Starfish, correctly known as sea stars, are marine invertebrates belonging to the class Asteroidea, characterized by their radial symmetry. They are found across the globe, inhabiting every one of the world’s oceans, from shallow intertidal zones to the deepest abyssal plains. With over 2,000 distinct species recognized, this diverse group exhibits an enormous range in physical dimensions. The difference between the smallest and largest known species represents one of the most extreme size disparities in the marine environment.
Defining Starfish Size
Scientists use a precise method to standardize the measurement of sea star size, accounting for variability in arm length and body shape. The primary measurement is the radius (R), which is the distance from the center of the body disc to the tip of one arm. Total diameter is often reported for comparison, representing the distance from the tip of one arm, through the center, to the tip of the opposing arm. This diameter is essentially twice the radius (2R) for five-armed species. Weight is generally a less reliable metric because the star’s mass is highly variable depending on its feeding and reproductive state.
The Smallest Starfish Species
The smallest known species is Parvulastra parvivipara, native to the intertidal rock pools of South Australia. A fully grown adult reaches a maximum radius of only about 4.7 millimeters, translating to a total diameter of less than one centimeter. This minute size is an adaptation to its specific and often ephemeral habitat of shallow, granite rock pools.
The small size of P. parvivipara is linked to viviparity, a reproductive strategy where the female retains the developing embryos inside her body. This brooding ensures offspring survival in the harsh, fluctuating intertidal environment, bypassing the need for a free-swimming larval stage. This unique life cycle allows the species to maintain its population within its limited geographic range, unlike most other sea stars which release eggs and sperm directly into the water column.
Giants of the Deep and Shallow Seas
The largest sea stars are measured in meters rather than millimeters. The largest known sea star by diameter is Midgardia xandaros, a deep-sea species collected from the Gulf of Mexico. This giant is notable for its long, slender arms relative to its small central disc, resulting in a recorded diameter of up to 1.4 meters (over 4.5 feet). Because of its deep-water habitat, little is known about the living habits or typical maximum size of this specimen.
The largest known shallow-water species, the Sunflower Sea Star (Pycnopodia helianthoides), is the largest by mass and volume. Found along the Pacific coast of North America, this creature can grow up to 1 meter (3.3 feet) in diameter and weigh up to 5 kilograms (11 pounds). The Sunflower Sea Star can possess up to 24 arms, giving it a massive, fleshy appearance compared to the five-armed Midgardia. Another giant is the Giant Sea Star (Pisaster giganteus), which is common in the North Pacific and reaches a diameter of 60 centimeters (2 feet).
Environmental Drivers of Size Variation
Size variation in sea stars results from environmental and ecological factors that influence growth rates and maximum lifespan. Deep-sea gigantism explains the size of species like Midgardia xandaros, where the stable, cold environment allows for a slow metabolism and extended growth. Colder water temperatures correlate with larger body sizes in many marine invertebrates, as organisms dedicate less energy to maintenance and more to growth.
Resource availability is a strong determinant of ultimate size. High-density populations often exhibit a smaller average body size due to increased competition for food. In habitats where food is abundant, such as nutrient-rich shallow waters or deep-sea areas with reliable food falls, sea stars can reach their maximum genetic potential. A longer lifespan, often found in cold, stable environments, also provides the necessary time for a sea star to achieve a large diameter.