The banded sea krait’s lethal venom packs a punch ten times more toxic than a rattlesnake’s, but fortunately these serpents are so meek that human bites are rare. Kraits cruise the shallow, tropical waters of coral reefs and mangrove swamps. But, alone among the sea snakes, they are amphibious and able to spend up to ten days at a time on land. Sea kraits hit the beach to digest their food (mostly eels and fish), mate, and lay eggs.
When a short-tail stingray stabbed a swimming Steve Irwin (the "Crocodile Hunter", the entire species acquired the undeserved reputation of killers. Irwin’s unlucky encounter notwithstanding, human stingray deaths are extremely rare—only one or two occur worldwide each year. The animals typically use their daggerlike tail spines for protection against predators like sharks or killer whales.
Short-tails are the largest of all stingrays and can reach lengths of 14 feet (4.25 meters) and top 750 pounds (340 kilograms).
The soft sea slugs known as nudibranchs have no shells and are defenseless from predators—or so it would seem. In fact the animals boast a toxic arsenal acquired by producing their own poisons or by ingesting them with meals of toxic sponges or anemones and then secreting them when threatened. Many nudibranchs boast brilliant color palettes that make them stand out on reefs or ocean floors and send would-be predators a clear message: I am not tasty.
The colorful lionfish presents an attractive appearance, but its arsenal of needlelike dorsal fins is actually meant to warn others away. Too-curious humans can receive a quite painful and potentially dangerous sting from these spines. Lionfish hail from Indo-Pacific reefs but they have recently spread to other warm waters, like the Caribbean, where they are causing some ecological concern by thriving as an invasive species.
This harmless-looking “beach ball” is actually a puffer fish, washed up on a Karnataka, India, shoreline. The fish, also known as a blowfish, attempts to make itself inedible by ingesting enormous amounts of water (or air) and swelling to several times its typical size. Lucky is the predator that heeds this warning. Those that don't might not live to repeat the mistake.
Puffer fish contain tetrodoxin, which is lethal to many fish—and to humans, who have yet to develop an antidote. Some Japanese consider the puffer a delicacy, though one fish holds enough toxins to kill 30 people.
Members of the large scorpionfish family live on the seafloor where they tend to blend in with their surroundings and hunt by ambush. These sit-and-wait predators adopt such effective camouflage that many of their meals likely never knew what hit them. Scorpionfish are also armed with venomous spines, which can deliver a dangerous sting to any animal trying to make prey of this predator.
Caribbean Fire Coral
Any diver who has handled fire coral knows where its “fire” descriptor comes from—the tiny barbs that produce a painful burning sensation. Calling this hydrozoan a coral, however, is a bit misleading—it’s more closely related to sea jellies. Fire corals can spread quickly across warm-water reefs, where they use their stinging polyps to persuade other animals not to grow on top of them.
One half of a deadly duo, the blue-ringed octopus enjoys an interesting symbiotic relationship with colonies of bacteria that inhabit its salivary glands. The bacteria produce potent neurotoxins that can kill a human in just a few minutes yet appear to have no harmful effect on the octopus. This toxic saliva, injected through shells broken with the octopus's birdlike beak, also helps it to digest crabs and mollusks.
Crown-of-Thorns Sea Star
The crown-of-thorns sea star carries a Biblical name, and these invertebrates can spread across coral reefs like an Old Testament plague. A single female, bristling with toxic spines, can produce up to 100 million eggs a year. The echinoderm feeds on living corals by spreading its stomach over them and secreting enzymes to liquefy the coral tissue. During outbreaks the sea stars can overwhelm Indian and Pacific Ocean reefs.
The reef stonefish is a master of camouflage often mistaken for a colorful rock or chunk of coral—an ability that helps it to ambush passing fish and crustaceans. When threatened the stonefish hunkers down and brandishes dorsal fin spines loaded with one of the most toxic venoms found in any fish—and one potentially deadly to humans.
Using its pectoral fins as shovels, the stargazer fish digs into the seafloor sand, leaving its eyes and mouth exposed to spot and eat passing prey. The fish can also produce a defensive electric shock of up to 50 volts, which it creates via a specialized organ located behind its eyes.
Striped Eel Catfish
The striped eel catfish, seen here in Malaysian waters, is the only catfish found on coral reefs—though these fish also swim in estuaries, tide pools, and open coastal waters across the Indo-Pacific. Each striped eel catfish is armed with venomous spines on its dorsal and pectoral fins that can deliver a very painful sting—though one rarely fatal to humans.