Internal and external factors influencing total segment number and posterior segment regeneration of the earthworm model Eisenia andrei

Abstract Posterior segment regeneration in earthworms is a complex process shaped by intrinsic physiological state and environmental conditions, yet the decisive factors determining regenerative output remain incompletely understood. Using the earthworm model Eisenia andrei (Annelida: Clitellata: Lumbricidae), we investigated how body condition, nutritional status, sexual maturity, and degree of tissue loss influenced posterior segment regeneration through quantitative, morphological, histochemical, and ultrastructural analyses. Our findings revealed a tendency for final regenerative output to converge toward a baseline body plan, with most individuals approaching approximately 100 total segments after regeneration. Sexual maturity (clitellate state) did not influence the regenerative output, suggesting that clitellum-associated regulation of regeneration may be context-dependent or species-specific rather than universal across earthworms. In contrast, nutritional status had a pronounced effect: starved animals regenerated approximately 82% more segments than fed animals despite substantial body weight loss, revealing an unexpected association between nutrient restriction and increased regenerative output. Regenerated segments also differed morphologically between nutritional conditions, suggesting that tissue maturation and differentiation may proceed differently under starvation. Ultrastructural analyses revealed extensive muscle dedifferentiation and a heterogeneous regeneration blastema, including cells lacking detectable mitochondria. Consistent with these observations, lactate dehydrogenase activity was increased particularly in regenerating starved animals, providing preliminary evidence for enhanced glycolytic metabolism and raising the possibility that lactate-dependent processes play a role in regeneration. Together, these findings reveal a strong influence of nutritional state on segment regeneration and provide new insights into the physiological and cellular mechanisms underlying total segment number and posterior regeneration in E. andrei .