Sea stars, commonly known as starfish, are marine invertebrates belonging to the class Asteroidea within the phylum Echinodermata. These organisms possess a unique biology, particularly concerning their methods of creating new life and recovering from injury. Sea stars exhibit remarkable flexibility in how they propagate their species, utilizing both conventional sexual reproduction and extraordinary physical regrowth. This dual capacity places them in a distinct biological category, ensuring their survival and population maintenance.
The Primary Role of Sexual Reproduction
The vast majority of sea star species rely on sexual reproduction as their main strategy for long-term population stability and genetic diversity. Most sea stars are dioecious, meaning they have separate male and female individuals, each possessing gonads located in their arms. These gonads produce either eggs or sperm, which are then released into the water column through small openings called gonoducts situated between the arms on the central disc. Reproduction generally occurs through a process known as broadcast spawning, where synchronized release of gametes increases the likelihood of external fertilization. The timing of this mass spawning event is often influenced by environmental cues like water temperature, light conditions, and the presence of phytoplankton.
After fertilization in the open water, the resulting zygote develops into a free-swimming, bilaterally symmetrical larva. The larval stage progresses through distinct forms, such as the bipinnaria and brachiolaria larvae, which swim for weeks or months while dispersed by currents. They then undergo metamorphosis, transforming from a bilaterally symmetrical form into the radially symmetrical juvenile sea star, which settles onto the seabed as a benthic adult.
Asexual Fission and Fragmentation
While sexual reproduction is the primary method, many sea stars intentionally employ asexual strategies. One method is fission, which involves the deliberate splitting of the main body along the central disc, resulting in two new, genetically identical individuals. Species like Coscinasterias tenuispina frequently use this technique, often resulting in individuals with an irregular number of arms reflecting the recent split.
Fragmentation is another active asexual process, where a sea star voluntarily sheds an arm that is pre-programmed to develop into a new organism. This form of autotomy, or self-amputation, is used for reproduction, not just defense, and is pronounced in species like Linckia. The shed arm must contain sufficient tissue from the central nervous ring or disc to initiate the growth of a new complete body, resulting in a clone that is a genetic duplicate of the parent.
The Biological Process of Regrowth
The ability of a sea star to regrow a lost arm is a complex process of injury repair. When an arm is lost due to predation or accident, the initial response is wound closure. The wound surface is quickly sealed off by muscular contractions and the migration of epidermal cells, forming a protective layer of new skin known as re-epithelialization. Beneath the new epidermis, a regenerative structure forms using existing cells near the injury site, where differentiated cells are thought to dedifferentiate and proliferate to rebuild the missing structures.
The process of arm regrowth is organized into an early and an advanced regenerative phase, which can take several months to over a year to complete. The radial water canal and radial nerve extend by outgrowth into the developing tip, guiding the formation of the new arm. For many species, successful regeneration of an entire arm requires that the central disc remains largely intact, serving as the necessary organizational center for the repair.
When Regeneration Becomes Replication
The line between injury repair and a new life blurs when regenerative capacity leads to the creation of a whole new organism. While most sea stars require part of the central disc to regrow a complete body, certain species achieve disk-independent bidirectional regeneration. In these cases, a single detached arm, containing no part of the original central disc, develops a new disc and the remaining arms. This detached, regenerating arm is often described as a “comet,” due to its appearance of a large arm with a small, developing central body trailing behind.