with oxygen.

4. Have high concentration of an oxygen storing protein called myoglobin in their muscle.

5. Ability to carry more oxygen per unit volume of blood, owing to increased red blood cell concentration and these cells carry more hemoglobin.

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6. Decreased heartbeat and oxygen consumption rate. 7. Reduction of blood flow to non-essential parts of the body, like extremities and

the intestine, but it maintained to vital organs like the brain, heart, spinal cord, eyes, adrenal gland and placenta (in pregnant seal).

8. After depletion the oxygen stored in their myoglobin, muscles have the ability to derive their ATP from fermentation.

It may be surprising that seals and toothed whales do not have extraordinary lung capacity. Uptake of large amounts of air at the surface would be dangerous to them (see bends below). After surfacing, diving mammals usually increase their metabolic rate and provide needed oxygen to tissues. Marine mammals dive deeper and stay down longer than human divers, so why don’t they get the bends? The answer is that they have adaptations that prevent nitrogen from dissolving in the blood in the first place. They have lungs with relatively small capacity, to prevent storage of any extra gas that might be released during drops in pressure. They have a flexible rib cage thus, when marine mammals dive, their lungs actually collapse by the pressure of the water. This squeezes the air in the lungs into the central space, where little or no air can dissolve into the blood. Air is moved instead into the central space, where little nitrogen is absorbed. These physical characters reduce the amount of nitrogen that enters the circulatory system. They also do not breathe compressed air at depths the way scuba divers do, so their bubble problems are fewer. Some pinnipeds actually exhale before they dive, further reducing the amount of air –and therefore nitrogen- in the lungs.

Seabirds Seabirds are those birds that spend a significant part of their lives at sea. They travel some great distance across the sea and typically breed on offshore islands or coastal areas. They have a salt gland that efficiently excretes salts gained from seawater and food. Seabirds can be conveniently divided into four major groups. 1. Penguins • Flightless and their flippers are modified forewings. • Their bones are heavier than those of other birds to reduce buoyancy and make

divining easier. • They are well insulated from cold air and water by means of a layer of blubber and a

thick layer of feathers. • They live in colonies that vary from a few pairs to thousands. • They range in size from the little blue, about a kilogram or two and standing about 40

cm, to the majestic emperor, which stands over a meter tall and weighs about 30 kg. 2. Petrels (طائر النوء) and their allies-the albatrosses (طائر القطرس), shearwaters(جلم الماء). • They have large external nostrils, which may be useful to smell prey, and a hooked

bill. • They range from the giant petrel, which preys on other birds to the small-fish-eating

puffins (طائر البفن) to the zooplankton-straining prions, which have comblike plates on each side of the mouth.

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3. Pelicans (طائر البجع) and their relatives-the gannets (طائر األطيش), and cormorants (الغاق) • Often heavy and include many brightly colored and ornamented species. • They are mainly tropical and have webbing between all four toes. • While some, such as frigate birds (الفرقاطة), fly far out to sea, most of this group stays

closer to land. • Feeding in this group is restricted to fishes.

4. Gulls (النورس), terns (طائر الخرشنة), and auks (طائر األوك) • Comprise by far the most diverse group of seabirds. • They feed mainly on small fish and zooplankton.

Fig. Some of the diversity of seabirds: (a) emperor penguin, (b) wandering albatross, (c) magnificent frigate bird, and (d) black-backed gull. Recorded seabirds in Gaza Strip Gulls, which include: Yellow-legged gull Black-headed gull Mediterranean gull Terns: Little tern Whiskered tern Cormorant

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BREEDING BEHAVIOR, NESTING GROUNDS, AND MIGRATION

Seabirds are nearly always monogamous (احادى الزواج). Monogamy appears to assure earlier breeding and increased survival of young. Care of young is almost always shared by the mother and father. Even egg incubation may be shared by the male; the male emperor penguin incubates a single egg for the entire Antarctic winter. Nearly all species nest in colonies, some numbering in the thousands.

Colonial breeding may allow sharing of information about feeding grounds among foraging parents, but the practice more likely arose from the shortage of suitable sites that are remote from predators of nesting birds and their eggs. Communal defense, such as the "dive-bombing" of herring gulls, also is an effective deterrent against predators. Colonial living, however, has the disadvantage of putting birds in intense competition for limited resources near the breeding colony.

Territoriality and combat are common in seabirds because high-quality nesting sites are in short supply.

Fighting is usually more intense in species that nest on cliffs or in holes, where space is more limiting than, say, on open-ground sites. Mating pairs may be formed before or after a nesting site is chosen.

Courtship often includes elaborate displays, which may involve groups of males.

Nesting sites vary in substratum type, degree of slope, and degree of isolation.

Nest sites are chosen for their suitability to avoid predators, for social interactions, and for proximity to local food The breeding cycle includes periods of nest site establishment, egg laying and

incubation, and fledging (ينبت ريشه). Seabirds migrate to maximize use of feeding and nesting areas.

After the breeding season birds may migrate to distant areas where food is more abundant.

Fig. Aggression in seabirds: (a) beak grabbing in Atlantic gannets and (b) flipper fighting between male penguins.

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Feeding Strategies: Seabirds employ two strategies for hunting food: flying and underwater swimming. Effi-cient flying however requires long and relatively inflexible wings, whereas underwater swimming requires short wings that are usable as flippers. The dichotomy of flying and underwater swimming influences the geographic location of seabirds. Efficient underwater swimmers cannot fly very far or indeed at all, as in the case of penguins. Thus one tends to find specialized underwater swimmers in coastal areas of very high productivity, such as upwelling and polar regions. By contrast, efficient flyers can be found throughout the relatively less productive tropical waters of the open Pacific. Methods by which seabirds obtain their prey (see the figure below)

Shorebirds Shorebirds differ from seabirds in that they have a greater dependency upon terrestrial sites for nesting. Shorebirds, include the sandpipers (طائر زمار الرمل), plovers ( طائر-and oystercatchers. They feed principally on intertidal soft-bottom and rocky ,(الزقزاقshore organisms and may exert profound effects on these ecosystems. Many species migrate in extremely large numbers and often stop for feeding at specific sites, where they devastate the local invertebrate populations. Feeding methods of shorebird Running and stabbing is done by plovers (الزقزاق), which pursue a prey (e.g., a crab) and stab it with the beak. Chiseling and hammering is employed by turnstones (قنبرة الماء) and oystercatchers. Ruddy turnstones use their bills to excavate clams from the sand and chisel them open. Oystercatchers may chisel a hole directly in an oyster or mussel.

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Probing and pecking نقر, is employed by Sandpipers, Godwits, and Sanderlings. Whirling and concentration Phalaropes swim in the water and use a whirling motion to concentrate crustaceans, which are pecked out of the water.

Fig. Some of the diversity of shorebirds: ruddy turnstone, purple sandpiper, marbled godwit, and northern Phalarope, with detail of webbed foot. (After Schneider, 1983.)

Sea Reptiles

Sea Snakes Sea snakes are relatives of the cobras and are found commonly throughout the Indo-Pacific region, although a single species occurs along the west coast of North America. None are to be found in the Atlantic or the Mediterranean. Sea snakes have fixed fangs and are largely venomous, although very few humans have died from sea snake bites and some species are surprisingly docile. Many snakes lay eggs on shore, but some complete their breeding cycle out at sea and young are born alive in the water. They are air breathers, and they must periodically go to the surface to gather air. Some snakes can stay submerged for several hours and must be tolerant of anoxia. Some species actively trap fish prey in coral crevices and then grab them with the mouth, whereas others lie quite still, like sticks, and strike fish that approach them.

Ruddy Turnstone

Marbeled GodwitPurple Sandpiper

Northern Phalarope

Webbed food of Phalarope

Ruddy Turnstone Northern Phalarope

Webbed food Phalarope

Ruddy Turnstone Northern Phalarope

Webbed food of Phalarope

MarbeledPurple Sandpiper Godwit

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Sea Turtles

There are only eight species of sea turtles, which live primarily in warm water. Sea turtles include: Kemp's ridley eats bottom-dwelling invertebrates. Leatherbacks consume jellyfish from the water column. Hawksbills feed on sponges. The estuarine diamond-backed terrapin may walk along the bottom and seize a mussel by means of a rapid, downward protrusion of the head. Two sea turtle species were identified as existing in the coastal region of Gaza. Caretta caretta (Loggerhead turtle) carnivore and eat bottom-dwelling invertebrates. Chelonia mydas (Green turtle) herbivore, feed on seaweeds and sea grasses.

* Female sea turtles lay eggs on specific beaches, and migrate between the beach and seasonal feeding grounds great distances away. Sea turtles are well known for their beach nesting and extensive migrations. Males and females mate near nesting beaches, and females come onto shore several times per breed-ing season. On each occasion, they lay approximately 100 eggs apiece, which are buried in the sand. After about 2 months, young hatch from the eggs, crawl to the surface, and scramble to the sea. Their movements are guided by three distinct signals. After hatching, they orient and move toward the horizon, which gets them to the strandline. They then move opposite to the direction of arriving waves, which moves them out to sea. They then oriented by the earth's magnetic field which may provide information on geographic position and migration direction. It is not known however how turtles manage to migrate long distances to feeding grounds or locate their natal beaches. Palestine and Gaza strip was reported as turtle nesting beaches of by the Coastal Zone Plan for Gaza (MOPIC, 1996). Unfortunately the two species found in Gaza coasts and their eggs are under extreme pressure from hunting and collecting. It is not known at present whether the turtles still breed along the beaches of Gaza Strip, but fishermen usually report regular sightings at seawaters. Sea turtles are protected species and hunting is prohibited by law.