Starfish, or sea stars, are marine invertebrates belonging to the class Asteroidea, characterized by their radial symmetry and often five-armed body plan. They inhabit all the world’s oceans, from shallow coastal zones to the deepest abyssal plains. Their unique anatomy supports varied reproductive strategies. Understanding how these organisms propagate requires examining the underlying reproductive mechanisms they employ.
Standard Sexual Reproduction
The majority of starfish species utilize sexual reproduction, meaning they are gonochoristic, with individuals being distinctly male or female. This strategy is the most common scenario across the estimated 1,900 species of sea stars. The reproductive organs, known as gonads, are typically paired and located within each arm, swelling with either eggs or sperm during the reproductive season.
The primary form of reproduction is broadcast spawning, where both males and females release their gametes directly into the water column. Starfish often aggregate in large groups to increase the density of eggs and sperm, enhancing the probability of successful external fertilization. This synchronized release is often triggered by environmental cues such as water temperature, tides, or the presence of phytoplankton. Males may initiate spawning first, releasing chemical signals that encourage females to follow suit. The resulting fertilized eggs develop into planktonic larvae before settling and metamorphosing into the familiar star shape.
True Hermaphroditism in Starfish
While separate sexes are the rule, a small number of starfish species exhibit true hermaphroditism, possessing both male and female reproductive capabilities. Hermaphroditism can be categorized into two main forms: simultaneous and sequential.
Simultaneous Hermaphroditism
Simultaneous hermaphrodites possess both functional male and female gonads, or sometimes a single organ called an ovotestis, that produces both eggs and sperm at the same time.
Sequential Hermaphroditism
Sequential hermaphroditism involves an individual changing its sex over the course of its life. For example, species like Asterina gibbosa begin life as functional males before transitioning into females as they grow larger, a pattern known as protandry.
Dual-sex physiology often occurs where finding a mate is challenging, such as in low-density populations or isolated habitats. The ability to function as either sex maximizes reproductive potential with any conspecific encountered. In species like Nepanthia belcheri, sex change can be linked to asexual reproduction, where a large female that divides by fission results in the smaller, new individuals developing into males.
Reproduction Through Fission and Regeneration
Separate from sexual methods, many starfish reproduce asexually through fission and regeneration. This strategy results in a new, genetically identical individual without the need for gametes or fertilization. Fission is the deliberate splitting of the body, which can involve the central disk breaking into two pieces or an arm detaching with a portion of the central disk attached.
The detached portion, sometimes called a “comet,” uses its regenerative capacity to regrow the missing parts of the body. In species like Coscinasterias tenuispina, the starfish may split into two halves, each regenerating the lost arms and disc, resulting in two complete individuals. This process of self-amputation, or autotomy, is a survival mechanism used for predator evasion, but it also functions as a form of reproduction.
Fission allows for rapid population increase and is a common method in some starfish genera, often occurring in response to environmental stressors. It is a distinct reproductive pathway from the production of eggs and sperm, relying on the organism’s ability to duplicate its entire structure from a fragment. For a new starfish to fully regenerate from a severed arm, it must contain at least a small portion of the central body.