WHEN SHARKS GET OLDER

Different species of shark reproduce in different ways; some lay eggs and others give birth to their pups (babies) alive.

Pups are born ready to swim and hunt. They have a full set of razor-sharp teeth from ‘day one’.

A baby shark and an egg case

Young sharks grow as fast as they can because the bigger you are, the safer you are. Lots of larger animals eat baby sharks. Predators include elephant seals, giant squid, big reef fish, and other sharks.

Sharks never stop growing throughout their lives. Bigger sharks do not grow as quickly as they did when they were young. Different species of sharks grow at different rates.

7 m and 2500 kg Great White wiki

Most sharks grow faster in summer than in winter and the change in growth rate leaves a mark in their vertebrae. Age can be determined by counting these rings like counting the growth rings in a tree.

Growth rings can be seen clearly using x-rays

Some sharks may not mature and start producing young until they are 30-40 years old.

Sharks keep growing new teeth throughout their life so they never run out of sharp eating equipment. Some sharks may grow 30,000 new teeth during their lifetime.

New teeth waiting to replace old ones.

Zoology UBC

There are about 440 species of sharks in the world but we only know how long some of them can live.

KIND OF SHARK	ESTIMATED MAXIMUM AGE
Great White Shark	Up to 100 years
Tiger Shark	Longer than 12 and perhaps 30 years
Nurse Shark	About 25 years
Bull Shark	About 25 years
Spiny Dogfish	Possibly up to 100 years
Basking Shark	Longer than 20 and perhaps 50 years
Whale Shark	From 70 to 100 years
Short-finned Mako Shark	From 15 to 25 years
White-tip Reef Shark	About 30 years

Record breaking tiger shark caught as a trophy

Diving with sharks on assignment

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WHAT DO TIGER SHARKS EAT?

Tiger Sharks have a bad reputation and are second only to the Great White Sharks in attacking humans.

Tiger sharks teeth are serrated with powerful jaws making them an ideal killing machine. They are able to bite through the shells of sea turtles.

Tiger Sharks will eat almost anything but dolphins are a favorite food. Studies have shown that 70% of wild dolphins have scars from shark bites (mostly as calves).

In sand tiger sharks, the biggest, strongest pups eat their brothers and sisters while still inside their mother’s body. This seems terrible but guarantees the birth of the most aggressive young.

A Tiger Shark’s teeth are not attached to the jaw, but grow in their flesh. In order to keep the biting equipment in order, teeth are constantly replaced throughout the shark's life.

In 1935, an Australian Tiger Shark spat out a human arm. Police discovered that the shark had not attacked anyone but had eaten the arm after a murder victim had been cut up and thrown into sea.

 


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DOLPHINS AND ECHOLOCATION

When the mammalian ancestors of whales began to move from land and back into the sea they were faced with huge problems. A major difficulty was that they had lost the evolutionary advantages necessary for a successful aquatic life; underwater vision was difficult, they were poor swimmers, and they were easy prey for the huge sharks that inhabited the ocean from 50-75 million years ago. In fact, sharks may have been their greatest problem since sharks had already perfected incredible senses of smell, sound detection, and the ability to receive the invisible electrical fields produced by other animals. Not only were the sharks more efficient fishermen but large sharks could detect, out-swim, and then capture ancient whales.

Many whales solved this problem with the help of evolution by becoming so large that even large sharks couldn’t eat them. They also stopped competing with sharks for food by changing to a plankton diet. Their descendants are the baleen whales of today: examples are the Right Whale, Gray Whale, and Humpback Whale.

Other whales evolved to compete with the sharks. They retained their teeth and fish diet but became faster and developed an incredible sense of echolocation which allowed them to ‘hear’ the approach of huge sharks long before they could be seen.

Because sound travels through water better than light does, the ability to make a sound and then interpret the ‘meaning’ of its echo, allowed the toothed whales to find food when sharks couldn’t and avoid predators before they could get too close. Whale echolocation today may be the most sophisticated sensory system in the entire animal kingdom. Unlike our own vision, echolocation for whales carries three dimensional information. Toothed whales can ‘see’ inside and through many objects and reflected sounds seem to allow them to ‘see’ around or behind things.

The information that can come back by echo depends on the frequency of the sound. Low frequency travels long distances and has less detail while high frequency is shorter range with high definition. To get the whole picture some whales ‘sweep’ the frequency range between high and low frequencies. When they home in on prey these ‘sweeps’ sound like a continuous creaking sound.

When humans send out radio signals for locating things (radar), we focus the signal into a beam using a specially shaped antenna. Toothed whales do the same with sound using special fat deposits in the top of their heads and in their jaws. This fat is different from the other fat deposits in the whale’s body and fits into specially shaped areas in the jaw and skull. We use facial muscles to frown or smile but whales seem to use their muscles to adjust the shape of the fat deposit and focus the sound beam.

These special fat deposits are most remarkable in the sperm whale where they may weigh several tonnes. In dolphins the deposit looks like a rounded lobe on the front of the head and is called the melon.

The frequency of toothed whale sounds ranges from 40 Hz to 325 kHz. A list of typical sound levels is shown in the table below (from Wikipedia). A level of 120dB causes hearing damage and pain in humans.

Kind of Whale	Sound	Broadband  level (dB)
Sperm whale	clicks	163–223
Beluga whale	echolocation clicks	206–225
White-beaked dolphin	echolocation clicks	194–219
Spinner dolphin	pulse bursts	108–115
Bottlenose dolphin	whistles	125–173

We still don’t know much about all this whale engineering but it seems to work like this:

• A powerful sound is generated by ‘vocal cords’ (phonic lips) within the whale
• As the sound radiates out, the melon focuses it like a lens focuses a beam of light
• The sound beam hits an object in the sea and is reflected back
• The teeth in the lower jaw act as ‘antennas’ collecting the echoes
• Fat deposits in the lower jaw carry the sound to the inner ears
• The complex brain interprets the echoes and constructs a ‘picture’ of the object

In this way a whale may see a picture with sound similar to the picture we see with light. It is clear though that the picture is only as good as the information processor that untangles the complex echoes coming back to the whale. To see with sound, whales have also evolved very large brains.

Many people believe that the large brain means that dolphins in particular are the philosophers of the sea. The truth is that a large part of that brain seems to be used for processing and remembering echo information necessary for feeding and navigation and relating to other whales with their own acoustic information.

The ancestors of whales had ears for hearing on land. Whales still have ear openings that lead to the inner ear. In baleen whales these openings are filled with a hard wax but in the toothed whales the hole is open. There is evidence that dolphins can hear in air and water with these openings but they probably play a very small part in echolocation.

The baleen whales are not hunters like the toothed whales and have not developed the elaborate clicks and whistles to ‘sweep’ their prey. It has been suggested that baleen whales may use their stored echo-soundings to create complex three-dimensional maps of the ocean floor thousands of meters below for use in their long annual migrations covering tens of thousands of kilometers.

Illustrations are from Wiki-commons.

SWIMMING WITH TIGER SHARKS

Mature tiger sharks can live 45-50 years and reach 6-7.5 metres long. They can weigh more than 900 kilograms.

Tiger Sharks get their name from the vertical stripes found on their bodies. These markings look like ripples of sunlight moving through the water and help them "blend in".

Photographing Tigers on assignment.

Most Tiger Sharks seem to be nomadic and cover long distances. One shark tagged in Hawaii was recaptured off the coast of South Africa. Others stay within a smaller range and may patrol a beach or bay.

Keep low and pay attention.

A Tiger Shark’s teeth make them dangerous to handle. One trick Reef Ninja uses is to roll them on to their backs. They seem to go to sleep (this is called tonic immobility).

Tiger Sharks can be unpredictable and divers must be very cautious. It is easy to believe that the sharks like our company and that they can be trusted.

Mature sharks have less pronounced strips compared to juveniles.

More people are struck by lightning than killed by sharks. You should be scuba diving instead of standing in the rain!

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THE INCREDIBLE GLASS SQUID

One small group of very strange squid prowls the mid-depths where light levels are very weak and being spotted by sea lions or toothed whales means a certain death. To avoid detection the glass squids ('cranch' squids) have learned to become invisible.

Most of the 60 species are transparent giving them their name. Some even have light organs to cancel out shadows cast by their eyes. Their digestive glands are visible but usually held vertically to cast the smallest shadow.

Some eyes are small and circular but a few species have eyes like ‘telescopes’ that can be extended away from the body. Scientists have no explanation for the telescopic eyes.

Most glass squid are small with rounded bodies but one species, the Colossal Squid, is the largest of all squid species reaching 14 metres in length and 500 kg.

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WEIRD SEAHORSE BIOLOGY

Seahorses are fish. But, very strange fish. Unfortunately, seahorses are disappearing from our oceans.

The Leafy Sea Dragon from South Australia.

Seahorses have no stomach. Food passes through their digestive systems so quickly, they must eat almost continuously to stay alive.

Seahorses eat at least 3,000 or more small shrimp and other plankton each day.

 They propel themselves by using a small fin on their back that flutters up to 35 times per second. Even smaller pectoral fins located near the back of the head are used for steering.

When they aren't swimming, they hang onto seaweed and coral with their prehensile tail. 

Males form territories of about one square meter. Females range over about one hundred times that area.

They use their long snouts and toothless mouths to suck in plankton and small crustaceans that swim by (like sucking up grains of rice through a straw).

Seahorses do not have scales; instead their skin covers a set of bony plates arranged in rings around their bodies. Each species has a distinct number of rings.

The smallest seahorse is called Denise's seahorse. It's only about a centimeter long.

The earliest known seahorse fossils date back about 13 million years.

Thanks to Dave Bretherton for the excellent seahorse photo.

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WHALE SONGS ACROSS THE OCEAN

 

This is the first in a series of marine ecology articles aimed at  teens and produced as a set of the most often asked questions together with answers.

Q: What is a whale song?

A: "Song" refers to the pattern of regular and repeated sounds made by some species of whales, especially the Humpback Whale. Surprisingly, these repeated sounds show a similarity to the repeated sounds used in human song.

 

Q: Do whales make sounds the same way we do?

A: Not quite. They do use the air in their lungs but don’t have vocal cords like ours. In dolphins, sound is made by the phonic lip membranes in the top of the head (below the blow hole) which vibrate the surrounding tissue. The rate of these vibrations and therefore the frequency of the sound can be controlled with muscles. 

 

Q: Can humans hear whale songs?

A: Humans hear sounds between about 20 and 20,000 Hz (the larger the number the higher the pitch). The frequency of toothed whale sounds ranges from 40 Hz to 325,000 Hz so we can hear their lower frequencies but not their very high pitched ‘squeaks’. The frequency of baleen whale sounds ranges from 10 Hz to 31,000 Hz so many of their lower frequency songs are sped up electronically so that we can hear them.

Q: Do all whales sing?

A: Most whales and dolphins produce sounds of varying degrees of complexity but nothing like the elaborate songs of the Humpback Whale. Another singing ‘star’ is the Beluga; sometimes called the "sea canary", because it produces such a variety of whistles, clicks, and pulses.

Q: Which whales are the best singers?

A: Two groups of whales, the Humpback Whale and the Blue Whale from the Indian Ocean, are said to produce the most complex series of repeated sounds at varying frequencies. The Humpback Whale’s song has been described (biologist Philip Clapham) as "probably the most complex in the animal kingdom".

Q: Why do whales sing?

A: Scientists believe that the songs (mostly seasonal) of the Humpback whale (and some blue whales) are used in finding a mate. The simpler sounds made by other whales have a year-round use. Toothed whales use echolocation to ‘see’ the size and nature of underwater objects, to catch fish, and probably to keep in touch with others in their pod.

Q: How long does a Humpback’s song last?

A: The whale can often repeat his ‘personal’ song, which last up to 30 minutes or more, over and over again for several hours or even several days. Whales that live in the same region (which can be as large as entire ocean basins) usually sing similar sounding songs. Whales from different regions sing quite different songs.

Q: How do scientists study whale songs?

A: The military developed underwater microphones to listen for and track enemy submarines. Scientists can also use these microphones (called hydrophones) to locate and track whale noises. Their equipment allows them to calculate how far through the ocean a sound travels. 

Q: How loud is a whale’s voice?

A: Humans can listen to loud sounds up to about 120dB (decibels) before they feel pain and damage their ears. Some whales make sounds up to about 200-220 dB which is about a billion times more powerful than your loudest shout. These very loud low frequency sounds can travel for thousands of kilometers through the water.

 

Q: Can humans mess up whale songs?

A: It looks like we can. Marine biologist Dr. C. Clark says that the amount of noise from ships doubles every 10 years making it harder and harder for whales to communicate, navigate, or find their food. Humans need to use ships to transport goods across the ocean but it may be time we tried to make them quieter for the sake of the whales.

 

Q: What is the longest distance a whale’s song can travel?

A: The Voyager Golden Records are recordings put in the two Voyager spacecraft launched in 1977. The recording is meant for any intelligent extraterrestrial life that might find the Voyager in the future. Without visiting us, extraterrestrials will hear our Earth sounds, which include whale songs. In about 40,000 years those songs will have traveled to a star in the Ophiuchus constellation. That whale song might one day be heard 15,000 trillion kilometers from where it was sung. Wouldn’t it be a tragedy if it is the only whale song left in the universe when it gets there?

Thanks to photographers at Wiki for their wonderful work.

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