CIRCADIAN RHYTHMS REVEAL CONSERVED PRINCIPLES OF ENDOCRINE INTEGRATION IN A TERRESTRIAL GASTROPOD MOLLUSC

Abstract

Circadian regulation of endocrine secretion coordinates reproduction, metabolism, and stress physiology across metazoans. While the temporal organisation of endocrine axes is well characterised in vertebrates, quantitative rhythmometric analyses of diurnal hormone dynamics in terrestrial gastropods remain limited. Here, we evaluated 24-h variation in circulating endocrine factors in the nocturnal pulmonate Achatina fulica using single-component cosinor analysis. Adult snails were sampled at 06:00, 12:00, 18:00, and 00:00 h across 24-h cycle every 24h, 48h, 72h and 96h (n = 4 per time point). Plasma concentrations of follicle-stimulating hormone (FSH), luteinizing hormone (LH), progesterone (P), testosterone (T), estradiol (E), growth hormone (GH), and glucocorticoid-immunoreactivity (CORT) were quantified by ELISA and fitted to a 24-h cosine function to estimate mesor, amplitude, and acrophase. Cosinor analysis revealed pronounced axis-specific temporal organisation. FSH exhibited high-amplitude rhythmicity (mesor 65.25 ng/mL; amplitude 90.50 ng/mL) with a nocturnal acrophase at 00:00 h, and progesterone showed a similarly robust rhythm (mesor 34.42 ng/mL; amplitude 57.67 ng/mL) peaking at 00:20 h. CORT displayed moderate 24-h variation (mesor 3.50 ng/mL; amplitude 2.24 ng/mL) with an acrophase at 19:46 h. Estradiol exhibited moderate rhythmicity (mesor 0.31 ng/mL; amplitude 0.25 ng/mL) with an acrophase at 21:39 h, whereas testosterone showed low-amplitude oscillation (mesor 7.28 ng/mL; amplitude 0.48 ng/mL). GH displayed modest rhythmicity (mesor 0.28 ng/mL; amplitude 0.17 ng/mL) with an estimated acrophase near 00:14 h. In contrast, LH remained arrhythmic across the 24-h cycle. These findings indicate phase-specific endocrine compartmentalisation in A. fulica, characterised by nocturnal reproductive hormone surges and a dusk-associated elevation in CORT. Given the limited temporal resolution (four sampling points; df = 1), rhythm parameters are interpreted as descriptive indicators of temporal structure rather than definitive inferential estimates.