As nocturnal organisms, horn sharks spend most of their day resting on a rocky bottom, in a kelp bed, or in the deep crevices of a cave or cavern. (Source) Due to their rock-like colors, the horn shark can remain in the same spot all day, blended in with its surroundings and safe from predators. (Source) Scientists believe that each matured horn shark returns to the same resting spot day after day.
While a horn shark may only be 15-17 cm when born, it can grow up to 3 meters by the time it has fully matured. (Source) This is still comparatively smaller than other sharks in the ocean and, because of that, many organisms—such as large fish or sharks—can be a threat to the horn shark. Horn sharks are known for their square box-like heads, two dorsal fins, a pectoral fin, and an anal fin. (Source) Of these distinguishing factors, the most well known features are the dorsal fins.
Each dorsal fin contains a large spine that is extends several centimeters above the shark’s body. Because the teeth of the horn shark are not sharp enough to penetrate and seriously hurt a predator, the spines located on the dorsal fins become the most important defensive mechanism of the horn shark. When a predator attempts to eat a horn shark, the shark stabs the inside of the predator’s mouth with its spines, causing intense pain to the predator. The predator then releases the unharmed horn shark—resulting in an injured predator, but a safe horn shark. (Source)
Horn sharks pose little threat to humans. It is rare that a human would ever come in unintentional contact with a horn shark because of their location in the deepest region of the ocean. Divers have gotten very close and have even hand fed the horn sharks safely. However, on occasion, a horn shark has been taunted and harassed resulting in an attack. To date, a horn shark has reportedly bitten only one human. (According to the International Shark Attack File) (Source)
Besides being of little danger to humans, they are also of little importance to humans. Fishermen only occasionally catch horn sharks when trawling on the bottom of the ocean. When this does occur, they either throw them back into the water or keep them as fish bait. (Source) The only other use of horn sharks by humans is for scientific study—due to their lifespan of up to 12 years—or in exhibit tanks at aquariums. (Source)
The lionfish (Pterois volitans) is a marine fish native to the Indo-Pacific region (Source). However, since the 1990s it has become common in other places around the globe due to the aquarium trade (Source). The P. volitans species is characterized by the red and white or brown and white stripes that cover the lionfish’s body and spines and serve as a warning to predators. Although beautiful, the sting of a lionfish is extremely painful and can lead to hospitalization for humans if they are stung (even by a dead lionfish!) On the spines (13 dorsal spines accompanied by 10-11 dorsal soft rays and 3 anal spines accompanied by 6-7 anal soft rays) glandular grooves are found extending from thebase of the spine to approximately three quarters of the way up the spine (Source). These apocrine-type venom glands are triggered when the spine of the lionfish enters the body of a predator or threatening animal. This disturbs the gland tissue and causes the outer covering of the spine to push out ventrally, tearing through the glandular tissue and releasing the venom, which then travels up the glandular groove and into the body of the other creature (Source).
Envenomation causes extreme pain and a range of cardiovascular, neuromuscular, and cytolytic (relating to the disruption of cells) effects in humans and can be fatal to other fish (Source). However, the venom glands were adapted for defense purposes and are not known to be used to capture or injure the lionfishs’ prey. While a few lionfish have been found in the stomachs of some grouper species, research has shown that even top predators, including sharks, and parasites avoid lionfish (Source). In addition, the mimic octopus has been known to mimic the lionfish, evidence that other marine animals are aware of and understand the danger of interaction with the lionfish. As a result, they continue to eat many smaller fish species and because the lionfish have no natural predators their numbers have been increasingly dramatically in the Atlantic (and in particular in the Bahamas).
Consequently, the lionfish poses a threat to the future of the marine food chain in several areas and a danger to divers. Attempts to slow down the proliferation of lionfish include consumption (by humans), the sport of physically removing the lionfish from the water, and research to explore what it is in the Indo-Pacific region that keeps the lionfish population at a reasonable and sustainable level (Source).
While the adaption of these venomous spines is for defensive purposes it would be interesting to note how often the lionfish actually uses this mechanism and how often its appearance and reputation protect it from potential threat.
One blue-ringed octopus (Hapalochlaena) weighing 25 grams possesses enough poison to fatally paralyze ten adult humans (source) Found in tide pools in the warm Pacific waters and with a body only five centimeters wide and an arm span of only ten centimeters, the blue-ringed octopus secretes a metabolic toxin called tetrodotoxin (source), abbreviated as TTX in its saliva. By sequestering this bacterially produced TTX in its tissues (source), the blue-ringed octopus manages to retain this deadly toxin in its tiny, molluscan body without harming itself.
Map of the Pacific Ocean- home to the BRO
One milligram of TTX is potent enough to kill a full grown human (source), a very adequate defense to overcompensate for its vulnerable size—nevertheless, the blue-ringed octopus is not the only animal able to isolate this toxin. TTX has been isolated in not only the blue-ringed octopus, but in some species of the pufferfish, Californian newt, parrotfish, frogs of the genus Atelopus, sea stars, angelfish, and the xanthid crab (source). Creating a symbiotic relationship with the common marine bacteria called Pseudoalteromonas haloplanktis tetraodonis (source), the blue-ringed octopus obtains this potent neurotoxin by creating ideal living conditions for the bacteria (source). TTX works by “blocking channels that control the movement of sodium ions across nerve and muscle cell membranes, halting their electrical activity;" (source) thus, the victim experiences overall numbness and paraesthesia before paralysis sets in and convulsions, mental impairment, and cardiac arrhythmia occur.
Death usually occurs anywhere between 20 minutes to eight hours after the victim has come into contact with the neurotoxin (source). With such extreme effects, the blue octopus’s ability to secrete this powerful toxin has scientists wondering why these animals are not affected by the toxin themselves. The most probable theory is simple: evolution. Upon examining these highly dangerous animals in more detail, it became apparent that their sodium channels had evolved to resist the dangers of the toxin. Whereas the pufferfish simply developed eight different versions of sodium channels, the blue-ringed octopus had developed a “slightly different sodium channel receptor" (source) which made it immune to the paralyzing effects of the TTX. These mutations make the blue-ringed octopus capable of withstanding roughly 500 to 1,000 times the concentration of TTX in comparison to these other non-toxic fish (source).
Blue-Ringed Octopus's body in partial defense mode.
However, these creatures are not the only ones that have adapted to the toxin, and predators such as garter snakes in the western United States have similarly evolved resistance to TTX and can feast on highly toxic newts (source); natural selection at its best. No longer affected by the poisonous newts, these snakes have been found to have “independently evolved TTX-resistant sodium channels… and are so resistant that the dose of toxin needed to immobilize them is sufficient enough to kill over 900 people" (source). Does this mean that the predators of the blue-ringed octopus have the potential to have a symbiotic relationship with the TTX-producing bacteria? Only time will tell. For the mean time, watch out when you go in a tide pool!
Being within the order Torpediniformes, the ray harbors the ability to electrocute its attacker or prey. This species uses two organs made of muscle cells called electrocytes, located on either side of the head, to send electrical currents up to 220 volts that can stun or kill sharks, mullets, dogfish and boney fish. Their diet consists of benthic or pelagic boney fish (Source). The cells of the torpedo ray have the ability to make electrical currents “through the flow of calcium ions” (Source). They stun their prey by wrapping their pectoral fins around the fish where the electrical shock is administered. After the prey is disoriented or killed, the ray opens its jaws to eat the fish. The Atlantic torpedo ray has the ability to distend its jaws in order to eat a fish half its size! They have been known to kill fish too large for consumption. A nocturnal predator, the ray uses “electric sensors to "image" and detect potential prey based on the electric fields they emit” (Source). The reason the ray uses this electrical shock is because of its slow nature. The torpedo ray is quite languid, and therefore the shock enables them to catch fast moving fish.
This brilliant unique organism has evolved beyond the cusp of human knowledge. Much is to be learned from the Atlantic torpedo ray in the coming years! Let us pray that it will not become extinct or succumb to the growing issue of global warming.
Although, in humans, pain is minimal initially, after about 30 minutes the pain worsens and the victim will experience severe pain in his/her muscles, and the area around the wound. The toxicity of the sea snake venom is so intense that the average snake could kill more than five men, while the two deadliest sea snakes, Oxyuranus microlepidotus and Oxyuranus scutellatus, could kill 125 men.
The Yellow Bellied Sea Snake is a highly venomous sea snake found in all tropical oceans excpet the Atlantic:
Mike Wells, 39, swam 395ft (120 metres) through Fish Rock Cave, a pitch-black ocean chamber in only two minutes and 40 seconds. It was the longest underwater cave swim as well as the first time that Fish Rock Cave had been freedived. The veteran of 19 years made it through the cave, 85ft below the surface, on his third attempt after cramp forced him to abandon the first two tries. He narrowly averted disaster on his final dive when his monofin became trapped in a narrow crevice. His son Jeremy, a member of the support team, freed him.
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