Sea stars, also known as starfish, are marine invertebrates belonging to the class Asteroidea within the phylum Echinodermata. They are found in all the world’s oceans, inhabiting diverse environments from tropical reefs to polar regions and from intertidal zones to abyssal depths. Sea stars typically exhibit a central disc with arms radiating outwards, often five, though some species can have many more. They have varied methods of reproduction, allowing them to thrive across these marine landscapes.
Sexual Reproduction
Most sea star species reproduce sexually, typically having separate male and female individuals. Some species are simultaneous hermaphrodites, producing both eggs and sperm, while others are sequential hermaphrodites, changing sex during their lifespan. Each arm of a sea star contains two gonads, which release gametes through openings called gonoducts located on the central disc. Fertilization generally occurs externally, with males and females releasing their eggs and sperm into the surrounding seawater in a process called spawning.
Spawning is a coordinated event, often influenced by various environmental cues. Water temperature changes, lunar cycles, and the presence of phytoplankton can all trigger this synchronized release of gametes. The first individual to spawn may release pheromones, chemical signals that attract other sea stars and prompt them to release their gametes simultaneously. This collective spawning increases the chances of successful fertilization. In some instances, a male and female may even engage in pseudocopulation, where the male climbs atop the female, intertwining arms, to ensure closer proximity during gamete release.
Asexual Reproduction
Sea stars also have asexual reproductive capabilities. One common method involves regeneration, where a single arm, provided it contains a portion of the central disc, can develop into an entirely new sea star. Regeneration as a reproductive strategy means that a genetically identical offspring is produced without the need for a mate.
Some sea star species engage in deliberate fission, actively dividing their bodies into two or more parts. Each of these fragments, as long as it includes a piece of the central disc, can then regenerate into a complete individual. This intentional splitting allows for rapid population increases. While regeneration can occur after accidental injury, deliberate fission is a specific reproductive behavior, distinguishing it from simple tissue repair.
The Sea Star Life Cycle
Following sexual reproduction, fertilized eggs, or zygotes, develop into free-swimming larval stages. These larvae, often known as bipinnaria and brachiolaria, are planktonic, drifting in ocean currents. During this phase, the larvae are bilaterally symmetrical and consume microscopic organisms like algae for energy. This planktonic existence allows for wide dispersal before they settle.
The larval stages culminate in metamorphosis, where the free-swimming larva undergoes significant physical changes. The larva develops the radial symmetry characteristic of adult sea stars and settles onto the seabed. From this point, it grows into a juvenile sea star, gradually maturing into an adult. For asexual reproduction, the detached arm, or fragment with a central disc, gradually regenerates the missing parts, forming a new central disc, arms, and internal organs over several months until it becomes a fully formed sea star.