The animal kingdom is divided into two major groups based on the presence or absence of a backbone. The vast majority of species belong to the latter category, existing without the internal skeletal structure characteristic of mammals, birds, and fish. These creatures are known collectively as invertebrates. The sheer size reached by some boneless animals challenges the common perception that only vertebrates can achieve massive proportions. Identifying the heaviest among them requires looking beyond land-dwelling insects and into the deep ocean.
Defining Invertebrates
An invertebrate is any animal that does not possess a vertebral column or backbone. This group encompasses approximately 97% of all animal species, ranging from microscopic organisms to enormous marine life. Lacking a rigid internal skeleton, these creatures must rely on alternative structural supports to maintain their body mass and shape.
Many large aquatic invertebrates, such as jellyfish and worms, utilize a hydrostatic skeleton. This is a fluid-filled cavity surrounded by muscle layers. Contraction of these muscles pressurizes the incompressible fluid, providing the necessary stiffness and internal support for movement. Arthropods, like crabs and lobsters, employ a chitinous exoskeleton, a hard outer shell that acts as a suit of armor and a point of muscle attachment. While this external skeleton imposes a mechanical limit on size, the buoyancy of water allows marine arthropods to grow significantly larger.
The Heaviest Boneless Animal
The heaviest reliably measured boneless animal is the Colossal Squid, Mesonychoteuthis hamiltoni. This cephalopod is primarily found in the frigid waters of the Southern Ocean around Antarctica. The largest intact specimen ever caught and scientifically examined weighed approximately 495 kilograms (1,091 pounds).
The Colossal Squid is often confused with its relative, the Giant Squid (Architeuthis dux), which can be longer in total length due to its extremely long tentacles. However, the Colossal Squid is significantly heavier because it possesses a much wider, stockier, and more muscular mantle. Its body mass is concentrated in its robust torso, giving it greater overall density and bulk. The species also possesses the largest eyes in the animal kingdom, measuring up to 27 centimeters across, an adaptation for hunting in the deep ocean twilight zone.
Why Invertebrates Grow So Large
The immense size of the largest invertebrates is explained by a phenomenon called Deep-Sea Gigantism. This biological principle suggests that animals living in the cold, deep ocean tend to evolve larger body sizes than their shallow-water relatives. The frigid temperatures of the deep sea are a major factor, as they dramatically slow down an animal’s metabolism.
A slower metabolism leads to a longer lifespan and delayed sexual maturity, which allows for a prolonged period of growth. Furthermore, food is scarce in the deep ocean, creating an evolutionary advantage for larger animals. Bigger organisms have more efficient energy storage and can travel greater distances to locate sparse resources, such as sinking carcasses. Finally, the lack of significant predation pressure in the deepest zones removes a constraint on body size, enabling these creatures to maintain their massive bulk.
Notable Contenders for Weight
While the Colossal Squid is the confirmed heavyweight, other invertebrate species reach impressive masses. The Lion’s Mane Jellyfish (Cyanea capillata), for example, is composed almost entirely of water, making accurate weighing challenging. Based on the volume of its bell, which can reach over two meters in diameter, and its tentacles that can trail over 30 meters, its maximum weight is estimated to potentially exceed one tonne (1,000 kilograms).
The Giant Clam (Tridacna gigas) is the largest living bivalve mollusk. Individual clams can live for over a century, with the largest specimens reliably recorded to weigh around 250 kilograms (550 pounds) alive. Another impressive example is the Japanese Spider Crab (Macrocheira kaempferi), the largest living arthropod by leg span. Although its legs can stretch up to 3.7 meters, its actual body mass is relatively small by comparison, with the largest individuals weighing a maximum of about 20 kilograms.