Sharks can be found throughout the world. From the giants like the Whale Shark, the Basking Shark, and the infamous White Shark to the smallest like the Dwarf Lantern Shark and the Dogfish are certainly recognized and often feared in oceans around the globe.
But at least one species of shark is not limited to marine habitats. Often found in freshwater estuaries, rivers and even lakes – the Bull Shark is rather unique among the planets top predatory fish.
A stocky heavy-bodied gray shark with a short bluntly-rounded snout. Female Bull Sharks may reach lengths of up to 11 feet long. Sexually dimorphic – females may weigh up to 700 pounds and grow much larger than male Bull Sharks, which grow to weigh just over 200 pounds and measure 7 and half feet long.
Juvenile Bull Sharks are usually brownish-gray in color but develop a more gray topside and lighter belly as the mature. This counter-shading coloration is common among many aquatic predators.
Bull sharks upper teeth are broad, jagged and triangular while the lower teeth are slender, pointed and edged with fine serrations. Bull Sharks may have up to 350 teeth in their mouth at one time, comprised of 50 rows of teeth with 7 teeth in each row.
Bull sharks are typically solitary animals, only coming together to mate. Offspring are usually born in the spring or summer, except in warm climates where young may be born year-round. Bull sharks become mature between 14 and 18 years of age. They are believed to live up to 30 years in the wild.
Bull Sharks are classified among the requiem sharks, which includes other species like Lemon Sharks, Blue Sharks and the Tiger Shark.
Bull Sharks, unlike nearly all other shark species have the ability to live and thrive in freshwater. The may even give birth in freshwater habitats. While Bull Sharks are generally found along coastal waters, it is not uncommon for them to venture up freshwater rivers.
Bull Sharks have been found as far north up the Mississippi River as the state of Illinois and more than 2000 miles up the Amazon River in South America.
Bull Sharks are also found in freshwater lakes and rivers in Africa where they are known as Zambezi Sharks – where they have even been known to attack hippos. Bull Sharks also inhabit Lake Nicarauga in Central America, known there as Lake Nicaragua Sharks – once believed to be a separate species of shark.
Perhaps one of the most unique freshwater populations of Bull Sharks can be found in the 52 acre lake on the Carbrook Golf Club in Queensland Australia – originally 6 sharks entered the lake during extreme flooding in 1996 – it is believed there may be as many as 12 present today.
Freshwater is typically considered toxic to most sharks since they require salt in their body to survive. Bull Sharks possess the ability to live in this rather abnormal habitat due to their kidney’s unique ability to recycle the salt within their bodies and special glands, located near their tails, also aid in salt retention. Bull Sharks can also survive in water with higher levels of salinity than normal ocean water as well.
They feed primarily on bony fish such as mullet, tarpon, gar, mackerel and smaller sharks. They are also known to feed on stingrays, sea turtles and sometimes dolphins.
Bull Sharks, like their relative the Tiger Shark are considered highly aggressive animals. Due to their tendency to live in shallow coastal waters, as well as many freshwater lakes and rivers, they often come in contact with people.
This frequent interaction with people in their native habitats, along with their aggressive nature, have led many to often describe the Bull Shark as the most dangerous shark in the world. Many documented shark attacks around the world are often attributed to the Bull Shark.
In the early 1900s off the coast of New Jersay a series of 5 shark attacks occured over 12 days – 4 of these attacks resulted in fatalities. Several attacks happened in shallow water and tidal rivers and over 15 miles from open ocean. This incident inspired the Peter Benchley novel “Jaws” which brought great attention and fear to the White Shark.
Despite a White Shark being caught shortly after the New Jersey attacks of 1916 – a 9 foot Bull Shark was also caught. Many factors and some evidence have led many to claim that these attacks were probably made by the aggressive and powerful Bull Sharks and not the jaws of the infamous White Shark.
Widely known as the “sharksucker” or “sucker fish” – the Remora is a rather odd looking marine fish known for attaching to other larger marine animals such as whales, manta rays, sea turtles and of course sharks.
Remoras are thin, elongated, rather dark fishes growing from 1 to 3 feet long, usually 11 or 12 times as long as it is wide. The lower jaw projects forward beyond the upper jaw. The Remora is most often found in the warmer parts of all oceans.
Remoras have no swim bladder but they require a swift passage of water over the gills and cannot survive in still waters. Therefore they most commonly attach to another ocean going animal as a means of easy transportation and meals. Remoras feed on food scraps as well as small parasites on the host animal’s skins, and in the case of sharks – sometimes their gills and mouths.
Remoras adhere by means of a flat, oval sucking disk on top of the head. This appendage contains a variable number of paired, sharp bony plates.
Many scientists have suggested the sucker disk was derived from the dorsal fin but in recent years a study conducted by David Johnson of the Smithsonian’s National Museum of Natural History and Ralf Britz of the Natural History Museum, London, have actually shown that the Remora’s sucker disc is in fact the Remora’s dorsal fin. By studying the development of the fish from the earliest larval stages to adulthood they discovered sucker disc in in fact the Remora’s modified dorsal fin.
Despite their normal mode of transportation, hitching a ride on other animals, the Remora is not considered to be a parasite. Instead they are considered to have a commensal relationship with their host, since they do not hurt the host. It has been suggested that the relationship is symbiotic since the Remora can obtain its food acting as a cleaner fish and removing parasites from the host, thus benefiting both. Some small Remora specimens have even been found clinging to the roof on the inside of a sharks mouths.
It is not known whether the sharks tolerate the Remora’s presence or simple have no means to remove them but some aquarists who have worked with sharks and remoras in managed care have mentioned that some Remoras due seem to be a considered a nuisance by some sharks. However, Remoras have no known natural predators and very few have ever been discovered in the stomach of a shark.
Spawning occurs during the spring and summer months throughout most of its range and during the autumn months in the Mediterranean Sea. When the embryos hatch, the young fish have a large yolk sac, non-pigmented eyes, and an incompletely developed body. During development of the newly hatched fish, the sucking disc begins to form.
Juvenile Remora sometimes act as cleaning fish, setting up cleaning stations where they clean parasites off parrotfishes. Remoras live freely for about one year until reaching 1 to 2 inches in length at which time they attach to a host animal.
Although considered harmless to humans, there have been some reports on Remoras following divers and attaching to divers’ legs. This can be extremely painful as the sucking disc is lined with numerous sharp ridges.
In ancient Greece, sailors feared that Remoras had mysterious magical powers and could slow down or stop their ships. In fact, the scientific name for the species means “to hold a ship.”
There are eight species of Remora found worldwide. The common Remora also known as the “sharksucker” but some remora species appear to specialize in specific hosts. The white suckerfish seems to prefers manta rays, the spearfish remora is often found on swordfishes and other billfishes and the marlin sucker is most common on marlin.
The species – Remora australis – is often known as the “Whalesucker” because it almost exclusively attaches to whales, dolphins and porpoises. Despite these apparent preferences, Remoras will attach to nearly any host when the need or opportunity arises.
While they measure only a few inches to a foot long, the Sea Dragon may be just as fascinating and mysterious as the dragons of legend and myth. But unlike the sea dragons of maritime folklore, these unique bony fish are rather peaceful, quite delicate and pose no threat to humans.
Closely related to Sea Horses and Pipefish, there are just three known species of marine fish known as Sea Dragons. The Leafy Sea Dragon, the Weedy, or Common, Sea Dragon, and the newly discovered Ruby Sea Dragon.
All three species are found exclusively off the coast of Southern Australia. They are typically found in shallow coastal waters, where they seem to prefer sea grasses, rocky kelp forests, and seaweed beds. Despite being found in common waters, Leafy and Weedy Sea Dragons are rarely found together.
Leafy Sea Dragons resemble floating pieces of seaweed with leaf-like appendages flowing along the length of their body. This appearance making them difficult to spot in their natural habitat. They are relatively large compared to other seahorse species, reaching lengths of up to one foot long. Leafy Sea Dragons are generally orange to yellow in body color with olive-tinted appendages.
Weedy Sea Dragons have long, slender reddish-brown bodies comprised of bony plates with yellow and purple accents, and several weed-like appendages used for camouflage. Small pectoral fins on their neck are used for body positioning, while long, shallow dorsal fins are their sole means of propulsion. Males are narrower and darker than females, and both have several short spines used for protection. Their snouts are elongated, and resemble other seahorses. Unlike other seahorses though, their tails are not prehensile.
The Ruby Sea Dragon was first described in 2015. Genetic studies from the Scripps Institution of Oceanography at UC San Diego have shown this to be a distinct species with a bright shade of red colored skin – distinctly different from the orange tint in Leafy Sea Dragons and the yellow and purple hues of Common Sea Dragons. Despite only being recently recognized as a unique Sea Dragon – a previous specimen had existed in the Western Australian Museum, unidentified for nearly 100 years.
Researchers believe the animal’s coloring suggests it inhabits deeper waters than the Leafy and Common Sea Dragons, as the red shading would be absorbed at depth and effectively serve as camouflage.
Sea Dragons breed once a year in the early Summer months. Like seahorses, Male Sea Dragons care for the fertilized eggs. However, they do not have a specialized pouch like male seahorses. Instead, the female deposits the eggs in the skin of the male, who will carry the eggs under the tail on a brood patch. The skin is soft at the time the eggs are first embedded but becomes hardened to form a cup around each of the 250 to 300 eggs. Each egg receives oxygen from the cup’s blood-red tissue.
At first the eggs are bright pink and darken in color as they develop. It may take several hours, even days for all of the eggs to hatch. Newly hatched Sea Dragons receive no further parental care. They reach sexual maturity in approximately two years.
Sea Dragons do not have teeth or stomachs. Due to the lack of a stomach, the feed almost continuously. Their primary food consists of shrimp, plankton and other small crustaceans. They use their long, thin, tubular snout to create a strong suction to rapidly suck in the food item. Special muscles in the snout can widen to accommodate different size prey.
Although they have no specific natural predators, Sea Dragons are under increasing pressure from human-related causes. They are taken as incidental by-catch by fisheries. One of their biggest threats is pollution—their sea grass homes are threatened by runoff of fertilizer and waste water. They have been collected for the pet trade and Chinese traditional medicine.
Sea Dragons are fully protected under Australia’s local, state and federal legislation. Special licenses are required to collect or export them. Very few facilities have bred Sea Dragons in managed care – the Aquarium of the Pacific in Long Beach, California was the first accredited facility in the world to successfully breed Weedy Sea Dragons. Researchers have learned that individual Sea Dragons can be identified by facial patterns.
With ongoing studies in the wild, cooperation among facilities to better understand the reproduction and care of these creatures and by protecting their natural habitats it may be possible to prevent these real-life dragons from becoming nothing more than a myth.
Special thanks to the following organizations for their education, research and conservation programs that provided information for this episode:
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