Adult fission as an auxiliary reproductive mode in holothuroids, with a detailed look at Holothuria atra and Stichopus chloronotus

Asexual reproduction by transverse fission is an additional reproductive mode in some holothuroid species. Currently, there are 18 species of sea cucumbers described with transverse fission. Five of those belong to the order Dendrochirotida. All remaining examples belong to two genera, Holothuria (10 species in the order Holothuriida) and Stichopus (three species in the order Synallactida). This review focuses on Holothuriida and Synallactida and more closely investigates distribution, mechanism, and effects on population structure in the most abundant Indo-Pacific coral-reef species Holothuria atra and Stichopus chloronotus. Nearly all species of Holothuriida and Synallactida identified as being capable of undergoing transverse fission are shallow-water, coral-reef species occurring in populations with high asexual rates and usually inhabiting the shallower part of the respective species' distribution. Asexual reproduction has important influences on population structure, creating high-density populations consisting of smaller individuals. In addition, these populations have low genetic diversity often dominated by a few clones (groups of individuals with the same multilocus genotype) and often exhibit biased sex ratios. We present a model suggesting that locations with presumed high mortality, low habitat stability, small optimum individual size, high food availability, and low larval supply are promoting asexual reproduction through fission. Because asexual reproduction leads to many members of certain clones, it can extend genotype longevity ad infinitum ("eternal life") and ultimately also increase sexual reproductive success for these broadcast-spawning echinoderms.