Why can some marine arthropods grow larger than land arthropods?

Marine arthropods can often reach larger sizes than their terrestrial counterparts due to the physical properties of their environment and their specific respiratory adaptations.

While all arthropods are constrained by the need to molt their exoskeleton—a process that limits how large an animal can grow before the mechanical difficulties of shedding and supporting a new, soft shell become insurmountable—the aquatic environment provides additional support. In water, buoyancy helps support body mass, similar to how it allows large vertebrates like the blue whale to grow much larger than the largest terrestrial mammals.12

Furthermore, respiratory efficiency plays a critical role. Aquatic arthropods, such as crustaceans, utilize gills for gas exchange. In contrast, terrestrial arthropods rely on tracheae and book lungs. Tracheal systems are highly efficient for small, terrestrial animals because they allow for passive diffusion, but they are not as effective at supporting the oxygen demands of larger body sizes. Consequently, the combination of buoyancy and gill-based respiration allows marine species to overcome some of the size limitations faced by land-dwelling arthropods.314