Most starfish do have separate sexes, with distinct male and female individuals that produce either sperm or eggs. The catch is that you almost never can tell them apart by looking. Males and females of the same species are virtually identical on the outside, and the only reliable way to determine sex in most species is to examine the gonads directly. Reproduction, meanwhile, is far more varied than most people expect, ranging from mass synchronized spawning events to quietly splitting in half.
Separate Sexes, but Not Always
The vast majority of starfish species are what biologists call gonochoric, meaning an individual is either male or female for its entire life. In studies of specific species, the ratio of males to females in a population tends to hover around one to one. Research on the tropical Pacific starfish Pharia pyramidatus, for example, found no significant difference from a balanced sex ratio throughout the year.1Cambridge University Press. Reproductive biology of the starfish Pharia pyramidatus (Echinodermata: Asteroidea) from the Mexican tropical Pacific But that tidy ratio is not universal. Populations of the fissiparous starfish Coscinasterias calamaria show dramatic site-to-site variation: two populations had roughly equal numbers of males and females, one was heavily biased toward females, and a fourth consisted almost entirely of males.2ScienceDirect. Sexual and asexual reproduction in geographically separated populations of the fissiparous asteroid Coscinasterias calamaria (Gray)
A smaller number of starfish species are simultaneous hermaphrodites, carrying both ovaries and testes at the same time. And at least one species has taken this to an extreme. Cryptasterina hystera reproduces through internal self-fertilization and brood protection, a suite of traits that evolved relatively recently compared to its close relative Cryptasterina pentagona, which is a standard broadcast spawner with separate sexes.3PubMed Central. Extraordinarily rapid life-history divergence between Cryptasterina sea star species So the short answer to “do starfish have genders” is yes, usually, but the group as a whole has explored nearly every possible arrangement.
Broadcast Spawning and the Problem of Open Water
The most common way starfish reproduce sexually is broadcast spawning. Males and females release sperm and eggs directly into the water, where fertilization happens externally. This sounds haphazard, and it is. Sperm dilutes quickly in open ocean, so the timing of release matters enormously. If males spawn hours before females, or vice versa, fertilization rates plummet.
To solve this, starfish rely on a combination of environmental and biological cues to synchronize spawning within a population. Research on crown-of-thorns starfish found that an abrupt temperature increase of about 4°C triggered spawning in males but had less effect on females. Males also spawned in response to phytoplankton in the water, while females did not respond to that cue at all. The presence of sperm already in the water column, however, induced both sexes to spawn, suggesting that males typically spawn first and their sperm acts as a chemical signal for females to follow.4PLOS ONE. Environmental and biological cues for spawning in the crown-of-thorns starfish This staggered system, with males acting as the first responders and females joining once chemical confirmation is in the water, helps ensure eggs meet sperm before it dilutes beyond usefulness.
Chemical Signals Between Males and Females
The coordination goes beyond just reacting to temperature or food. Gravid (reproductively ready) starfish produce sex-specific chemical signals, essentially pheromones, that help regulate when and where spawning aggregations form. Research on crown-of-thorns starfish identified distinct pheromone profiles in males and females, received primarily by sensory tentacles on the arm tips. Those tentacles express a range of receptors and signaling molecules in a sex-specific manner, meaning males and females are literally tuned to detect different chemical cues from each other.5PubMed Central. Sex-specific expression of pheromones and other signals in gravid starfish
This pheromone system helps explain why starfish that seem to be solitary animals for most of the year can suddenly form dense reproductive aggregations at precisely the right time. They are not stumbling into each other. They are chemically calling to one another across the reef or seabed.
What Happens Inside the Gonad
Inside a starfish’s body, spawning is triggered by a hormonal cascade that has been studied for decades and remains one of the best-understood examples of reproductive endocrinology in invertebrates. A hormonal peptide released from the nervous system acts on the ovary (or testis) to produce a molecule called 1-methyladenine, which serves as the direct trigger for egg maturation and release.6PubMed. 1-methyladenine biosynthesis in starfish ovary: action of gonad-stimulating hormone in methylation This molecule appears to work across many starfish species as a general inducer of both egg maturation and ovulation.7PubMed. Induction of spawning and oocyte maturation in starfish by 1-methyl-adenosine monophosphate
Sea stars have actually served as important model organisms for studying basic reproductive processes across the animal kingdom. Their eggs are large, transparent, and easy to work with in the lab, and the hormonal pathway from nervous-system signal to gamete release is well mapped, making them useful far beyond marine biology.8PubMed Central. Use of sea stars to study basic reproductive processes
Not All Starfish Scatter Their Eggs
Broadcasting is the dominant strategy, but some starfish take a completely different approach: brooding. A brooding starfish holds its fertilized eggs on or inside its body while they develop, rather than releasing them to drift in the plankton. This is relatively uncommon among echinoderms as a whole, but it has evolved independently in several lineages and is of real scientific interest as a departure from the norm.9Journal of Experimental Marine Biology and Ecology. Brooding in echinoderms: How can modern experimental techniques add to our historical perspective?
The cold-water species Leptasterias polaris is a well-studied example. Females arch their bodies upward over a clutch of fertilized eggs, holding them close to their underside for months. In lab conditions, the embryos go through their first cell division in about 45 hours, reach an advanced larval stage at around 40 days, and become fully developed juvenile starfish within roughly five and a half to six months. Interestingly, embryos develop at the same rate whether or not they are actually being brooded, which suggests the mother’s main contribution is not warmth or nutrition but simply keeping the eggs clean and safe from predators.10PubMed. Prespawning Behavior, Spawning, and Development of the Brooding Starfish Leptasterias polaris
Brooding species tend to produce far fewer offspring than broadcast spawners, but each offspring gets a much better start in life. A broadcast spawner might release millions of eggs, most of which die as larvae drifting in the plankton. A brooding mother might produce dozens of well-developed juveniles that settle directly onto the seabed, skipping the risky planktonic stage entirely.
Reproducing Without a Mate
Some starfish can reproduce without sex at all. The most dramatic form of asexual reproduction in starfish is fission: the animal literally splits its body in two, and each half regenerates the missing parts. Certain species rely heavily on this strategy. At least one tropical asterinid sea star maintains stable populations through asexual propagation alone, even in geographically isolated settings.11Marine Biology. Forever fissiparous: asexual propagation and stable demography in a tropical and geographically isolated asterinid sea star
Fission comes with trade-offs, though. A starfish that has recently split has fewer arms to catch food with, and it must divert energy toward regrowing its missing body parts rather than toward reproduction or growth. Studies on the sea star Allostichaster capensis found that recently split individuals had the lowest feeding rates, the least stored lipids, and the lowest gonad production, regardless of how much food was available.12Journal of Experimental Marine Biology and Ecology. Fission, regeneration, gonad production and lipids storage in the pyloric caeca of the sea star Allostichaster capensis In other words, splitting is not free. The animal pays for it by temporarily losing reproductive capacity.
Arm Loss and the Cost to Reproduction
Even when a starfish loses arms through predation or self-amputation rather than fission, the energy toll on reproduction is severe. Work on the multi-armed sun star Heliaster helianthus showed that adults that had lost arms had five to seven times lower reserves of carbohydrates, lipids, and proteins in both their food-storage organs and their gonads compared to intact animals. Arm loss did not just reduce the animal’s ability to eat; it forced a reallocation of energy away from growth and reproduction and toward regeneration.13Journal of Experimental Marine Biology and Ecology. Effect of the degree of autotomy on feeding, growth, and reproductive capacity in the multi-armed sea star Heliaster helianthus
This means that in a population experiencing heavy predation or disease-related arm loss, reproductive output can drop significantly even if the animals survive. Each individual may look healthy once it has regrown its arms, but the months spent regenerating represent a reproductive gap.
From Egg to Juvenile
For broadcast spawners, fertilized eggs develop into free-swimming larvae that drift in the plankton, feeding on tiny algae. Species with these feeding larvae typically pass through a distinctive bipinnaria stage and, in most cases, a subsequent brachiolaria stage before they are ready to settle.14Methods in Cell Biology. Echinoderms, Part A – Section: 3.1 Asteroidea The brachiolaria larva has specialized adhesive structures that help it grip a surface when it finally drops out of the water column.
Settlement is not random. Competent larvae (those that are developmentally ready to metamorphose) respond to biological cues on the seabed, particularly bacterial biofilms coating rocks, shell fragments, or other hard surfaces. Once attached, the larva undergoes a dramatic metamorphosis, reorganizing its body plan from a bilaterally symmetric swimming form into the radially symmetric crawling juvenile starfish. The whole process, from fertilized egg to settled juvenile, can take weeks to months depending on species and water temperature. During that planktonic phase, mortality is enormous. Larvae face starvation, predation, and the real possibility of being swept into unsuitable habitat by ocean currents.
When Species Cross
Because many starfish species spawn by releasing gametes into the same water at roughly the same time of year, hybridization between closely related species is a real possibility. Cross-fertilization experiments with three species of Patiriella sea stars living in the same waters off New South Wales found that the gametes of P. calcar and P. gunnii were fully compatible in both directions, producing viable hybrid juveniles in the lab.15PubMed. Hybridization of sympatric Patiriella species (Echinodermata: Asteroidea) in New South Wales
Genomic evidence suggests hybridization also happens in the wild. Analysis of Asterias sea stars in the North Atlantic documented introgression (gene flow through hybridization) occurring between northern New England and the southern Canadian Maritimes, alongside ecological divergence and signs of adaptation to different environments.16PubMed. Evidence of introgression, ecological divergence and adaptation in Asterias sea stars Hybridization zones can introduce genetic variation that helps populations adapt to changing conditions, or they can blur species boundaries in ways that complicate conservation efforts.
Threats to Starfish Reproduction
The reproductive success of starfish is increasingly affected by environmental change. Ocean acidification poses a direct threat to the earliest stages of reproduction. Experiments on the coral-eating crown-of-thorns starfish showed that elevated carbon dioxide levels in seawater significantly reduced sperm motility and, to a lesser extent, velocity. At environmentally realistic sperm concentrations, this translated into strongly reduced fertilization rates. Larvae raised under these conditions also showed significantly lower rates of normal development by day ten.17PLOS ONE. Impacts of Ocean Acidification on Early Life-History Stages and Settlement of the Coral-Eating Sea Star Acanthaster planci For a broadcast spawner that depends on sperm finding eggs in open water, anything that slows sperm down or reduces its effectiveness can have outsized consequences.
Disease presents another challenge. Sea star wasting disease caused massive die-offs of Pisaster ochraceus and other species along the North American Pacific coast in recent years. Recovery depends on recruitment of new juveniles and their survival to reproductive size, but data show that both recruitment and survivorship vary dramatically from place to place. Juveniles need to be cryptic to survive, and sick individuals do not persist long in the harsh intertidal zone, making population recovery uneven and difficult to predict.18PLOS ONE. Large-scale impacts of sea star wasting disease (SSWD) on intertidal sea stars and implications for recovery
Why Crown-of-Thorns Outbreaks Keep Happening
On the opposite end of the spectrum from struggling populations, the crown-of-thorns starfish is notorious for periodic population explosions that devastate coral reefs. A major reason these outbreaks are so hard to prevent is the species’ extreme reproductive potential. A single large female can produce tens of millions of eggs in a season. Combine that with larvae that are flexible in their diet, resilient to variable food conditions, and capable of rapid, plastic growth, and you get a species that can capitalize on even small environmental advantages to boom in numbers.19PubMed Central. Crown of thorns starfish life-history traits contribute to outbreaks, a continuing concern for coral reefs
Nutrient runoff from agriculture, which fuels phytoplankton blooms that feed crown-of-thorns larvae, is one of the leading hypotheses for what tips these populations from stable into outbreak mode. The link between spawning cues and phytoplankton presence in the water, described earlier, adds another layer: more phytoplankton may not only feed larvae better but also stimulate more males to spawn in the first place, amplifying the entire reproductive cycle. Reef managers trying to control outbreaks are essentially fighting against a reproductive system that evolved to be prolific and opportunistic.