Starfish, also known as sea stars, are invertebrates belonging to the class Asteroidea, found in all the world’s oceans from the intertidal zone to abyssal depths. They utilize both sexual and asexual methods to maintain their populations. Starfish often act as predators that help regulate the populations of other benthic invertebrates, such as mussels and clams. This dual reproductive strategy contributes to their widespread presence and ecological importance.
How Starfish Reproduce Sexually
The majority of starfish species reproduce sexually, which involves the release of reproductive cells called gametes. Starfish are typically dioecious, meaning individuals are either male or female, though a few species are hermaphroditic. The reproductive organs, or gonads, are situated within the arms, with two gonads found in each arm.
Sexual reproduction relies on a process called spawning, where individuals release sperm and eggs directly into the surrounding water column. This method is known as external fertilization. To maximize successful fertilization, starfish often form large groups called spawning aggregations.
The timing of these mass spawning events is regulated by environmental cues to ensure synchronization. Factors such as water temperatures, the phase of the moon, or changes in daylight hours can act as triggers for gamete release. Chemical signals, or pheromones, released by the first individual to spawn also prompt others to release their gametes simultaneously. In some species, a male may climb on top of a female, intertwining arms in a behavior sometimes referred to as pseudocopulation, to ensure the sperm and eggs are released close together.
Asexual Reproduction: Fission and Regeneration
Starfish can reproduce asexually, a process that does not require gametes. This method is less common but allows for rapid population growth. One primary form is fissiparity, or fission, where the central disc intentionally splits into two or more parts.
Each resulting fragment then regenerates the missing portions, including arms and internal organs, until a complete, new individual is formed. Certain species, such as those in the genera Coscinasterias and Stephanasterias, are well-known for this fissiparous ability. Six-armed starfish that undergo fission, for example, often split their disc into two three-armed halves, with each half regenerating to form a new six-armed individual.
This reproductive fission is distinct from the general ability of a starfish to simply regenerate a lost arm after an injury. However, some species can reproduce asexually through autotomy, which is the self-amputation of an arm along with a portion of the central disc. This detached arm, sometimes called a “comet,” can then regenerate a full new body, while the original starfish regrows the lost arm.
Development Stages: The Starfish Life Cycle
Following external fertilization, the resulting zygote begins the starfish life cycle. The fertilized egg undergoes rapid cell division, developing into a ciliated, free-swimming larva. This initial stage is the bipinnaria larva, characterized by bilateral symmetry and ciliated bands used for movement and feeding.
The bipinnaria progresses into the brachiolaria larva, which remains part of the plankton community. The brachiolaria stage is identifiable by three short arms that contain adhesive tips and a central sucker. The larva uses these structures to find and attach itself to a suitable substrate on the ocean floor.
Once attached, the larva undergoes metamorphosis, shifting from its bilaterally symmetrical, free-swimming form to the radially symmetrical body plan of the adult. The larval structures degenerate, and a miniature, five-rayed juvenile starfish develops from the hind-part of the larva. This juvenile then detaches from the substrate and begins life as a bottom-dwelling organism, growing until it reaches sexual maturity, typically after about a year.