Starfish, also known as sea stars, are marine invertebrates with unique reproductive strategies. Unlike birds or reptiles, starfish do not lay traditional eggs. Instead, they primarily reproduce sexually by releasing their reproductive cells, known as gametes, directly into the surrounding water.
Sexual Reproduction in Starfish
Most starfish species engage in external fertilization, a process where male and female individuals release their gametes into the seawater. Each arm contains two gonads that produce these gametes. Gametes are released through small openings called gonoducts, located on the central disc between the arms. This synchronized release of eggs and sperm into the water column is often referred to as broadcast spawning.
Successful fertilization in broadcast spawning relies on a high concentration of gametes in the water. To achieve this, starfish often gather in groups during their breeding season. Environmental cues play a significant role in triggering and synchronizing spawning events. Changes in water temperature, food sources like phytoplankton, and lunar cycles can influence when starfish release gametes. Some species also use chemical signals, such as pheromones, to attract others and induce simultaneous spawning, ensuring a higher chance of fertilization.
After male and female starfish release their gametes, the buoyant eggs and sperm mix in the water. If a sperm successfully fertilizes an egg, a zygote is formed. This external fertilization process means that the initial development of the starfish begins in the open ocean. While most species utilize external fertilization, a few species have evolved internal fertilization, and some even brood their eggs, keeping them within specialized structures until they are more developed.
Asexual Reproduction Strategies
Beyond sexual reproduction, some starfish species possess the ability to reproduce asexually. One common asexual strategy is fission, where the central disc divides into two or more pieces, and each portion regenerates the missing parts. This process leads to genetically identical offspring, effectively cloning the parent. For example, a six-armed starfish might split into two three-armed halves, each regenerating into a complete six-armed individual.
Another form of asexual reproduction involves autotomy, the intentional shedding of an arm. If a detached arm contains a portion of the central disc, it can regenerate an entire new starfish. This regenerative capacity is also used for recovery from injury or as a defense mechanism against predators. While not all starfish species reproduce asexually, it is a significant aspect of the life cycle for many, allowing rapid population growth and recovery in certain conditions.
From Larva to Adult Starfish
Following fertilization, the starfish zygote undergoes cell divisions, leading to the formation of a free-swimming, bilaterally symmetrical larval stage. The initial larval form for most starfish is called a bipinnaria larva. These microscopic larvae are equipped with bands of cilia, which they use for locomotion and filtering phytoplankton, their primary food source, to fuel their growth.
As the bipinnaria larva develops, it progresses into a second larval stage, the brachiolaria larva. This stage is characterized by three short, adhesive arms and a sucker, which allow the larva to attach to a suitable substrate on the seabed. This attachment marks a pivotal moment, as the larva then undergoes metamorphosis.
During metamorphosis, larval structures are reabsorbed, and the adult starfish body plan develops, transitioning from bilateral to radial symmetry. The juvenile starfish, resembling a miniature adult, detaches from its stalk and begins life as a bottom-dwelling organism. This developmental journey from a fertilized egg to a juvenile starfish can take several weeks or months.