Diserens, N. | bioRxiv Science

bioRxiv2026

Suitability of clove oil for the euthanasia of four ornamental fish species: Goldfish (Carassius auratus), Bristlenose catfish (Ancistrus sp.), Midas cichlid (Amphilophus citrinellus) and Three-spot gourami (Trichopodus trichopterus)

Euthanasia of ornamental fish is usually performed by immersion in an anaesthetic bath. However, it may be difficult for private fish owners to access anaesthetics, and alternative methods are limited. Clove oil is commonly available in drugstores and its use for fish euthanasia has been reported. However, there is a lack of scientific literature supporting its suitability for this purpose and of available species-specific protocols. Our study aimed to assess the suitability of clove oil to euthanise goldfish (Carassius auratus), bristlenose catfish (Ancistrus sp.), Midas cichlid (Amphilophus citrinellus) and three-spot gourami (Trichopodus trichopterus). Time to loss of reflexes and to cessation of opercular movements were recorded. Aversive reactions and histological gill lesions were compared with positive and negative controls, and reversibility of euthanasia attempts was assessed. Opercular movements did not reliably cease in goldfish, even under high clove oil concentrations (1000 mg/L). Clove oil caused loss of indicators in Midas cichlid at a concentration of 1000 mg/L, and in bristlenose catfish and three-spot gourami at a concentration of 400 mg/L. Five Midas cichlid recovered after treatment with clove oil. Clove oil caused severe gill lesions to goldfish and Midas cichlids. The results show that clove oil is not suitable for humane euthanasia of goldfish and Midas cichlid. Additionally, some studies have questioned the ability of clove oil to induce loss of consciousness in fish. Therefore, we recommend avoiding euthanasia with clove oil when suitable alternatives are available and using a secondary killing method, as euthanasia attempts are not always irreversible.

zoology↗

bioRxiv2026

Early-life conditions shape baseline immune gene expression, but not pathogen-induced immune activation, in brown trout

Early-life environment and parental background can shape immune phenotypes, but whether such differences persist during infection and influence the immune response remains unclear. We used single-cell RNA sequencing to characterize kidney immune responses in brown trout (Salmo trutta) from three origins differing in parental history and early rearing: wild parents with offspring reared in the wild; wild parents with offspring reared in a hatchery; and hatchery parents with offspring reared in a hatchery. All fish originated from the same river and showed no detectable genetic population structure. Fish were exposed to Tetracapsuloides bryosalmonae, the causative agent of proliferative kidney disease, and analyzed 25 days later alongside unexposed controls. Across 18 scRNA-seq datasets, we identified 30 cell clusters and found a clear transcriptional imprint of developmental and parental history in the resting immune system. In unexposed fish, origin was associated with pronounced differences in gene expression, particularly in B cells. When fish were subsequently exposed to the pathogen and developed subclinical infections, both immune-cell composition and transcriptional state experienced pronounced changes: T cells increased in relative abundance, while B cells, neutrophils and proliferating progenitors showed the strongest transcriptional responses, consistent with an active but controlled subclinical response. Strikingly, the strong origin-dependent differences present before exposure largely disappeared after infection. Fish from all three backgrounds converged on a shared transcriptional response, and genes associated with origin showed little overlap with those responding to infection. Thus, at single-cell resolution in vivo, we identify infection-induced transcriptional convergence across distinct environmental and parental backgrounds. Our findings show that substantial baseline immune variation can persist under resting conditions yet be overridden by pathogen exposure, constraining distinct immune states towards a common response.

immunology↗