3 The open ocean is vast. It covers an area of 361 million sq. km (139 million sq. miles) and more than 70% of the world’s surface. Much of the ocean lies in the world’s tropical and sub-tropical areas, which contains comparatively few nutrients and plankton. This massive three-dimensional space is home to some of the most extraordinary creatures on the planet, some of which must travel long distances up and down the water column or across the open ocean to find food and mates. VERTICAL LAYERS The open ocean is known as the pelagic realm. It contains several vertical layers or zones with different characteristics. The layers form a physical and chemical barrier to different organisms and few organisms can move easily between them. The main vertical zones are: 1. Neuston Layer The topmost meter of the ocean is completely different from the rest of the water mass. It is called the “neuston layer” and is comparatively rich in nutrients because many of the waste chemicals excreted by plankton in deeper water float to the surface and concentrate there. This chemically enriched environment provides and ideal habitat for bacteria, unicellular protozoans and microscopic algae.
4 8. OPEN OCEAN 2. Photic Zone (Surface) The top 200 m (656 ft) of water is warm, highly mixed, and effectively floats upon the colder, denser water below. It is the sunlit zone of the ocean and is limited to the maximum depth sunlight can penetrate. The photic zone is where phytoplankton is found since they need sunlight for photosynthesis. It is also where most animals are found at night. Under water, the colours of the visible spectrum are absorbed by water with increasing depth. Red will disappear at a depth of around 6 m (20 ft), orange at 9 m (30 ft), yellow at 18 m (60 ft) and green at 21 m (70 ft). By 30 m (100 ft), everything appears blue or greyish green. At greater depths, all visible light is absorbed and everything appears dark blue or black. 3. Twilight Zone (Intermediate) Below the photic zone lies the colder and denser intermediate layer called the twilight zone which lies between 200 m and 1000 m (656 ft and 3,300 ft). The twilight zone is where most animals are found during the day. 4. Dark Zone (Deep) Below 1,000 m (3,300 ft), the water is extremely heavy, dense, and cold. It is also completely dark. In spite of these difficult conditions, however, deep sea life can be found here.
5 8. OPEN OCEAN TEMPERATURE & OXYGEN BARRIERS The different layers of the ocean are physically and chemically different. The water temperature and the amount of dissolved oxygen play important role in partitioning life vertically in the ocean. For example; 1. Temperature The layers have different temperatures and thermoclines (lines of temperature differences) form. In the tropics, the surface zone water may be as warm as 25°C (77°F) while the intermediate zone water temperature may be just 11°C (50°F). In the dark zone, the water becomes much colder, 5°C (41°F). The main ocean thermocline lies within the intermediate zone. This is a permanent feature that is rarely broken down and it proves an impenetrable barrier to most marine life because it cannot cope with the sudden change in temperature. By and large, this thermocline separates the ocean’s upper-water organisms from those in the deep. 2. Oxygen Minimum Layer Another boundary exists within the intermediate zone. This is the oxygen minimum zone and marks the level below which dissolved oxygen in the water is at its minimum. Few organisms from the warm oxygen-rich surface waters above can survive in these oxygen-depleted conditions.
6 8. OPEN OCEAN OVERTURN Anything that is heavier than water sinks under gravity so many dying organisms and nutrients fall through the thermocline into the deep zone below. If the ocean was static, these sources of food would be lost to the life in the surface layer. Fortunately, in polar and temperate oceans, in a process called “overturn”, winter chilling causes the surface water to become so dense that it sinks into the deep ocean forcing nutrient-rich water up from the bottom. Winter storms further mix the water layers, ensuring that oceans in the far north and south of the world have high nutrient levels and can support the massive plankton populations in the summer months. Tropical and sub-tropical oceans, however, do not experience sufficient cooling to allow overturn to occur so here the stable surface zone contains vastly fewer nutrients than the deep zone. REFERENCES & FURTHER READING Byatt, Andrew, Fothergill, Alastair and Holmes, Martha, The Blue Planet: Seas of Life, Chapter 6, DK Publishing Inc., (2001), ISBN 0- 7894-8265-7
8 8.1.1 Vertical Migrations In the open ocean during the day, there are very few places animals can hide from predators. In order to avoid predators, therefore, most animals hide in the twilight zone (200-1,000 m / 656-3,300 ft) where they are less easily seen by predators that hunt mainly by sight. Thus, during the day, the photic zone only contains about 10% of the total marine life, while 75% is found in the twilight zone. At night, however, the amount of life at the surface quadruples to 40% as a result of mass vertical migrations. Every night, millions of tonnes of animals undertake the largest mass migration on Earth journeying up from the ocean’s twilight zone to the photic zone in search of food. In the middle of the night, the top 30 m (98 ft) of the ocean teems with feeding plankton. At dawn, the same animals will return to the twilight zone. The extent of the vertical migration up and down the water column varies between species. The smallest plankton probably travel just 10-20 m (33-66 ft) while larger animals may travel as much as 1000 m (3,300 ft). As the zooplankton travel up and down the water column, so follow their predators, including fish such as anchovies, mackerel and herring, and jellyfish. In turn, predators of these fish and jellyfish, such as sharks, dolphins and turtles, follow.
10 8.1 LIFE IN THE OPEN 8.1.2 Migrations Across Oceans Some animals undertake immense journeys across the oceans in search of food and breeding grounds. The following are some of the extraordinary travelers that undertake these great migrations across the oceans: 1. Arctic Terns Arctic terns fly between the Arctic and Antarctic regions to enjoy two summers a year. They travel further than any other bird. Some Arctic terns fly more than 35,000 km (21,000 miles) in a year from the Arctic to the Antarctic and back! 2. Grey Whales Grey whales swim more than 22,000 km (13,600 miles) a year between their Arctic feeding grounds and their breeding grounds off California. No other mammal migrates as far. 3. Wandering Albatrosses The wandering albatross spends most of its life circling the globe north of Antarctica. This seabird travels up to 12,000 km (7,500 miles) before returning briefly to land to breed. 4. Green Turtles Every 2-3 years, female green turtles leave their feeding grounds off Brazil and travel to Ascension Island in the middle of the Atlantic Ocean where they were born. Here, they lay their eggs before returning across the sea again. How they navigate this 2,000 km (1,250 mile) journey is not known but some possible mechanisms include their using currents, seamounts or the Earth’s magnetic field.
11 8.1 LIFE IN THE OPEN 5. Spiny Lobsters Migrating spiny lobsters form “lobster trains” and match head to tail along the seabed to protect themselves from enemies. 6. European Eels The European eel has an amazing life cycle. From 4 to 18 years of age, it lives in freshwater rivers and lakes. When it reaches adulthood, it heads downstream towards the ocean. On reaching the sea, it changes to saltwater life and heads into the open ocean. Heading south and west, it swims 6,000 km (3,700 miles) to the Sargasso Sea. Here, at a depth of 700 m (2,300 ft), it meets up with thousands of other eels and spawns in the deep cold water. The effort is terminal and all adult eels die. Their microscopic larvae spend 3 years growing up in the plankton and following the Gulf Stream past the Caribbean and back across the Atlantic. In their 4 th year, the young eels wriggle and slip their way up the rocky slopes of European rivers to spend their next 14 years in freshwater. 7. Salmon Salmon hatch in freshwater streams but mature at sea. They will return just once to the streams of their birth to spawn and die. Some adults will travel thousands of miles to reach the river where they hatched. Interesting! The longest recorded migration of any marine vertebrate goes to a leatherback turtle. Scientists from NOAA initially tagged an animal in Papua, Indonesia and tracked its movements for 647 days to the coast of Oregon in the USA – a one-way swim of 20,558 km (12,774 miles) across the Pacific Ocean.
14 8.1 LIFE IN THE OPEN REFERENCES & FURTHER READING http://www.bbc.co.uk/radio4/worldonthemove/ - Migrations http://www.seaturtlestatus.org – Sea Turtle migrations Byatt, Andrew, Fothergill, Alastair and Holmes, Martha, The Blue Planet: Seas of Life, Chapter 6, DK Publishing Inc., (2001), ISBN 0-7894-8265-7
16 8.1 LIFE IN THE OPEN ALBATROSS An albatross (Diomedeidae spp.) aloft is a magnificent sight. These birds weigh up to 10 kg (22 lb) and have the longest wingspan of any bird – up to 3.4 m (11 ft). The wandering albatross is the biggest of some two dozen different species. Albatrosses use their huge wings to ride the ocean winds and can glide for hours without rest or even a flap of their wings. They also float on the sea's surface though this makes them vulnerable to aquatic predators. Albatrosses drink salt water, as do some other sea birds. Albatrosses feed on squid or schooling fish but are familiar to mariners because they sometimes follow ships in the hope of dining on handouts. These long-lived birds reach 50 years of age. They are rarely seen on land and gather only to breed at which time they form large colonies on remote islands. Some species mate for life. Mating pairs produce a single egg and take turns caring for it. Young albatrosses may fly within 3-10 months and then leave land behind for some 5-10 years until they themselves reach sexual maturity.
17 8.1 LIFE IN THE OPEN PORTUGUESE MAN-OF-WAR The Portuguese man-of-war (Physalia physalis) is a siphonophore – an animal (cnidarian) made up of a colony of organisms working together. A man-of-war comprises four separate polyps. It gets its name from the uppermost polyp, a gas-filled bladder called a float or pneumatophore, which sits above the water and resembles an old warship at full sail. Man-of-wars are also known as bluebottles for the purple-blue colour of their pneumatophores. The float may be 30 cm (12 in) long and 12.7 cm (5 in) wide. The tentacles are the man-of-war's second organism. These long, thin tendrils can extend 50 m (165 ft) in length below the surface, although 10 m (30 ft) is more average. They are covered in venom-filled nematocysts used to paralyze and kill fish and other small creatures. For humans, a man-of- war sting is excruciatingly painful but rarely deadly. Muscles in the tentacles draw prey up to a polyp containing the digestive organisms (gastrozooids) while the fourth polyp contains the reproductive organisms. Man-of-wars are found, sometimes in groups of 1,000 or more, in warm waters of throughout the world's oceans. They have no independent means of propulsion and either drift with the currents or catch the wind with their pneumatophores. To avoid surface threats, they can deflate their air bags and submerge.
18 8.1 LIFE IN THE OPEN LEATHERBACK TURTLE Leatherback sea turtles (Dermochelys coriacea) are the world’s largest turtles growing up to 2.6 m (8.5 ft) in length and 907 kg (2,000 lb) in weight. While all other sea turtles have hard, bony shells, the smooth, black carapace of the leatherback is soft and almost rubbery to the touch. They may live over 45 years but human threats, such as fishing lines and nets, mean most leatherbacks meet an early end. Other threats include illegal egg harvesting and loss of nesting habitat. Hatchlings often die when beachfront lighting draws them away from the ocean and hundreds die at sea when they swallow plastic which they mistake for jellyfish – their main food. They can dive to 1,230 m (4,035 ft) and remain submerged for 35 minutes. In all, only about 1 in 1,000 leatherbacks survives to adulthood. The worldwide population is estimated at about 26,000-43,000 nesting females annually, but they are suffering exponential declines and are critically endangered throughout their range and now teeter on the brink of extinction. Their enormous range comprises the tropical and temperate waters of the Atlantic, Pacific, and Indian oceans. Unlike other reptiles, their body temperature stays well above the surrounding water and they have been found in the icy seas as far north as British Columbia, Canada, and as far south as the Cape of Good Hope, South Africa.
19 8.1 LIFE IN THE OPEN OCEAN SUNFISH The ocean sunfish (Mola mola) is the heaviest bony fish in the world. It has an average length of 1.8 m (5.9 ft) and an average weight of 1,000 kg (2,200 lb) although individuals up to 3.3 m (10.8 ft) in length and weighing up to 2,300 kg (5,100 lb) have been observed. The species is native to tropical and temperate waters around the globe. It resembles a fish head without a tail and its main body is flattened laterally. Sunfish can be as tall as they are long when their dorsal and anal fins are extended. Sunfish mainly eat jellyfish. As this diet is nutritionally poor, they consume large amounts in order to develop and maintain their great bulk. Females of the species can produce more eggs than any other known vertebrate. Sunfish fry resemble miniature pufferfish with large pectoral fins, a tail fin and body spines uncharacteristic of adult sunfish. Adult sunfish are vulnerable to few natural predators but sea lions, orcas and sharks will consume them. Among humans, sunfish are considered a delicacy in some parts of the world, including Japan and Taiwan but sale of their flesh is banned in the European Union. Sunfish are often accidentally caught in gill nets and are also vulnerable to injury or death from encounters with floating rubbish, such as plastic bags.
20 8.1 LIFE IN THE OPEN BOTTLENOSE DOLPHIN Bottlenose dolphins (Tursiops truncatus) are well known as the intelligent and charismatic stars of many aquarium shows. In the wild, these sleek swimmers can reach speeds of over 30 kph (18 mph). They surface often to breathe, doing so two or three times a minute. They reach 4.2 m (14 ft) in length and weigh 500 kg (1,100 lb) and can live 50 years. Dolphins often eat bottom-dwelling fish as well as shrimp and squid. Bottlenose dolphins track their prey through the use of echolocation. They can make up to 1,000 clicking sounds per second. These sounds travel underwater until they encounter objects, then bounce back to their dolphin senders, revealing the location, size, and shape of their target. Bottlenose dolphins travel in social groups and communicate with each other by a complex system of squeaks and whistles. Schools have been known to come to the aid of an injured dolphin and help it to the surface. Bottlenose dolphins are found in tropical oceans and other warm waters around the globe. They were once widely hunted for meat and oil (used for lamps and cooking) but today only limited dolphin fishing occurs. However, dolphins are threatened by commercial fishing for other species, like tuna, and can become entangled in nets and other fishing equipment. Interesting! All dolphins, including the bottlenose, are porpoises. Although some people use these names interchangeably, porpoises are actually a larger group that also includes animals like killer whales (Orca) and beluga whales.
21 8.1 LIFE IN THE OPEN WHALE SHARK The whale shark (Rhincodon typus) is the largest fish in the sea with a length of 18.3 m (60 ft) length. The whale shark's flattened head sports a blunt snout above its mouth with short barbels protruding from its nostrils. Its back and sides are gray to brown with white spots among pale vertical and horizontal stripes, and its belly is white. Its two dorsal fins are set rearward on its body, which ends in a large dual-lobbed caudal fin (or tail). Whale sharks eat plankton which they filter from the water. They scoop tiny animals and plants, along with any small fish that happen to be around, with their colossal gaping mouths while swimming close to the water's surface. It then shuts its mouth, forcing water out of its gills and filtering out the food. Whale sharks are found in all tropical seas. They migrate every spring to the continental shelf of the central west coast of Australia. The coral spawning of the area's Ningaloo Reef provides the whale shark with an abundant supply of plankton. Whale sharks are currently listed as a vulnerable species. However, they continue to be hunted in parts of Asia, such as Taiwan and the Philippines.
22 8.1 LIFE IN THE OPEN BLUE MARLIN The blue marlin (Makaira nigricans) is the largest of the Atlantic marlins and one of the biggest fish in the world. Females, which are significantly larger than males, can reach 4.3 m (14 ft) in length and weigh more than 900 kg (1,985 lb). Females can live up to 27 years in the wild. Native to the tropical and temperate waters of the Atlantic, Pacific, and Indian Oceans, blue marlins are among the most recognizable of all fish. They are cobalt-blue on top and silvery-white below with a pronounced dorsal fin and a long, lethal spear-shaped upper jaw. They are so-called blue-water fish, spending most of their lives far out at sea. They are also highly migratory and will follow warm ocean currents for thousands of kilometers. Blue marlins prefer the higher temperature of surface waters, feeding on mackerel and tuna, but will also dive deep to eat squid. They are among the fastest fish in the ocean, and use their spears to slash through dense schools, returning to eat stunned and wounded victims. Their meat is considered a delicacy, particularly in Japan, where it is served raw as sashimi. Although not currently endangered, conservationists worry that they are being unsustainably fished, particularly in the Atlantic. Interesting! It is a blue marlin that the old fisherman battles in Ernest Hemingway's classic story “The Old Man and the Sea”.
23 8.1 LIFE IN THE OPEN THRESHER SHARK The thresher shark (Alopias vulpinas) is an oceanic deep-water shark. It has a pointed snout and 5 gills slits in front of each pectoral fin. It also has an extremely long upper lobe in caudal fin that often exceeds length of body. The tail used to herd and stun prey. It is often found in large numbers and is considered a large and dangerous shark especially during maritime disasters. The thresher shark feeds on fishes and squids and can reach 6.1 m (20 ft) length and weigh 454 kg (1,000 lbs). SHORTFIN MAKO The shortfin mako (Isurus oxyrinchus) is a member of the mackerel shark family. It is a slender, bullet-nosed shark that is bright blue to slate blue above and white below and can reach 3.7 m (12 ft) in length. Its front teeth are long, narrow, curved with no cusps at base. It is a very swift and active shark that hunts tuna and other fish. It is possibly the fastest swimming shark.
24 8.1 LIFE IN THE OPEN ATLANTIC BLUEFIN TUNA The Atlantic bluefin tuna (Thunnus thynnus) is one of the largest, fastest and most beautiful of the world’s fishes. Their torpedo- shaped, streamlined bodies are built for speed and endurance. Their colouring (metallic blue on top and shimmering silver-white on the bottom) helps camouflage them from above and below. Their voracious appetite and varied diet pushes their average size to 2 m (6.5 ft) in length and 250 kg (550 lb) although they can reach twice this size. Atlantic bluefins are warm-blooded, a rare trait among fish, and are comfortable in the cold waters off Newfoundland and Iceland, as well as the tropical waters of the Gulf of Mexico and the Mediterranean Sea, where they go each year to spawn. They are among the most ambitiously migratory of all fish, and some tagged specimens have been tracked swimming from North American to European waters several times a year. They can live 15 years in the wild. They are prized among sport fishermen for their fight and speed, shooting through the water with their powerful, crescent-shaped tails up to 70 kph (43 mph). They can retract their dorsal and pectoral fins into slots to reduce drag. Some scientists think the series of “finlets” on their tails may reduce water turbulence.
25 8.1 LIFE IN THE OPEN Bluefins attain their enormous size by gorging themselves almost constantly on smaller fish, crustaceans, squid, and eels. They will also filter-feed on zooplankton and other small organisms and have even been observed eating kelp. The largest tuna ever recorded was an Atlantic bluefin caught off Nova Scotia that weighed 679 kg (1,496 lb). Bluefin tuna have been eaten by humans for centuries. In recent years demand and prices for large bluefins soared worldwide, particularly in Japan, and commercial fishing operations found new ways to find and catch these sleek giants. As a result, bluefin stocks, especially of large, breeding-age fish, have plummeted, and international conservation efforts have led to curbs on commercial takes. Nevertheless, at least one group says illegal fishing in Europe has pushed the Atlantic bluefin populations there to the brink of extinction. Interesting! We often say that reptiles or fish are cold-blooded and mammals are warm-blooded. However, many scientists try not to use these terms because they are not precise and, in some cases, inaccurate. More precisely, some animals, such as mammals are able to regulate their body temperature above that of the environment’ – these animals are “homeothermic” – which means they are able to keep their body temperature constant. In contrast, animals, such as most reptiles and fish, cannot regulate their body temperature and are called “poikilothermic”. These animals rely on warmth from the sun and the environment to warm them up. While most reptiles and fish cannot regulate their body temperature, there are exceptions. For example, the Leatherback turtle is a reptile that is able to regulate its temperature while the bluefin tuna is a fish that is able to do so.
26 8.1 LIFE IN THE OPEN ATLANTIC MANTA The Atlantic Manta (Manta birostris) is an oceanic ray and is a member of the devil ray family. Its body is dark brown or black above and white below, and it has long and pointed pectoral fins (their “wings”). Manta rays also have two large cephalic (head) fins that look like “horns” hence their “devil” ray name. They have short whip-like tails with no spines on them. They also have a wide, terminal mouth wide which they use to feed on plankton and small fishes. They can reach 6.7 m (22 ft) across their wings and weigh 1,814 kg (4,000 lbs).
27 8.1 LIFE IN THE OPEN ELEPHANT SEAL There are two species of elephant seals (Mirounga spp.) – northern and southern. Northern elephant seals can be found in California and Baja California though they frequent offshore islands rather than the North American mainland. Northern elephant seals live an average of 9 years in the wild. Southern elephant seals live in sub-Antarctic and Antarctic waters. These are cold waters but they are rich in the fish, squid, and other marine foods these seals enjoy. Southern elephant seals breed on land but spend their winters in Antarctic waters near the Antarctic pack ice. Southern elephant seals are the largest of all seals. Males can be over 6 m (20 ft) long and weigh up to 4,000 kg (8,800 lb). However, these massive pinnipeds aren't called elephant seals because of their size but take their name from their trunk-like inflatable snouts. Southern elephant seals can dive over 4,921 ft (1,500 m) deep and remain submerged for up to two hours. Southern elephant seals live 20 to 22 years.
28 8.1 LIFE IN THE OPEN When breeding season arrives, male elephant seals define and defend territories. They collect a harem of 40-50 females, which are much smaller than their enormous mates. Males battle each other for mating dominance. Elephant seals give birth in late winter to a single pup after an 11 month pregnancy and nurse it for approximately a month. While suckling their young, females do not eat. Both mother and pup live off the energy stored in the reserves of her blubber. Elephant seals were aggressively hunted for their oil and their numbers were reduced to the brink of extinction. Fortunately, populations have rebounded under legal protections.
29 8.1 LIFE IN THE OPEN SAILFISH Sailfish (Istiophorus platypterus) range throughout the warm and temperate parts of the world’s oceans. They are blue to gray in colour with white underbellies. They get their name from their spectacular dorsal fin that stretches nearly the length of their body and is much higher than their bodies are thick. Sailfish are members of the billfish family and, as such, have an upper jaw that juts out well beyond their lower jaw and forms a distinctive spear. They are found near the ocean surface usually far from land feeding on schools of smaller fish like sardines and anchovies which they often shepherd with their sails. They also feed on squid and octopus. Their meat is tough and not widely eaten but they are prized as game fish. These powerful, streamlined fish can grow to more than 3 m (10 ft) and weigh up to 100 kg (220 lb). In the wild, they live about 4 years. Sailfish are fairly abundant throughout their range and their population is considered stable. They are under no special status or protections. Interesting! Sailfish are the fastest fish in the ocean and have been clocked leaping out of the water at more than 110 kph (68 mph).
30 8.1 LIFE IN THE OPEN REFERENCES & FURTHER READING http://animals.nationalgeographic.com/animals/birds/albatross.html - Albatross http://animals.nationalgeographic.com/animals/invertebrates/portuguese-man-of-war.html http://animals.nationalgeographic.com/animals/reptiles/leatherback-sea-turtle.html http://animals.nationalgeographic.com/animals/mammals/bottlenose-dolphin.html http://animals.nationalgeographic.com/animals/mammals/elephant-seal.html - Elephant seal http://animals.nationalgeographic.com/animals/fish/bluefin-tuna.html - Atlantic Bluefin Tuna http://animals.nationalgeographic.com/animals/fish/blue-marlin.html - Blue Marlin http://animals.nationalgeographic.com/animals/fish/whale-shark.html - Whale shark http://en.wikipedia.org/wiki/Ocean_sunfish - Ocean sunfish
32 8.2 OCEAN LIFE 8.2.1 Sharks & Rays SHARKS DESCRIPTION Sharks have 5-7 pairs of gill slits – usually 5 pairs Gill slits on lateral side of body Most sharks have several rows of sharp pointed teeth Fusiform shape – cylindrical and tapered at both ends Extremely varied in size Heterocercal caudal fin – upper tail fin lobe longer than lower May or may not have a spiracle behind each eye May or may not have a nictitating membrane (“eyelid”) over eye Predators include other sharks, killer whales, and Man Marine but a few species (e.g. Bull Shark) can enter fresh water Sharks swim with their caudal fin (tail) and form an “S” shape with their body and tail SIZE Largest – Whale shark – 18.3 m (60 ft) length Smallest (probably) – Spined pygmy shark (Squaliolus laticaudus) – 0.2 m (6 in) Sexual dimorphism – females grow ~25% larger than males in most shark species Of 355 species, only 39 exceed 3.1 m (10 ft) in length while 176 species stay under 0.8 m (39 in)
33 8.2 OCEAN LIFE RAYS (BATOIDS) DESCRIPTION Nearly all rays have 5 pairs of gill slit openings Gill slits and mouth on the ventral side of the body Pectoral fins enlarged & attached to sides of head Rhomboid or circular shape Caudal and dorsal fins reduced, sometimes absent No anal fin All rays have large spiracles to take in water for respiration All lack nictitating membrane on the eye Predators include sharks and Man Largest family within the rays are the skates Electric ray family can generate powerful electrical charges Sawfish family use their “saws” for hunting Most rays are bottom dwellers but some are pelagic (e.g. mantas) As a group, very successful in colonizing the deep sea Mostly marine but some (e.g. sawfishes) can enter fresh water; a few live only in fresh water Most rays swim by flapping their pectoral fins (“wings”) but guitarfishes & sawfishes swim like sharks SIZE Largest – Manta Ray – 6.7 m (22 ft) across wings and 1,814 kg (4,000 lbs)
34 8.2 OCEAN LIFE SCALES Sharks and batoids have placoid scales which are also often called “dermal denticles” (“skin teeth”). Placoid scales and teeth have the same structure, consisting of 3 layers: Outer layer of vitro-dentine (an enamel) Dentine Pulp cavity Placoid scales give the skin a tough sandpapery texture. Shark and batoid skin was formerly valued as a source or leather and as an abrasive called “shagreen” Placoid scales are arranged in a regular pattern in sharks and an irregular pattern in batoids. Some rays are covered with denticles (small prickles) while others are naked or have only small patches of denticles. Many have a median (middorsal) row of enlarged denticles (spines) down the back and tail. In stingrays and their relatives, some tail denticles are modified into long barbed spines – these and the spines at the front of the dorsal fins are commonly called “stings” and can cause severe wounds.
35 8.2 OCEAN LIFE TEETH & JAWS Teeth are modified, enlarged placoid scales Size and shape of teeth vary enormously Sharks Can be serrated wedges, smooth & pointed or blunt for crushing Bite force exerted by some sharks up to 8,000 PSI Often possess multiple rows of teeth Rows of teeth roll forward replacing old, broken or missing teeth A shark's jaws are loosely connected to the rest of the skull at two points. As the upper jaw extends forward from the mouth, teeth of the lower jaw first encounter prey. The lower jaw teeth puncture and hold prey while the upper jaw teeth slice. Rays Stingrays and eagle rays have teeth that are fused into plates Flattened teeth suited to grinding or crushing shellfish Some skates have many rows of teeth – Winter Skate >72 rows Filter-feeders have reduced non-functional teeth. Devil rays, basking and megamouth sharks strain plankton from the water with gill rakers (filaments composed of thousands of tiny “teeth”). Whale sharks strain plankton through a spongy tissue supported by cartilaginous rods between the gill arches instead of using gill rakers.
36 8.2 OCEAN LIFE DIET Sharks eat almost anything: fishes, crustaceans, molluscs, marine mammals, sea birds, and other sharks and rays. Most rays eat clams, mussels, and oysters but rays also eat a variety of fishes, squids, and crustaceans. For example: Sharks Bull sharks eat fishes & other sharks Great white sharks eat sea lions & other marine mammals Hammerhead sharks eat fishes & stingrays Wobbegongs eat shrimps Tiger sharks eat several species of sea turtles & sea birds Whale and basking sharks feed on plankton Rays Sawfishes – eat fishes Electric rays – eat bottom organisms, flounders and small sharks Stingrays – eat clams and oysters Eagle rays – eat molluscs Manta rays – strain plankton from the water
37 8.2 OCEAN LIFE BREEDING & REPRODUCTION All sharks and rays utilize internal fertilization “Claspers” – male sex organs in cartilaginous fish; modified pelvic fins Clasper is turned forward and inserted into female cloaca Sharks can be oviparous, ovoviviparous or viviparous Skates lay eggs but other rays are ovoviviparous Eggs are often called “Mermaid’s Purses” Oviparous (“egg-laying”) – e.g. Horn Shark, Skates Eggs are laid by the female and develop outside her body. The eggs contain yolk and provide nourishment to the growing embryo. Ovoviviparous (“egg & live-bearing”) – e.g. Sand Tiger Shark Fertilized eggs develop within the female but the embryo gains no nutritional substances from the female. As a result, once the egg yolk runs out, young must eat other eggs or one another to survive (“uterine cannibalism”). Viviparous (“live-bearing”) – e.g. all Hammerhead and Requiem Sharks (except the Tiger Shark which is ovoviviparous). Shark embryo develops inside the body of the female from which it gains nourishment through a complex yolk-sac placenta. Female then gives birth to live offspring.
38 8.2 OCEAN LIFE SENSES Sight Sharks have a basic vertebrate eye but it is laterally compressed; lens is large and spherical Eyes are particularly sensitive to moving objects In clear water, a shark's vision is effective at a distance up to ~15.2 m (50 ft) Unlike those of other fishes, a shark's pupil can dilate and contract Cone cells are present indicating sharks may have some colour vision Eyes have numerous rods that detect light intensity changes making sharks sensitive to contrast Sharks see well in dim light. Eye has a layer of reflecting plates (“tapetum lucidum”) behind the retina. These plates act as mirrors to reflect light back through the retina a second time. Smell Sharks have an acute sense of smell & can detect minute quantities of substances in water (e.g. blood) Can detect concentrations as low as one part per billion of some chemicals (e.g. certain amino acids) A shark’s sense of smell functions up to hundreds of meters (yards) away from a source Nurse sharks have “barbels” near their nostrils which enhance tactile or chemo receptors Taste Sharks and batoids have taste buds inside their mouths Taste may be responsible for a shark's final acceptance or rejection of prey items Some sharks prefer certain foods and will spit out things that have an unpleasant taste
39 8.2 OCEAN LIFE Acoustic Sharks have an inner ear only; use sound to initially detect prey The lateral line system is a series of fluid-filled canals just below the skin of the head and along the sides of the body. The canal is open to the surrounding water through tiny pores. The lateral line canals contain a number of sensory cells called neuromasts. Tiny hair-like structures on the neuromasts project out into the canal. Water movement created by turbulence or vibrations displaces these hair-like projections, stimulates the neuromasts, and triggers a nerve impulse to the brain. Like the ear, the lateral line senses low-frequency vibrations. Sensory Pit A sensory pit is formed by the overlapping of two enlarged placoid scales guarding a slight depression in the skin. At the bottom of the pit is a sensory papilla: a small cluster of sensory cells that are probably stimulated by physical factors such as water currents. Sensory pits are distributed in large numbers on the back, flank, and lower jaw. Electrical – Ampullae Of Lorenzini External pores dot the surface of a shark's head. Each pore leads to a jelly-filled canal that leads to a membranous sac called an ampulla. In the wall of the ampulla are sensory cells innervated by several nerve fibres. These ampullae detect weak electrical fields generated by all living organisms and are used to help detect prey in the final stages of prey capture. The ampullae may also detect temperature, salinity, changes in water pressure, mechanical stimuli, and magnetic fields.
40 8.2 OCEAN LIFE INTERESTING FACTS Sharks Sharks can detect electrical impulses generated by the muscles of other animals through the ampullae of Lorenzini. Sharks are often known as “obligate ram ventilators” – this means sharks are obliged to keep moving to ram or force water through their gills in order to ventilate or breathe. However, it is not true that ALL sharks must constantly keep moving in order to breathe – some (such as whitetip reef shark and nurse shark) can rest on the sea floor and pump water over their gills. Only a comparatively small number of sharks are dangerous to Man – of 355 species, about 25 are known to have attacked Man and another 40 are potentially dangerous. Sharks may or may not have a nictitating membrane. This is an eyelid-like structure which is drawn over the eye to give it extra protection from injury caused by thrashing prey. When a shark, such as the Great White, bites its prey, it does not actually see what it is biting as each eye is covered by a nictitating membrane at the last moment. Rays do not have nictitating membranes over their eyes.
41 8.2 OCEAN LIFE Rays Some skates have numerous rows of teeth. For example, Winter Skates have 72 or more rows of teeth (usually > 80) in upper jaw! Sawfish use their “saws” to scythe through schools of fish when hunting prey. Sawfishes are ovoviviparous – eggs are retained in uterus and live young are born with their saw encased in a sheath to protect the mother. Some rays, such as the Atlantic Torpedo, have electric organs capable of delivering powerful electric shocks (~200V).
42 8.2 OCEAN LIFE THREATS Sharks have been around for hundreds of millions of years but they are seriously threatened today. Sharks reach sexual maturity slowly and have few young. In addition, as a top predator, their actual numbers are comparatively small. The primary threat to their continued existence is probably humans. Threats include: Pollution Sharks are often caught as a by-catch and discarded Sport and trophy hunting Use of shark cartilage and other body parts in both Eastern and Western medicine Fishing – people eat the pectoral fins (“wings”) of certain rays; fins also used to make Shark’s Fin Soup “Finning” – cutting the fins off sharks and then discarding the body Perhaps the greatest challenge with shark conservation is convincing people of the need to protect them. Sharks often eat sick and weak prey. This actually improves the gene pool for the stronger, healthier individuals that go on to reproduce. Shark over-fishing removes this vital link in the delicate balance of the ocean ecosystem. When sharks were over-fished around Australia, the octopus population increased dramatically. The octopus then preyed heavily on spiny lobsters and decreased that population. By destroying sharks, therefore, humans may unwittingly be removing a key that keeps ocean populations healthy.
43 8.2 OCEAN LIFE CONSERVATION Fisheries management programs are necessary for a sensible shark harvest. As of 2007, only the United States, New Zealand, and Canada have started shark management plans. In 1993, the National Marine Fisheries Service (NMFS) implemented a plan to manage U. S. shark fisheries of the Atlantic Ocean, Gulf of Mexico, and Caribbean Sea. This plan included: o Annual commercial quotas, which are divided into half- yearly quotas. o Provisions for closing a fishery for a species group when the semi-annual quota is met. o Catch limits for recreational anglers. o Permit requirements for commercial vessels that catch sharks. o A requirement that vessels land fins in proportion to carcasses (effectively prohibiting the practice of shark finning). o A requirement that when sharks are not kept, they are released in a manner that ensures the probability that they will survive. The state of California passed a law the same year totally protecting the great white shark. The law set forth by NMFS placed limits on 22 species of large coastal, seven species of small coastal, and 10 pelagic species of sharks. A yearly catch limit of 5.4 million pounds of sharks still failed to stop declining shark populations. In the spring of 1997, NMFS cut the quota of large sharks to 1.285 metric tons, limited the catch of small coastal sharks, and banned the commercial harvest of whale, great white, basking, sand tiger, and bigeye sand tiger sharks.
45 8.2 OCEAN LIFE SHARK ORDERS & FAMILIES Approximately 8 orders, 30 families and 355 species Order Hexanchiformes Family Hexanchidae – cowsharks, 6-gill & 7-gill sharks * Family Chlamydoselachidae – frilled sharks Order Pristiophoriformes Family Pristiophoridae – sawsharks Order Squatiniformes Family Squatinidae – angelsharks Order Heterodontiformes Family Heterodontidae – horn sharks * Order Orectolobiformes Family Orectolobidae – wobbegong sharks Family Ginglymostomatidae – nurse sharks * Family Rhincodontidae – whale shark * Family Parascylliidae – collared carpet sharks Family Brachaeluridae – blind sharks Family Hemiscylliidae – bamboo sharks * Family Stegostomatidae – zebra sharks * Discussed here Order Squaliformes Family Squalidae – dogfishes Family Echinorhinidae – bramble sharks Family Oxynotidae – rough sharks Order Lamniformes Family Odontaspididae – sand tiger sharks * Family Mitsukurinidae – goblin shark Family Lamnidae – mackerel sharks * Family Cetorhinidae – basking shark * Family Alopiidae – thresher sharks * Family Pseudocarchariidae – crocodile sharks Family Megachasmidae – megamouth shark Order Carcharhiniformes Family Scyliorhinidae – catsharks Family Proscylliidae – ribbontail catsharks Family Psoudotriakidae – false catsharks Family Leptocharildae – barbeled houndsharks Family Triakidae – smoothhound sharks * Family Hemigaleidae – weasel sharks Family Carcharhinidae – requiem sharks * Family Sphyrnidae – hammerhead sharks *
46 8.2 OCEAN LIFE RAY ORDERS & FAMILIES Approximately 4 orders, 11 families, and 470 species Order Rajiformes Family Rajidae – skates * Family Rhinobatidae – guitarfish * Order Torpediniformes Family Torpedinidae – electric rays * Order Pristiformes Family Pristidae – sawfish * * Discussed here Order Myliobatiformes Family Dasyatidae – stingrays * Family Myliobatidae – eagle rays * Family Mobulidae – devil rays * Family Rhinopteridae – cownosed rays * Family Urolophidae – round stingrays Family Gymnuridae – butterfly rays Family Potamotrygonidae – river rays
47 8.2 OCEAN LIFE 1. WHALE SHARK – RHINCODON TYPUS Whale Shark Family Grey-brown with distinctive pattern of yellow or white spots and bars Caudal fin nearly vertical; upper lobe much longer 3 prominent ridges along each side of back Teeth tiny and numerous Feeds mainly on plankton but will eat fishes and squid Each eggs is in a large, horny case Largest fish and also largest shark To 18.3 m (60 ft) length 2. BASKING SHARK – CETORHINUS MAXIMUS Basking Shark Family Dark grey or slate-coloured above; lighter below Gill slits very long – across entire side & nearly meeting below Each gill has long, closely set gill rakers that strain plankton from water Mouth large, teeth tiny Sheds gill rakers in winter, goes to bottom and fasts while new gill rakers grow Generally harmless but its size can make it hazardous to small boats Often hit by ocean-going ships To 13.7 m (45 ft) length SHARK SPECIES
48 8.2 OCEAN LIFE 3. WHITE SHARK – CARCHARODON CARCHARIAS Mackerel Shark Family Slate blue or leaden grey above, dirty white below Heavy body, large head, pointed snout Teeth large and triangular with serrated edges Largely oceanic but will stray in coastal waters Large & dangerous shark made infamous through “Jaws” To 7.9 m (26 ft) length but usually less than 4.9 m (16 ft) 4. SHORTFIN MAKO – ISURUS OXYRINCHUS Mackerel Shark Family Bright blue to slate blue above, white below Slender bullet-nosed shark Front teeth long, narrow, curved with no cusps at base Very swift and active shark Possibly the fastest shark – hunts tuna Important game fish Frequently marketed as “swordfish” To 3.7 m (12 ft) length
49 8.2 OCEAN LIFE 5. TIGER SHARK – GALEOVERDO CUVIERI Requiem Shark Family Brownish grey above, whitish below with conspicuous dark blotches and bars. Snout short and broadly rounded from below Spiracle present Teeth broad and coarsely serrated with deep notch on outer edge Mostly pelagic but commonly enters shallow bays to feed Large and dangerous shark, known to attack Man To 7.3 m (24 ft) length 6. BULL SHARK – CARCHARHINUS LEUCAS Requiem Shark Family Gray to dull brown above, white below Heavy body Snout short, very broad and rounded from below Upper teeth nearly triangular, serrated Common large shark in coastal water Also found in rivers and lakes; survives well in fresh water. Implicated in several attacks on humans in NJ rivers To 3.5 m (11.5 ft) length
50 8.2 OCEAN LIFE 7. SANDBAR SHARK – CARCHARHINUS PLUMBEUS Requiem Shark Family Heavy body Dark grey to brown above; becoming paler below Infamous relatives are bull, tiger and oceanic whitetip sharks Bears live young (like all requiem sharks) To 3.1 m (10 ft) length 8. WHITETIP REEF SHARK – TRIAENODON OBESUS Requiem Shark Family Small, slender shark Extremely short, broad snout, oval eyes Spiracles usually present Gray above, lighter below and sometimes with dark spots on sides First dorsal-fin lobe and dorsal caudal-fin lobe with conspicuous white tips Second dorsal-fin lobe and ventral caudal-fin lobe often white-tipped Viviparous Sluggish inhabitant of lagoons and seaward reefs Often found resting in caves or under coral ledges during the day More active at night Feeds on benthic animals such as fishes, octopi, spiny lobsters and crabs To 2.1 m (7 ft) length
51 8.2 OCEAN LIFE 9. BONNETHEAD SHARK – SPHYRNA TIBURO Hammerhead Shark Family Head spade shaped Small harmless species Feeds mainly on crustaceans Bears live young (like all hammerheads) To 1.5 m (5 ft) length 10. NURSE SHARK – GINGLYMOSTOMA CIRRATUM Nurse Shark Family Rusty brown with small yellowish eyes No nictitating membrane on eye Mouth small; teeth in a crushing series Feeds on crustaceans and shellfish Use their thick lips to create suction and pull prey from holes & crevices Has “barbels” (“whiskers”) front edge of each nostril Barbels used to find food on the ocean bottom. Caudal fin with no distinct lower lobe Will bite if provoked but otherwise relatively harmless Large eggs in horny capsules To 4.3 m (14 ft) length but usually less than 3.1 m (10 ft)
52 8.2 OCEAN LIFE 11. SAND TIGER – ODONTASPIS TAURUS Sand Tiger Family Greyish brown or tan above with dark spots especially towards the tail Paler below All 5 gill slits in front of pectoral fins Teeth long and curved, pointed, non-serrated – feeds on fish Sluggish species Not known to attack man, in spite of ferocious appearance 1 or 2 young retained in oviducts; feed on eggs produced by mother To 3.2 m (10.5 ft) length 12. THRESHER SHARK – ALOPIAS VULPINAS Thresher Shark Family Oceanic deep-water sharks Extremely long upper lobe in caudal fin (often exceeds length of body) Pointed snout 5 gills slits in front of each pectoral fin Tail used to herd and stun prey Often occur in large numbers Considered dangerous especially during maritime disasters Feeds on fishes and squids Corral schools of fish using the long upper lobe of their tails To 6.1 m (20 ft) length and 454 kg (1,000 lbs)
53 8.2 OCEAN LIFE 13. SIXGILL SHARK – HEXANCHUS GRISEUS Hexanchidae Family Six gill slits Long caudal fin Six large trapezoidal teeth on each side of lower jaw Coffee-coloured to brown or greyish on back, paler below Mostly in deep water Spiracle present Live bearing To 4.9 m (16 ft) length and 590 kg (1,300 lbs) 14. HORN SHARK – HETERODONTUS FRANCISCI Horn Shark Family Tan to dark brown with black spots above; pale yellowish below Snout short and blunt Ridge above eye 2 dorsal fins each with a spine Sluggish, solitary, active at night Eats crabs and small fishes Lays large (to 5”) eggs – eggs have spiral flanges To 0.9 m (3 ft) length
54 8.2 OCEAN LIFE 15. WHITE-SPOTTED BAMBOO SHARK – CHILOSCYLLIUM PLAGIOSUM Bamboo Shark Family Nostrils sub-terminal on snout Caudal fin with a pronounced notch but without a ventral lobe Transverse dark bands and numerous white or bluish spots Oviparous A common but little-known inshore bottom shark Utilized for human consumption and used in Chinese medicine To 0.8 m (2.5 ft) length 16. LEOPARD SHARK – TRIAKIS SEMIFASCIATA Smoothhound Shark Family Broad black bars, saddles and spots Snout short, bluntly rounded Feed mainly on crabs, shrimps, bony fish – large variety of food in diet Strong-swimming, nomadic Ovoviviparous with 4-29 in a litter Good for human consumption To 2.1 m (7 ft) length
55 8.2 OCEAN LIFE RAY SPECIES 1. ATLANTIC MANTA – MANTA BIROSTRIS Devil Ray Family Dark brown to black above, white below Pectoral fins long and pointed Two large cephalic (head) fins – look like “horns” (hence “devil” rays) Mouth wide, terminal Tail whip-like but short and no spine Oceanic Feeds on plankton and small fishes To 6.7 m (22 ft) across wings and 1,814 kg (4,000 lbs) 2. SPOTTED EAGLE RAY – AETOBATUS NARINARI Eagle Ray Family Disk dark grey to brown above with a pattern of white spots and streaks Whitish below Long graceful wings (pectoral fins) Long whip-like tail with a long spine near base Frequently seen in large schools during non-breeding season To 2.4 m (8 ft) across wings and 227 kg (500 lbs)
56 8.2 OCEAN LIFE 3. COWNOSE RAY – RHINOPTERA BONASUS Eagle Ray Family Dark brown to olive above with no spots or marks Snout squarish with an indentation in the centre Free-swimming ray that swims by flapping its pectoral fins Eats molluscs Whip-like tail with venomous spine (note: spine venomous, not the tail itself) Oceanic; sometimes jumps out of the water and slaps back down Several theories as to why this occurs; e.g. parasite removal but probably more to do with territorial display Frequently gathers in large schools Bears live young (like all eagle rays) To 0.9 m (3 ft) across disk 4. SOUTHERN STINGRAY – DASYATIS AMERICANA Stingray Family Disk almost perfect rhombus Most stingrays are bottom-dwellers Often lie submerged in sand except for the eyes Whip-like tail with venomous spine (note: spine venomous, not the tail itself) Generally stingrays will not bite, sting or hurt you although you may be stung if you inadvertently step on one Bears live young (like all stingrays) To 1.5 m (5 ft) across disk
57 8.2 OCEAN LIFE 5. ROUGHTAIL STINGRAY – DASYATIS CENTROURA Stingray Family Disk with numerous scattered spines Tail with numerous rows of small spines One of the largest stingrays Similar to Southern Stingray To 2.1 m (7 ft) across disk and 4.3 m (14 ft) long 6. YELLOW STINGRAY – UROLOPHUS JAMAICENSIS Stingray Family Disk almost round Tail stout with spine placed back Disk yellowish with dark spots Common ray from Florida to northern South America To 0.4 m (14 in) across disk and 0.7 m (26 in) long but usually much smaller
58 8.2 OCEAN LIFE 7. WINTER SKATE – RAJA OCELLATA Skate Family Disk broad, rhombic-shaped and spotted Tail thick – not whip-like – tail never has a large spine or “sting” Usually 1-4 ocelli on each side of disk At least 72 rows of teeth (usually > 80) in upper jaw Bottom-dweller Good for human consumption To 1.1 m (43 in) 8. ATLANTIC GUITARFISH – RHINIOBATOS LENTIGINOUS Guitarfish Family Pectoral fins joined in front to head Spiracles large Gill slits and mouth on underside Long triangular disk; thick, tapered body Brownish above usually with many small white spots below To 0.8 m (30 in)
59 8.2 OCEAN LIFE 9. SMALLTOOTH SAWFISH – PRISTIS PECTINATA Sawfish Family Pectoral fins joined in front to head Spiracles large Gill slits and mouth on underside “Saw” is about ¼ length of the fish 24 or more teeth on each side of “saw” Caudal fin has no distinct lower lobe Not aggressive and no threat to man unless caught and handled Saws are dried and sold as souvenirs Moves its head from side to side and scythes/strikes prey with its long rostrum May also use the front of its snout to dig for prey buried under sand To 5.5 m (18 ft) 10. ATLANTIC TORPEDO – TORPEDO NOBILIANA Electric Ray Family Disk round without spines Chocolate to dark grey above without spots, whitish below Produce 200V but not generally aggressive Kidney shaped electric organ found on each side of the head Sluggish bottom-dwellers Feeds on other bottom-dwellers including flounders and small sharks To 1.8 m (6 ft) and 91 kg (200 lbs)
60 8.2 OCEAN LIFE REFERENCES & FURTHER READING http://www.elasmodiver.com - Sharks, rays and chimaeras http://www.seaworld.org/animal-info/info-books/sharks-&-rays/index.htm - Sharks and rays Robins, C.R. et al., A Field Guide to Atlantic Coast Fishes of North America, Houghton Mifflin Pub. (1986)
62 8.3 ACTIVITIES 8.3.1 Sharks CORE ACTIVITY (a) Colour the great white shark slate grey above and white below, and add the following labels to your picture: Dorsal Fin Caudal Fin (Tail) Pectoral Fin Gill Slits Jaws Snout Eye
63 8.3 ACTIVITIES (b) Name several dangerous sharks (c) What is the world’s largest shark? What does it eat? (d) How many senses does a shark have and what are they? How many senses does a human have?
64 8.3 ACTIVITIES ANSWERS (a) Colour the great white shark slate grey above and white below, and add the following labels to your picture: Dorsal Fin Caudal Fin (Tail) Pectoral Fin Gill Slits Jaws Snout Eye
65 8.3 ACTIVITIES (b) Name several dangerous sharks The great white, tiger, and bull sharks are known to be dangerous sharks (c) What is the world’s largest shark? What does it eat? The world’s largest shark (and fish) is the whale shark. It primarily eats plankton but it will also eat fishes and squid. (d) How many senses does a shark have and what are they? How many senses does a human have? A shark has 6 senses Sight Smell Touch Hearing Taste Electrical A human has 5 senses; we have all the senses a shark has except the electrical sense.
66 8.3 ACTIVITIES 8.3.2 Rays CORE ACTIVITY (a) Colour the Southern stingray dark grey, and add the following labels to your picture: Spines Tail Sting (barb) Eye Pectoral Fin Rhomboid Disk (b) What is the world’s largest ray? What does it eat?
67 8.3 ACTIVITIES ANSWERS (a) Colour the Southern stingray dark grey, and add the following labels to your picture: Spines Tail Sting (barb) Eye Pectoral Fin Rhomboid Disk (b) What is the world’s largest ray? What does it eat? The world’s largest ray is the manta ray. It eats plankton and small fishes.