Micro‐ CT reveals host‐controlled symbiosis of the Jurassic serpulid–hydroid association

Symbiosis impacts ecological interactions and evolutionary innovations, yet, direct evidence of host–symbiont dynamics is rarely preserved in the fossil record. The study of the Middle Jurassic hydroid Protulophila gestroi bioclaustrated within the tubes of serpulid Propomatoceros lumbricalis provides valuable insights into syn vivo interactions and palaeoecological conditions that shaped this symbiosis. Specimens from four Bajocian–Callovian localities derive from different palaeoenvironments of the Polish Basin. Hydroids are preserved in the external tube portions as small apertures surrounded by bulges formed during bioclaustration, reflecting skeletal overgrowth by the serpulid host. High‐resolution micro‐CT imaging enabled reconstruction of internal hydroid morphology and morphometric quantification. The preserved traces consist of thin, branching and anastomosing stolons and elongated cavities corresponding to polyp chambers. Comparative analysis within individual serpulid specimens and across environments reveals strong host–symbiont interdependence; external aperture morphology correlates with the depth of colony incorporation, indicating significant influence of host biomineralization. Variation among hosts suggests that, besides host–symbiont dynamics, environmental and behavioural factors such as food availability and water dynamics also contributed to shaping these relationships. These findings provide rare, direct evidence of host‐regulated symbiosis in deep time, demonstrating the complex physiological interdependence in Mesozoic mutualistic relationships.