Despite its name and armored shell, the horseshoe crab's closest living relatives are which animals?
- Crabs and lobsters
- Spiders and scorpions
- Clams and sea snails
- Sea turtles
Why B? And why not the others?
Correct answer: B. Spiders and scorpions
Horseshoe crabs are chelicerates, the group that contains spiders, scorpions, mites and ticks — modern genetic studies place them firmly among or beside the arachnids, and they even breathe with 'book gills' that mirror the book lungs of spiders. The name and the hard shell make crabs and lobsters the natural guess, but crustaceans have antennae and chewing mandibles that horseshoe crabs completely lack. Clams and snails share the seashore and a shelled appearance, yet they are molluscs, an entirely different branch of the animal kingdom. Sea turtles resemble them only in the domed silhouette and the habit of hauling ashore to lay eggs on beaches. As a bonus twist, horseshoe crab blood is pale blue and so exquisitely sensitive to bacterial toxins that medicine has relied on it for decades to safety-test vaccines and injectable drugs.
Source: Encyclopaedia Britannica & National Geographic: horseshoe crabs are chelicerates related to spiders and scorpions