Crustaceans are a diverse group of invertebrates found across various aquatic environments, from shallow coastal waters to the deepest ocean trenches. This group includes familiar creatures such as crabs, lobsters, and shrimp. They are characterized by their hard exoskeletons, segmented bodies, and jointed appendages, playing significant roles in marine ecosystems.
The Japanese Spider Crab
The Japanese spider crab (Macrocheira kaempferi) is the largest known living crustacean, primarily due to its extraordinary leg span. This span can reach up to 12 feet (3.7 meters) from claw to claw, with some reports suggesting a maximum of 13 feet (4 meters). Its body, or carapace, measures up to 15 inches (38 cm) wide.
This creature can weigh as much as 44 pounds (20 kg). It features an orange body with white spots on its legs and a pear-shaped carapace covered in small, spiny projections. It primarily inhabits the sandy and rocky continental shelf and slope off the Pacific coast of Japan, typically found at depths ranging from 160 to 1,970 feet (50 to 600 meters). These slow-moving omnivores are scavengers, feeding on dead and decaying matter, including fish, invertebrates, and algae found on the seafloor. The Japanese spider crab is thought to have a long lifespan, potentially living for 50 to 100 years.
What Defines the Largest?
The designation of “largest” among crustaceans can be understood through different measurements, primarily leg span and body mass. The Japanese spider crab earns its title as the largest crustacean due to its unparalleled leg span.
However, if “largest” is defined by weight, the American lobster (Homarus americanus) holds the record for the heaviest crustacean. A specimen caught off Nova Scotia, Canada, weighed a 44.4 pounds (20.14 kg). While the American lobster can reach substantial body lengths, sometimes over 24 inches (61 cm), its leg span does not compare to that of the Japanese spider crab. This distinction highlights that “largest” is a relative term, depending on the specific physical characteristic being measured.
Other Notable Large Crustaceans
While the Japanese spider crab is known for its leg span, other crustaceans also exhibit impressive sizes. The American lobster is notable for its significant mass. It is found along the Atlantic coast of North America and can reach lengths of over 2 feet. These lobsters are often dark green or brownish-green and inhabit rocky and sandy seafloors.
The coconut crab (Birgus latro) is the largest terrestrial arthropod. These crabs can weigh up to 9 pounds (4.1 kg) and have a leg span of over 3 feet (1 meter). Found on islands across the Indian and Pacific Oceans, they are known for their ability to crack open coconuts with their powerful claws. The giant isopod (Bathynomus giganteus) is a deep-sea scavenger resembling a large pillbug. These isopods can grow up to 16 inches (40 cm) in length, with some reaching nearly 30 inches (76 cm), inhabiting the cold, deep waters of the Atlantic, Pacific, and Indian Oceans.
The Biology of Crustacean Growth
Crustaceans grow through a process called molting, or ecdysis, which is fundamental to their development. Unlike animals with internal skeletons that grow continuously, crustaceans are encased in a rigid external skeleton, or exoskeleton, that does not expand. To increase in size, they must periodically shed this old exoskeleton and grow a new, larger one.
The molting cycle involves several stages, including forming a new, soft shell beneath the old one, then shedding the old shell. During this vulnerable period, the crustacean absorbs water to expand its body before its new exoskeleton hardens. This cyclical process allows crustaceans to achieve their impressive dimensions over time, with larger species like the Japanese spider crab undergoing this transformation repeatedly throughout their long lives.