Influence of various reproduction substrates on the survival, health, and reproductive output of broodstock Mediterranean medicinal leech (Hirudo verbana).

Abstract

The captive breeding of non-traditional aquatic species is becoming increasingly important for both niche animal production and the conservation of overexploited taxa; however, breeding protocols for these species remain largely undocumented. The Mediterranean medicinal leech (Hirudo verbana), which is clinically used in reconstructive surgery and listed on CITES Appendix II, deposits its cocoons on a moist terrestrial substrate; this makes the reproductive substrate a critical but untested management variable. We compared four substrates available to leech breeders, namely unamended fresh lake peat (FLP), unamended commercial terrestrial peat (CTP), commercial lake peat (CLP), and vermicompost (VC). Each substrate was characterized across 14 physicochemical parameters and evaluated for its effects on the survival, health, and reproductive output of broodstock leeches. Substrate type significantly altered survival (log-rank χ²(3) = 53.3, P < 0.001): all leeches died within 2 days in VC and within 4 days in CLP; in contrast, 50% (FLP) and 60% (CTP) survived the entire 75-day experiment. Principal component analysis and hierarchical clustering distinguished lethal substrates from viable substrates along an ionic and osmotic gradient driven by electrical conductivity up to 10.14 dS/m and high levels of sodium, copper, and zinc, indicating that the immediate cause of death was ionic and osmotic stress rather than organic richness. No cocoons were produced on the lethal substrates, whereas on the two viable substrates, the number of cocoons, cocoon morphology, number of offspring per cocoon, and offspring body weight were equivalent. These findings reveal that commercially processed horticultural growing media pose an uncharacterized acute threat to captive reproduction and support the use of electrical conductivity as a low-cost, quantitative pre-use screening criterion; this principle is likely applicable to other exotic and wild species reared in semi-terrestrial breeding environments as well.