Functionally convergent anti-predator morphologies arise through divergent cellular strategies in Daphnia

Phenotypic plasticity exemplifies how environmental signals can shape organismal development, yet the cellular mechanisms translating ecological cues into adaptive morphologies remain incompletely characterized. Many species of Daphnia (freshwater crustaceans) develop a diverse array of inducible defenses in response to predator chemical cues (kairomones), providing a tractable system for examining how ecological pressures are transduced through developmental mechanisms. Daphnia lumholtzi produce elongated head and tailspines in response to kairomones. How this occurs at the cellular level, through changes in cell size, proliferation, or both, is currently unknown. To address this question, we quantified the temporal dynamics of cell proliferation in D. lumholtzi across 72 hours following kairomone exposure using EdU incorporation (marking proliferating cells) and DAPI staining (to quantify total nuclei). Predator cue exposure induced a three-phase proliferative response: initiation within 24 hours (slightly increased proliferation and total cells), a transitional plateau at 48 hours (minimal effects in both treatments), and commitment by 72 hours (strong increases in both proliferation and cell accumulation). Headspines exhibited higher proliferation than tailspines, suggesting anterior- posterior developmental prioritization. Treated animals maintained smaller average cell sizes throughout the response, consistent with continuous addition of newly divided cells rather than cell enlargement. Unlike the delayed-division strategy in D. longicephala or bilayer formation in D. pulex, D. lumholtzi employs sustained hyperplasia, demonstrating that the same selective pressure produces similar ecological outcomes through mechanistically distinct developmental programs. Our findings bridge ecological signals with cellular responses, exemplifying eco-evo- devo integration whereby environmental pressures (predation) are transduced through developmental processes (cell proliferation dynamics) to generate adaptive and heritable morphological diversity across ecological and evolutionary timescales.