The term “worm” is an informal label applied to numerous distantly related invertebrates with long, soft, and limbless bodies. This broad category includes creatures from several different animal phyla, such as the segmented annelids (like earthworms) and the unsegmented nemerteans (or ribbon worms). While most people picture small earthworms, the true giants of this group push the limits of animal size, with the largest specimens measured in feet or even dozens of meters. The size extremes are found in two habitats: the moist soils of the earth and the deep, cold waters of the ocean.
The Largest Terrestrial Wonders
The largest known worms living on land are members of the segmented annelid phylum, which includes earthworms. The African Giant Earthworm (Microchaetus rappi) holds the record for maximum length among terrestrial species. While most adults of this South African species average between 4 and 6 feet long, one exceptional specimen reportedly reached a stretched length of 22 feet (6.7 meters).
The Giant Gippsland Earthworm (Megascolides australis) of Australia is often considered the largest by mass, despite being shorter than its African cousin. This species, found in the clayey, wet subsoils of Victoria, averages about 3.3 feet (1 meter) in length. Individuals can stretch to nearly 9.8 feet (3 meters).
Measuring these giant annelids is difficult because their bodies are highly elastic, allowing them to contract or stretch significantly. This flexibility helps them navigate their deep, complex burrow systems, which can extend over six feet into the soil.
Deep Sea Giants and Record Holders
The absolute record for animal length among all worms belongs to a marine species, the Bootlace Worm (Lineus longissimus), a member of the Nemertea phylum, commonly known as ribbon worms. This unsegmented predator inhabits the shallow waters and rock pools of the North Atlantic. It is recognized as one of the longest animals on Earth, surpassing the size of the largest terrestrial worms.
Specimens of the Bootlace Worm are commonly found coiled under rocks, but they can extend dramatically when disturbed. The typical length for an adult is between 16 and 49 feet (5 to 15 meters), but individuals can reach 98 feet (30 meters). The most extreme measurement, a specimen washed ashore in Scotland in 1864, was reported to be over 180 feet (55 meters) long. This record is considered with caution due to the worm’s ability to stretch far beyond its natural size.
Despite its length, the Bootlace Worm is narrow, typically only 0.20 to 0.39 inches (5 to 10 millimeters) in width. This ribbon-like body plan allows for extreme elongation, a trait amplified in the marine environment. Other marine giants, such as some polychaete worms, can also exceed one meter in length.
Biological and Environmental Factors Driving Extreme Size
The large size of these worms results from biological adaptations interacting with specific environmental pressures. For terrestrial giants, size is supported by stable, resource-rich environments, specifically clayey soils with high moisture and abundant organic matter. Stable temperatures and consistent moisture deep underground allow these species to grow continuously without the need for rapid reproduction.
The immense length of marine species like the Bootlace Worm is partly a consequence of the aquatic environment, which provides buoyancy to support a large, soft body structure. For other deep-sea invertebrates, gigantism is often linked to cold water and high pressure, contributing to deep-sea gigantism. These conditions lead to slower metabolic rates and delayed maturity, allowing organisms to grow larger over a longer lifespan.
A lack of strong predation pressure also contributes to larger body sizes over evolutionary time. Terrestrial giants burrow deep to avoid threats, while the Bootlace Worm releases a highly toxic, neurotoxin-containing mucus when irritated, which serves as a chemical defense. This combination of environmental stability, low metabolic demand, and specialized defense mechanisms allows these creatures to circumvent biological constraints on size.