The direct answer to whether a spider can change its sex is overwhelmingly no for the vast majority of species. A spider’s biological sex is determined very early in development and remains fixed throughout its lifetime. True, functional sex reversal is not a known biological process in adult spiders. The idea of a sex change is often confused with developmental changes that occur as they mature.
The Biological Reality: Fixed Sex in Spiders
Spiders are classified as gonochoristic, meaning individuals are distinctly either male or female, a state also referred to as dioecy. This arrangement is the norm across the entire order Araneae, which encompasses over 50,000 known species. This fixed nature contrasts sharply with some other animal groups, like certain fish or mollusks, which exhibit sequential hermaphroditism and can functionally change sex during their lives. Once a spider reaches sexual maturity, its role as a male or female is permanent.
How Spider Sex is Determined
The biological foundation for this fixed sex lies in the spider’s genetic makeup, which establishes sexual identity at the moment of fertilization. Spiders possess complex chromosomal sex determination systems that are highly varied across different families. The most frequently observed system in spiders is the X1X20 system.
In this system, females typically carry two pairs of X chromosomes (X1X1X2X2), while males possess only one pair of X chromosomes (X1X2) and no Y chromosome, hence the “0” designation. Other groups, like some cellar spiders, utilize a simpler X0/XX system or even an XY/XX system, where males have a Y chromosome. This chromosomal configuration dictates the developmental pathway, ensuring that the spider’s sex is locked in long before it hatches. The process is a hard-wired genetic mechanism, unlike some environmental sex determination methods found in other arthropods.
Atypical Development and Misunderstood Phenomena
Observations that might suggest a spider has changed sex are usually instances of developmental anomalies or misinterpretations of normal maturation. The most striking anomaly is gynandromorphism, a rare condition where an individual exhibits both male and female characteristics clearly separated within the body. A spider may appear half-male and half-female, sometimes split perfectly down the middle, resulting from a genetic error during early cell division. Gynandromorphs are mosaics where different parts of the body express different genetic sexes, often due to the loss of a sex chromosome in one cell line.
Another related anomaly is intersexuality, where the spider displays characteristics that are intermediate between the sexes. Both gynandromorphism and intersexuality represent errors in initial determination or subsequent development, not a functional reversal of sex.
A common misconception arises during the final molt when a juvenile spider reaches adulthood. Males often undergo a significant change in appearance, such as developing the bulbous reproductive organs (emboli) on their pedipalps or changing color and size, which is mistaken for a sex change.
Furthermore, the presence of endosymbiotic bacteria, such as Wolbachia, can profoundly affect the sex ratio in some spider populations, sometimes causing a female-biased brood. While these bacteria can manipulate the sexual expression or survival of offspring in arthropods, there is no evidence that they cause a fully functional adult spider to reverse its sex.