Vertebrates
Fish can harm humans in three possible ways: by biting, stinging with a poisonous spine, or being poisonous as food. When it comes to bites, only a few species of sharks have attacked humans with fatal consequences. Among the dangerous species, only the tiger shark (Galeocerdo cuvier) regularly ventures onto coral reefs. During the day, it usually stays at great depths near steep slopes facing the sea, and occasionally moves to shallower waters at night. The oceanic species, the oceanic whitetip shark (Carcharinus longimanus), lives in open waters and only occasionally visits remote islands and reefs far from the shore. The famous great white shark inhabits cold temperate waters and rarely ventures into the tropics. The bull shark (Carcharinus leucas), on the other hand, is a coastal species that often resides in murky river mouths and can even swim far upstream.
Most commonly, divers encounter dusky gray sharks (Carcharinus amblyrbynchos). They are territorial and occasionally display aggression. They may perceive humans as competitors, and there have been numerous cases of attacks without serious consequences. Before attacking, a shark performs distinct ritual movements, known as threat display. If any shark starts behaving overly curious or persistent, it is best to retreat or even exit the water. It is strongly advised not to intentionally attract sharks with spearfishing or bait.
Barracudas, moray eels, and large triggerfish can also pose some danger in terms of bites. Most barracuda attacks have occurred in murky water, likely by mistake. Bites from moray eels are often the result of inserting hands into crevices and reef caves or swimming with speared fish; however, large moray eels can occasionally display aggression on their own. Large triggerfish, such as Balistoides viridescens and Pseudobalistes flavimarginatus, fiercely defend their nests with deposited eggs and aggressively attack any approaching creatures. Their bites can sometimes leave serious wounds that require stitching. There was a case where a triggerfish struck a dive master on the head, causing him to almost lose consciousness and requiring rescue. In another instance, it took 30 stitches to close the wounds on the cheek of a bitten individual.
Scorpionfish and triggerfish have venomous spines. The sturdy dorsal spines of the wartfish (Synanceia verrucosa) can penetrate the sole of tennis shoes. A sting from the wartfish causes excruciating pain and without proper treatment can be fatal. The colorful and peculiar flying lionfishes do not hide or avoid divers. Their stings are very painful but usually not deadly. Sting from triggerfish is comparable in terms of pain to the wartfish sting but has never resulted in death. Surgeonfish have sharp movable blades or two pairs of sharp immovable shields at the base of their tails. Porcupinefish have protruding cheek spines. Stingrays have one or several detachable spines at the base of their long tails, which can cause severe wounds. Many other fish can injure humans with their sharp spines, scales, and teeth, so it is important to handle them with caution.
Some reef fish are poisonous when consumed. The toxins are either produced by the fish itself or accumulated in its body from consumed organisms (ciguatoxin). Many boxfish (Ostraciidae) and cowfish (Aracanidae) have highly poisonous skin and internal organs. The toxin contained in them, tetrodotoxin, is one of the strongest known neurotoxins and has caused fatal poisonings on multiple occasions. Grammistids (Epinephelinae) and, possibly, young sweetlips (Plectorhinchus) secrete a skin toxin that tastes unpleasant and repels predators. Certain fish species can cause a form of food poisoning known as ciguatera. Toxins reach their highest concentration in the bodies of top predatory fish in the food chain. These toxins do not harm the fish, but they cause a severe and prolonged illness in humans, sometimes ending in death. Symptoms vary from tingling lips and limbs to altered skin sensitivity to cold and heat, weakness, vomiting, diarrhea, respiratory and cardiac disturbances. There is no specific treatment, but intravenous mannitol can help alleviate symptoms. The greatest danger of ciguatera lies in its unpredictable occurrence, with outbreaks happening in different places and at different times. Ciguatoxin is produced by single-celled dinoflagellates Gambierdiscus toxicus, which inhabit empty surfaces. Herbivorous fish consume them along with filamentous algae, and in turn, predatory fish eat them. The toxin is not digested or excreted but accumulates in muscles, particularly in the liver and reproductive organs. Each time a predator consumes a small fish, it accumulates the amount of toxin it has accumulated throughout its life. Consequently, the highest concentrations of toxins are found in the bodies of large predators. In the Indo-Pacific, ciguatera is most commonly caused by the bohar snapper (Lutjanus bohar) and the giant moray eel (Gymnothorax javanicus). When consuming very large groupers, snappers, emperors, jacks, barracudas, and triggerfish, caution should be exercised. It is always helpful to consult with locals.
Marine snakes: none of the 54 species of marine snakes are found in the Red Sea, but at least one species is found in the Gulf of Aden, and several in the Arabian Sea and the Gulf of Oman. They are non-aggressive, but their venom is highly potent: theoretically, 1.5 mg is a lethal dose. In the Red Sea, the banded eel (Myrichthys colibrinus) is often mistaken for a snake.
Invertebrates
Sponges: Many sponges contain tiny siliceous spicules or irritant substances in their bodies, which can cause skin irritations and painful dermatitis.
Hydroids: All stinging organisms (fire corals, hydroids, jellyfish, anemones and corals) are armed with nematocysts - stinging cells with ejected barbed threads and venom. Fire corals (Millepora spp.) and hydroids can cause intense stings. Fire coral is abundant along reef edges and can be easily touched while snorkeling or diving. It can be recognized by its mustard color and tiny "hairs" on the surface.
Portuguese Man o' War (Physalia physalis): This floating siphonophore is often mistaken for jellyfish. The tentacles of the Portuguese Man o' War can extend several meters and cause severe stings. It is frequently washed ashore.
Jellyfish: All jellyfish can irritate the skin. In our region, several small species of box jellyfish (Cubomedusae) cause painful stings, but there are no dangerous ones like the Australian Chironex fleckeri, which can kill within minutes.
Sea Anemones: The small (2 cm) Triactis producta stings intensely, while the more common Alicia pretiosa causes inflammation and rash. Most species can cause irritation on sensitive areas of the skin.
Corals: Most hard corals have sharp-edged skeletons that can easily cause cuts. These cuts heal poorly and are prone to infection. Wounds should be thoroughly disinfected, and antibiotics may be necessary.
Cone Shells (Conus spp.): These mollusks hunt using a venomous harpoon located at the tip of their long proboscis, which can reach any part of their shell (do not startle other mollusks with a respiratory siphon). The venom causes paralysis, and stings from several species (C. geographus, C. striatus, C. textile) can be fatal. Handling cone shells should be done with extreme caution, and never touch the soft parts. If stung, the venom should be suctioned out, and immediate medical assistance sought.
Cephalopods: All octopuses have a venomous beak in the center of their lower side, where their tentacles converge. None of the local species are dangerous, but if handled, they can deliver painful bites. The bite of the small Australian blue-ringed octopus (Hapalochlaena lunulata) leads to death within an hour. Its neurotoxin paralyzes the respiratory muscles. The only remedy is artificial lung ventilation until symptoms subside.
Crown-of-Thorns Starfish (Acanthaster planci): The spines of this predatory starfish are covered with venomous skin. Their stings are very painful and can even cause paralysis, especially if they break off. The venom can be neutralized with very hot water (>50°C).
Sea Urchins: Many species have very sharp and thin spines that easily penetrate the skin and can break off. Although these stings are unpleasant, they usually heal within a couple of days, and any embedded fragments dissolve. The spherical tips of the spines of the venomous sea urchin Astenosoma marisrubri cause intense pain. The long spines of the diadem sea urchin are mildly venomous, but in allergic individuals, the sting can cause respiratory disturbances.
