Friday, August 7, 2009

Cartilaginous Fishes (Sharks & Rays)

Sharks and their relatives differ from other types of fishes in that they don't have any true bone in their bodies. Their skeleton is made of cartilage. Their teeth are the hardest part of their body and they grow continually all their lives, always with a new one waiting to replace any that are wrenched out when they feed. Some sharks have very sharp teeth while other members of this family have teeth that are flattened plates more suited to crushing. The type of teeth they have depends on their diet. I find them really interesting because not only do they have teeth in their mouths, their whole body is covered with denticles (tiny teeth) which makes their skin really tough and abrasive. If just about any shark rubs against your skin it will take the skin off as though it was being rubbed with sandpaper, and can be used as such.
Although when most of us think about sharks we think of White pointers, Makos and Tiger sharks there are many more types of sharks and rays with diverse appearance, habitat and diets. Very few of them are considered to be dangerous to people if left alone. Many members of this family, especially those without the really sharp teeth, have a venomous barb/barbs which are used solely for defence.

These two photos were taken at an Australian territory - Christmas Island, in the Indian Ocean. The fish in these images is a whale shark, the biggest fish in the oceans. The can easily grow to 45 feet and they are totally harmless. This biggest fish in the world feeds on some of the smallest marine creatures- plankton, krill and other tiny creatures. It swims with its mouth open, filtering out everything edible. We were approached by two of these amazing fish while we were diving at Christmas Island. They swam between us and even though we were dwarfed by them they moved around us slowly and gracefully, close enough to touch but with out flicking us with their tails or huge fins.

This is a close up of a Port Jackson shark. The denticles on its skin are the little raised cross shaped light areas. Their teeth have evolved into flat plate like structures to accommodate their diet of crustaceans and bivalve molluscs (crabs and mussels) which they crush.

This is a white tipped reef shark taken at Heron Island. It is asleep in a gully. Some sharks need to keep moving so that the water is continuously washing over their gills supplying them with oxygen. This is not the case with reef sharks as well as many others.

The above photo and below are images of black tipped reef sharks. I took them at Heron Island during the season that turtles hatch on the beach and make their way to their ocean. These sharks are in about 300mm or 1 foot of water and they are waiting for any turtle hatchlings to arrive. If a lot of hatchlings all make it to the water at once some of them have a chance at getting through all the sharks to deeper water. It sounds harsh but each turtle lays many eggs each season that they mate. They have long lives and each female only really needs for two of her offspring to make it to adulthood to keep turtle numbers stable.

Black-tipped Reef shark, Qld.

Eastern Sawshark, Vic. These sharks are found in NSW and eastern part of Victoria. They are found in deep water from 100 m to over 600m so are not seen in their natural habitat by divers. They give birth to live young and the teeth are already present. Fortunately the teeth fold back during the birth process so that the mother is not injured.

Elephant Fish- Victoria. They are called this because of their nose. They are shark like in appearance but they have smooth skin over most of their body instead of the rough teeth like scales that other sharks and rays have. The markings on the top of the head are part of its sensory system.

These sharks are bottom dwellers and use their nose to find molluscs and other invertebrates in the sand.

Elephant Fish, Vic. It is part of a group of fish known as Ghost Sharks and this is the only species found in southern waters. It is found in sandy areas like estuaries and inshore areas and also in deeper water up to 200 meters.

Banjo Shark or Fiddler Ray - so named as their body shape resembles that of a banjo. Vic.

This is the underside of a Banjo shark. The gill slits and the mouth are easily visible. The small nostril like holes are part of their sensory system called spiracles and lead into the mouth and gills. Vic.

Blue-spotted Stingray, NSW. This is a tropical species that is sometimes found as far south as Sydney, NSW.

This is an Eagle Ray (above and below) Vic.

This is the underside of a large stingaree. The two nostril like holes are spiracles and lead into the mouth and gills. The mouth is the small slit behind them and the gills are obvious as well. This is a male - Only males have the modified fins at the base of their wing disc obvious in the lower left of the photo.

Manta Ray, Heron Island, Qld. The flaps at the side of the mouth are used to funnel the water (full of the tiny invertebrates that they eat) into their mouths when they are feeding. They are big, graceful and harmless to everything bigger than micro organisms. Sometimes they are in groups that swim in a spiraling manner upwards and then come back down and repeat it. As a diver it leaves you with a real feeling of being privileged to see such beautiful animals in their natural environment. It is easy to get so engrossed that you stop thinking about about how much air you have in your tank and some divers have been known to suck their tanks almost dry before they leave for the surface. They have a wing span of 2 - 3 meters and they seem to fly through the water rather than swim. They live in tropical waters in many parts of the world.

Leopard Shark, Heron Island, Qld.

These two photos of leopard sharks (above and below) were taken at the Melbourne Aquarium.

Some sharks give birth to live young while others lay eggs. These three images are from sharks found in Victorian waters. This egg case is from a Draughtboard shark.

Elephant shark egg case. Vic. These sharks take six - eight months to hatch.

This is an egg case from a Port Jackson Shark. They lay their eggs in rocky crevices and the screw shape keeps them wedged in place until they hatch about twelve months later. It takes ten years to reach maturity but even fully grown they are much less than a meter long.

Port Jackson Shark - above and below

Although these sharks are placid and not a threat to people, they have two sharp venomous barbs placed at the front of each dorsal fin that can inflict injury if the creature is handled incorrectly. They are visible in the above pictures.

Port Jackson Shark -close up.

Varied Catshark . These are a bottom dwelling shark that spends its days hidden in crevices, becoming active at night. The barbels hanging from the corner of its mouth help it find food on the sea floor. It belongs to a family known as the Collared Catsharks. It uses its barbels (the things hanging from the corner of its mouth) as sensory organs to help it find small bottom dwelling invertebrates on which it feeds.

This shark is found in NSW and further north in warmer water. It is a Blind shark.

This is a shark egg case. They are often washed up on the beach with seaweed, usually empty. The reason it looks so much like seaweed is for camouflage as these eggs are laid in amongst it.

This is a draughtboard shark. They are another species of egg laying sharks common in Victorian waters. They are also known as swell sharks because they can inflate their stomachs!

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