Chemical Defense Mechanisms in Freshwater Fish

Several groups of animals produce poisons, either to use as venom to subdue their prey or for protection against their own predators, or even both. Poison arrow frogs are particularly notorious for their potent chemical defenses. But whereas a number of marine fishes use chemicals to deter overly inquisitive hunters, comparatively few freshwater fish have adopted this technique. However, those that do are worth treating with caution.

How do pufferfish avoid predators?

Pufferfish have not one but two novel methods of overcoming predators, possibly as a result of their slow swimming speed, which might otherwise make them an easy target. The first line of defense is to swell up rapidly, taking huge volumes of water into their expandable stomachs. This sudden expansion usually leaves predators nonplussed, even unable to handle what has just become an unexpectedly large mouthful. But if this fails to produce the desired result, the back-up plan is virtually foolproof. The internal organs of pufferfish, especially their livers and ovaries, contain a powerful neurotoxin called tetrodotoxin, which is 1,200 times more potent than cyanide. The amount produced varies by species; some produce a sufficient amount to kill 30 humans.

Do catfish spines contain venom?

As many flshkeepers know, a catfish removed from the water will often lock out its leading pectoral and dorsal fin rays. This defensive reaction is in itself as damaging to predators in the wild as it is to a fish net at home, but some catfish species reinforce this physical defense with a chemical backup. For instance, some fish in the Pimelodella genus have poison glands located at these spines which cause a reaction akin to a severe insect sting. However, perhaps the most dangerous of venomous catfish is the aptly named stinging catfish, Heteropneustes fossilis. A puncture wound from its pectoral spine can be fatal, even to humans. If you are unfortunate enough to be stung by a catfish, bathe the wound for as long as possible in the hottest water you can stand (which causes the venom to break down), while someone else calls the doctor.

Are stingrays dangerous?

The name of the animal is the stingray, so perhaps it should come as no surprise that it packs a punch. That said, most stingray injuries are the result of unwary bathers stepping on the fish, which are usually concealed in the substrate — a situation that is clearly unlikely to occur in the home aquarium. The sting is a defensive mechanism, usually located towards the base of the tail, and jabbed upwards by the alarmed fish and into the leg of the victim. The fish are very seldom aggressive towards aquarium keepers; nonetheless, the increasing popularity of freshwater stingrays in the aquarium hobby increases the probability of an accidental injury and the animals should be handled with caution.

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