Post-Fission Regeneration and Life-History Traits in the Fissiparous Brittle Star Ophiactis savignyi (Ophiuroidea: Amphilepidida: Ophiactidae).

Fissiparity, a form of asexual reproduction, involves the loss of significant body parts followed by regeneration. In echinoderms, this mode of reproduction has been documented in numerous species, particularly among holothurians, asteroids, and ophiuroids. Fissiparous brittle stars typically exhibit six-fold symmetry and restore this body plan by regenerating the missing half of the disk and three arms after fission. However, the detailed regeneration process and its dynamics in natural populations remain poorly understood. Here, we characterized post-fission regeneration in the brittle star Ophiactis savignyi by defining eight stages based on external morphology and histological features. Our observations revealed that, during early disk regeneration, the paired lateral arm primordia developed prior to the central primordium, suggesting that regenerating individuals may transiently pass through a five-armed configuration before re-establishing six-fold symmetry. We further quantified the frequency of each regeneration stage in a wild population and examined its relationships with season and body size. Early regeneration stages were less frequent from winter to spring and among large individuals, indicating that fissiparity is more active during warmer periods and in smaller individuals. In addition, swelling of interradial regions containing gonads was observed in some large individuals, suggesting a size-dependent life-history strategy in which asexual reproduction predominates in smaller individuals and sexual reproduction becomes more prominent with growth.