A starfish, correctly known as a sea star, is a marine invertebrate belonging to the phylum Echinodermata, a group that also includes sea urchins and sea cucumbers. Determining a single, typical weight for a sea star is impossible because mass fluctuates dramatically across the roughly 2,000 known species. The weight of any individual animal is a product of its specific species, overall size, and body composition. Factors like arm count and body thickness contribute to the final measurement.
The General Weight Range of Common Starfish
The majority of sea stars encountered in coastal and intertidal zones fall into a moderate weight category, often measuring in ounces up to a few pounds. Many common species, such as the Purple Sea Star (Pisaster ochraceus), reach maturity at a weight between 2.5 and 3.2 ounces (70–90 grams). These moderate-sized animals typically have a diameter of up to a foot (30 cm).
The Common Sea Star (Asterias rubens), found widely across the North-East Atlantic, usually grows to 4 to 12 inches (10–30 cm) across. The general population of mid-sized sea stars ranges from a few ounces to several pounds. This range represents the bulk of species a person might observe in a typical marine environment.
Larger, bulkier species push this general average up, with some massive common types reaching between 3.3 and 6.6 pounds (1.5–3 kg) when fully grown. This weight represents a fully hydrated animal, with mass distribution depending on the species’ body plan. Weight changes as the animal feeds or endures periods of starvation, which can cause significant mass loss.
Weight Extremes: Identifying the Lightest and Heaviest Starfish
At the lighter end of the spectrum, the smallest sea star species, such as Patiriella parvivipara, measures only about 0.4 inches (1 centimeter) in diameter, giving it a negligible weight. These tiny creatures represent the minimum possible mass for an adult sea star.
Conversely, the heaviest sea star is generally considered to be Catala’s Sea Star (Thromidia catalai), an Indo-Pacific species that can reach an astonishing weight of up to 13 pounds (6 kilograms). This immense mass is due to its thick, cylindrical arms, which give the animal significant bulk despite its diameter being only about 24 to 26 inches (60–65 cm).
Another contender for the heaviest title is the Sunflower Sea Star (Pycnopodia helianthoides), considered the largest known echinoderm by mass, often weighing up to 11 pounds (5 kg). This species has an arm span reaching up to 3.3 feet (1 meter) and can possess up to 24 arms. The deep-water Midgardia xandaros provides an interesting contrast; it has the largest known arm span at 53 inches (134 cm), but its slender arms mean it does not approach the bulk of the true heavyweights.
Biological Factors Determining Starfish Mass
The physical mass of a sea star is determined by the composition of its body, primarily divided into two components: water and a calcareous skeleton. Sea stars possess a unique hydraulic system called the water vascular system, which accounts for a substantial percentage of their total weight. This system circulates seawater throughout the body, powering the thousands of tube feet used for locomotion and feeding.
The high proportion of water within the body means that a sea star’s weight is highly sensitive to its immediate environment and physiological state. Since the water vascular system powers movement and respiration, the water content is a direct determinant of its living mass. If the animal is out of the water or dehydrated, its mass can drop significantly.
The solid structure that gives the sea star its shape and rigidity is its endoskeleton, composed of thousands of tiny plates called ossicles. These ossicles are made of calcium carbonate, specifically a macroporous form of calcite. The dense arrangement and chemical composition of these ossicles are the primary determinants of the long-term, dry mass of the animal.
Ossicles are embedded within a connective tissue and are connected by muscles, allowing for a remarkable range of movement and flexibility. The density and size of these skeletal elements vary greatly between species, directly influencing whether a sea star is a lightweight, flat species or a heavy, bulky one. Environmental factors, such as food availability and temperature, influence growth rate and ultimate size, affecting the final mass an individual reaches.