Life underwater is a whole different ballgame from life on land: it presents unique challenges but may also relax some evolutionary constraints. Molting and the structure of the exoskeleton may have kept terrestrial arthropods from evolving large body sizes — but do these same constraints apply to aquatic arthropods?

• Marine Molting: Molting involves passing through a squishy stage that is dangerous on land. Would this process be easier for large arthropods if it happened underwater? Yes. The surrounding water helps support body parts that have just lost their armor. So underwater, even a large “squishy” arthropod can move around a bit and won’t slump into a deformed puddle without its armor.
• The Benefits of Buoyancy: When a terrestrial arthropod is scaled up, its body weight increases faster than what its exoskeleton can support, causing the exoskeleton to crumple. Would a giant aquatic arthropod suffer the same fate? Just imagine your own experience in a pool. Underwater, one feels almost weightless. A large aquatic arthropod would feel the same thing — the surrounding water would help support its body, taking some of the stress off its exoskeleton, and making its legs less likely to break under the strain.

Living underwater does seem to have relaxed some of the size constraints on arthropods:

• The marine Japanese spider crab (Macrocheira kaempferi) has an arm span of almost four meters (more than 13 feet)!
• The American lobster (Homarus americanus) can weigh in at 19 kg (about 40 lbs)!
• Extinct eurypterids — water “scorpions” — grew to almost three meters (about 10 feet) long!